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Planetary Geophysics: What is it? What can it teach us about finding life beyond Earth?

March 19th, 2024

Universe Today has examined the importance of studying impact craters, planetary surfaces, exoplanets, astrobiology, solar physics, comets, and planetary atmospheres, and how these intriguing scientific disciplines can help scientists and the public better understand how we are pursuing life beyond Earth. Here, we will look inward and examine the role that planetary geophysics plays in helping scientists gain greater insight into our solar system and beyond, including the benefits and challenges, finding life beyond Earth, and how upcoming students can pursue studying planetary geophysics. So, what is planetary geophysics and why is it so important to study it?

“Planetary geophysics is the study of how planets and their contents behave and evolve over time,” Dr. Marshall Styczinski, who is an Affiliate Research Scientist at the Blue Marble Space Institute of Science, tells Universe Today. “It is essentially the study of What Lies Below, focusing on what we can’t see and how it relates to what we can see and measure. Most of the planets (including Earth!) are hidden from view—geophysics is how we know everything about the Earth below the deepest we have dug down!”

As its name implies, geophysics is the study of understanding the physics behind geological processes, both on Earth and other planetary bodies, with an emphasis on interior geologic processes. This is specifically useful for planetary bodies that are differentiated, meaning they have several interior layers resulting from heavier elements sinking to the center while the lighter elements remain closer to the surface. 

The planet Earth, for example, is separated into the crust, mantle, and core, with each having its own sub-layers, and understanding these interior processes help scientists piece together what the Earth was like billions of years ago and even make predictions regarding the planet’s environment in the far future. These interior processes drive the surface processes, including volcanism and plate tectonics, both of which are responsible for maintaining the Earth’s temperature and recycling materials, respectively. So, what are some of the benefits and challenges of studying planetary geophysics?

Dr. Styczinski tells Universe Today, “Geophysics gives us the tools to determine what exists beneath the visible surface of planetary bodies (planets, moons, asteroids, etc.). It’s our only way to learn about what we can’t see! Finding out what is inside a planet, and under what conditions, like how much pressure and heat for each layer, helps us build a history for the planet and know how it will continue to change over time.”

In contrast, Dr. Styczinski also emphasizes to Universe Today the challenges, noting the difficulty in reproducing geologic conditions that occur over millions of years, even with the most sophisticated laboratories in the world, due to their slow movements over vast amounts of time. Additionally, he notes that particle accelerators are sometimes required to reproduce the extreme conditions within gas giants, which are also differentiated, though with gas and liquid layers, as opposed to rock. 

Artist’s illustration of gas giant interiors. (Credit: NASA/Lunar And Planetary Institute)

But Earth is not the only rocky world in our solar system that exhibits differentiation, as all four rocky planets (Mercury, Venus, Earth, and Mars) exhibit some form of interior layering that has occurred over billions of years, though at smaller scales due to their sizes. In addition to the planets, many rocky moons throughout the solar system also exhibit differentiation, including Jupiter’s Galilean moons, Io, Europa, Ganymede, and Callisto, and several of Saturn’s moons, including Titan, Enceladus, and Mimas. Of those moons, Europa, Titan, and Enceladus are currently targets for astrobiologists, as Europa and Enceladus have been confirmed to possess interior liquid water oceans, with Titan presenting strong evidence, as well. Additionally, Titan is the only moon with a dense atmosphere, and like Earth, it likely has interior geophysics driving it. But what can planetary geophysics teach us about finding life beyond Earth?

Artist’s illustration of terrestrial (rocky) planet interiors. (Credit: NASA)
Artist’s illustration of the interior of Jupiter’s icy moon, Europa. (Credit: NASA/JPL-Caltech/Michael Carroll)
Artist’s illustration of the interior of Saturn’s icy moon, Enceladus. (Credit: NASA/JPL-Caltech)

“We’ve learned from studying Mars that the surfaces of planets can be quite hostile to life as we know it,” Dr. Styczinski tells Universe Today. “If and when we are able to find life elsewhere in the solar system that we didn’t bring there ourselves, it will probably be found beneath the surface, where it can be protected from the harsh environment at the surface. Geophysics gives us the means to plan for expeditions into the subsurface, and the only method of finding liquid water that’s hidden from view on icy moons. These are the best places we know of to look for life beyond Earth.”

The reason why the surface of Mars is inhospitable to life as we know it is due to its lack of a thick atmosphere, which is responsible for preventing the Sun’s charged particles in the solar wind from reaching the planetary surface. While Mars once had a powerful magnetic field, Dr. Styczinski notes to Universe Today that “Some researchers think magnetic fields can actually strip away the atmosphere”, while quickly noting this “is a topic of fierce debate.” Mars once had a thicker atmosphere, which was lost along with its magnetic field over billions of years as the Red Planet’s interior cooled.

In addition to our solar system, Dr. Styczinski tells Universe Today that planetary geophysics also does an excellent job of helping scientists better understand exoplanets, specifically multi-planet systems like our own. While no exoplanet surface has yet been imaged, better understanding the geophysical processes of planetary bodies within our solar system helps scientists gain insights into how these same processes could occur on planets throughout the cosmos, including the magnetic field, as well. 

A planet’s magnetic field is driven by the internal processes occurring in its outer core, which for Earth is comprised of churning, liquid metal fluid, whereas the inner core is a solid ball of compressed metal. As this outer core’s fluid churns and circulates, it creates electrical currents that produce the massive magnetic field that envelopes our small, blue world in a bubble of protection from harmful space weather. The Earth’s magnetic field traps charged particles in radiation belts in space nearby. The way the magnetic field protects our planet can be seen during magnetic storms from the Sun, when the magnetosphere bends and flexes in response, sending particles from these radiation belts close to the surface in the high northern and southern latitude regions. There, they interact with the Earth’s atmosphere to produce the breathtaking auroras often observed in Alaska, the Nordic countries, and Antarctica. 

Rendition displaying the solar wind interacting with Mars, which does not possess a magnetic field, versus Earth and its very active magnetic field. The lack of a magnetic field means Mars is constantly bombarded by space weather, exposing its surface to harmful radiation, whereas Earth’s surface is almost entirely protected, allowing life to both survive and thrive across the planet. (Credit: NASA)

However, while the Earth’s magnetic field is impressive, it’s only fitting that the largest planet in the solar system, Jupiter, equally has the largest magnetic field, whose “tail” extends as far as Saturn’s orbit, or approximately 400 million miles. Additionally, the internal processes responsible for generating magnetic fields on gaseous planets like Jupiter, Saturn, Uranus, and Neptune could be starkly different than on Earth. Therefore, given all of these variables and processes, what is the most exciting aspect of planetary geophysics that Dr. Styczinski has studied during his career?

“The part of planetary geophysics that I find the most exciting is using the invisible magnetic field to sense subsurface oceans,” Dr. Styczinski tells Universe Today. “I continue to be blown away by how it all works when I really think about it. Salty ocean waters partially reflect the fields they are exposed to from their parent planet, as in Jupiter and its moon Europa. We use these measurements along with laboratory studies here on Earth and geophysics to understand the material layers inside Europa to work out the properties of the ocean. It still blows my mind that this process works as well as it does.”

Like most scientific fields, planetary geophysics encompasses a myriad of scientific disciplines and backgrounds with the goal of answering the universe’s toughest questions through constant collaboration and innovation. Geophysics is a combination of geology and physics but also incorporates mathematics, chemistry, atmospheric science, seismology, mineralogy, and many others with the goal of better understanding the interior processes of the Earth and other planetary bodies throughout the solar system and beyond. Therefore, what advice can Dr. Styczinski offer upcoming students who wish to pursue studying planetary geophysics?

“There are many paths into geophysics, and many different things to study and ways to study them,” Dr. Styczinski tells Universe Today. “Your past studies don’t have to be specific to geophysics or even involve geology at all. Perhaps the most productive move you can make is to ask for help, especially from someone studying a topic that interests you. Computer programming skills are invaluable. I recommend learning Python—it’s free and widely used all across science. There are many tutorials available, also for free. While not all geophysics will require a lot of programming, I think all geophysicists will benefit from having those skills.”

How will planetary geophysics help us better understand our place in the cosmos in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

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This New Map of 1.3 Million Quasars Is A Powerful Tool

March 18th, 2024

Quasars are the brightest objects in the Universe. The most powerful ones are thousands of times more luminous than entire galaxies. They’re the visible part of a supermassive black hole (SMBH) at the center of a galaxy. The intense light comes from gas drawn toward the black hole, emitting light across several wavelengths as it heats up.

But quasars are more than just bright ancient objects. They have something important to show us about the dark matter.

Large galaxies have supermassive black holes at their centers. Even those only casually familiar with space know that black holes can suck everything in, even light. But as black holes draw nearby gas towards themselves, the gas doesn’t all go into the hole, past the event horizon and into oblivion. Instead, much of the gas forms a rotating accretion disk around the black hole.

SMBHs aren’t always actively drawing material to them, an act known as ‘feeding.’ But when an SMBH is actively feeding, it’s called an active galactic nucleus (AGN.) When the material in the disk rotates, it heats up. As it heats, it emits different wavelengths of electromagnetic radiation. It can also emit jets.

When astronomers first began to detect this light, they only knew they were seeing objects that emitted radio waves. The name quasar means quasi-stellar radio source. But as time went on astronomers learned more, and the term active galactic nucleus was adopted. The term quasar is still used, but they’re now a sub-class of AGN that are the most luminous AGN.

Quasars inhabit galaxies that are surrounded by enormous haloes of dark matter. Astronomers think there’s a link between the dark matter haloes (DMH) and the quasars. The DMH may direct more matter toward the center of the galaxy, feeding the SMBH and igniting a quasar, and even aiding the formation of more massive galaxies.

Artist rendering of the dark matter halo surrounding our galaxy. For quasars, the dark matter halos are much more massive. Credit: ESO/L. Calçada
Artist rendering of the dark matter halo surrounding our galaxy. Credit: ESO/L. Calçada

A team of researchers has created a new catalogue of quasars that will be a powerful tool for probing quasars, DMHs, and SMBHs. Their results are in a new paper in The Astrophysical Journal titled “Quaia, the Gaia-unWISE Quasar Catalog: An All-sky Spectroscopic Quasar Sample.” The lead author is Kate Storey-Fisher, a postdoctoral researcher at the Donostia International Physics Center in Spain.

“This quasar catalogue is different from all previous catalogues in that it gives us a three-dimensional map of the largest-ever volume of the universe,” said map co-creator David Hogg, a senior research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City and a professor of physics and data science at New York University. “It isn’t the catalogue with the most quasars, and it isn’t the catalogue with the best-quality measurements of quasars, but it is the catalogue with the largest total volume of the universe mapped.”

This infographic helps explain Quaia, the new catalogue of 1.3 million quasars. Image Credit: ESA/Gaia/DPAC; Lucy Reading-Ikkanda/Simons Foundation; K. Storey-Fisher et al. 2024
This infographic helps explain Quaia, the new catalogue of 1.3 million quasars. Image Credit: ESA/Gaia/DPAC; Lucy Reading-Ikkanda/Simons Foundation; K. Storey-Fisher et al. 2024

The fact that the new catalogue captures the largest total volume of the Universe mapped and all the quasars in that space is key to understanding its purpose. It’s not meant as a survey that captures the largest number of quasars. The catalogue is meant to be a tool astrophysicists can use to understand the relationships between quasars, dark matter, black holes, and galaxies.

They call their catalogue Quaia because the data comes from the ESA’s Gaia spacecraft. Gaia’s mission is to map about one billion objects in the Milky Way, mostly stars. And it’s going about its mission with extreme accuracy. But among the multitudes of stars Gaia has mapped is a large number of quasars well beyond the Milky Way. That generated the name “Quaia.”

“We were able to make measurements of how matter clusters together in the early universe that are as precise as some of those from major international survey projects — which is quite remarkable given that we got our data as a ‘bonus’ from the Milky Way–focused Gaia project,” Storey-Fisher says.

Dark matter tends to clump in haloes around galaxies, and studying the distribution of quasars can help explain the distribution of dark matter. In the large scale of the Universe, dark matter is organized as a web, and the catalogue of quasars helps map that web.

The Cosmic Microwave Background (CMB), a strong piece of evidence for the Big Bang, is also part of this. As the light from the CMB travels toward us through space, the dark matter web’s massive gravitational power bends the light. Scientists can compare the CMB light we receive with the map of quasars and compare the two. The comparisons will them about the relationship between dark matter and quasars and how matter clumps together in the Universe.

Since quasars trace the cosmic web, their distribution gives information about the web that other sources can’t. For example, it can trace the distribution of matter at higher redshifts than galaxies can. And since it’s space-based, it avoids some of the data contamination that other quasar surveys suffer from, such as the Sloan Digital Sky Survey (SDSS.)

This is not the first quasar map/catalogue to be created. There are several others, including one from the Sloan Digital Sky Survey.

This figure shows five different quasar maps created by scientists using different data and methodologies. The creators of Quaia say that its redshifts are more accurate than the others, along with other properties. Image Credit: K. Storey-Fisher et al. 2024
This figure shows five different quasar maps created by scientists using different data and methodologies. The creators of Quaia say that its redshifts are more accurate than the others, along with other properties. Image Credit: K. Storey-Fisher et al. 2024

As the animation below shows, Quaia is more complete than the SDSS’s DR16Q, the SDSS’s quasar catalogue that accompanied its data release 16.

via GIPHY

Though the Gaia mission itself doesn’t generate many of its own headlines, it’s at the foundation of modern space science. Its data is behind lots of published research.

“This quasar catalogue is a great example of how productive astronomical projects are,” says Hogg. “Gaia was designed to measure stars in our own galaxy, but it also found millions of quasars at the same time, which give us a map of the entire universe.”

Now, the new Quaia catalogue is playing a similar role. The data it contains is already being used by other researchers.

“It has been very exciting to see this catalogue spurring so much new science,” Storey-Fisher says. “Researchers around the world are using the quasar map to measure everything from the initial density fluctuations that seeded the cosmic web to the distribution of cosmic voids to the motion of our solar system through the universe.”

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Webb Finds Hints of a Third Planet at PDS 70

March 18th, 2024

The exoplanet census now stands at 5,599 confirmed discoveries in 4,163 star systems, with another 10,157 candidates awaiting confirmation. So far, the vast majority of these have been detected using indirect methods, including Transit Photometry (74.4%) and Radial Velocity measurements (19.4%). Only nineteen (or 1.2%) were detected via Direct Imaging, a method where light reflected from an exoplanet’s atmosphere or surface is used to detect and characterize it. Thanks to the latest generation of high-contrast and high-angular resolution instruments, this is starting to change.

This includes the James Webb Space Telescope and its sophisticated mirrors and advanced infrared imaging suite. Using data obtained by Webb‘s Near-Infrared Camera (NIRCam), astronomers with the MIRI mid-INfrared Disk Survey (MINDS) survey recently studied a very young variable star (PDS 70) about 370 light-years away with two confirmed protoplanets. After examining the system and its extended debris disk, they found evidence of a third possible protoplanet orbiting the star. These observations could help advance our understanding of planetary systems that are still in the process of formation.

The MINDS survey is an international collaboration consisting of astronomers and physicists from the Max-Planck-Institute for Astronomy (MPIA), the Kapteyn Astronomical Institute, the Space Research Institute at the Austrian Academy of Sciences (OAW-IFW), the Max-Planck Institute for Extraterrestrial Physics (MPE), the Centro de Astrobiología (CAB), the Institute Nazionale di Astrofisica (INAF), the Dublin Institute for Advanced Studies (DIAS), the SRON Netherlands Institute for Space Research, and multiple universities. The paper that describes their findings will appear in the journal Astronomy & Astrophysics.

This spectacular image from the SPHERE instrument on ESO’s Very Large Telescope is the first clear image of a planet caught in the very act of formation around the dwarf star PDS 70. Credit: ESO/A. Müller et al.

PDS 70 has been the subject of interest in recent years due to its young age (5.3 to 5.5 million years) and the surrounding protoplanetary disk. Between 2018 and 2021, two protoplanets planets were confirmed within the gaps of this disk based on direct imaging data acquired by sophisticated ground-based telescopes. This included the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) and GRAVITY instruments on the ESO’s Very Large Telescope (VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA).

In recent years, the MINDS team has used Webb spectral data to perform chemical inventories on protoplanetary disks in multiple star systems. In a previous study based on data from Webb‘s Mid-Infrared Instrument (MIRI), the MINDS team detected water in the inner disk of PDS 70, located about 160 million km (100 million mi) or 1.069 AU from the star, a find that could have implications for astrobiology and the origins of water on rocky planets (like Earth). These results showcased Webb’s impressive capabilities and how it can observe the cosmos in infrared (IR) wavelengths inaccessible to ground-based observatories.

Valentin Christiaens, an F.R.S-FNRS Postdoctoral Researcher at the University of Liège and KU Leuven, was the lead author of this latest paper. “The advantage of Webb’s instruments is that they observe at infrared wavelengths that cannot be observed from the ground because of our atmosphere, which absorbs most of the infrared spectrum,” he told Universe Today via email. “Thanks to Webb we can obtain measurements of planets in formation (called protoplanets) in infrared, which allow us to better constrain our models of planet formation.”

For their latest study, the MINDS team examined PDS 70 using data from Webb‘s NIRCam as part of the MIRI Guaranteed Time Observations program on planet formation. Christiaens and his team were motivated to study PDS 70 further because previous research indicated the possible detection of a third protoplanet. This makes the system an ideal laboratory to study planet-disk interactions and search for accretion signatures. The presence of a possible third signal was detected in 2019 by a team using the VLT/SPHERE instrument but remained unconfirmed since.

This artist’s illustration shows a compact protoplanetary disk and an extended one. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

One possible interpretation for this signal was that it traces a third planet. Using NIRCam data, Christiaens and his colleagues sought to redetect this signal and confirm that it was a third planet in the system. The JWST is especially well-suited to this task, thanks to its advanced optics and coronograph, which removes interference from Webb’s images by blocking the star’s light. He and his colleagues were also aided by advanced algorithms that help separate starlight from other point sources in orbit (like exoplanets) and debris disks. As Christiaens explained:

“The observation of another star, called a reference star, can be used to subtract the light from the star of interest and look for exoplanets there. In our study, we instead opted for a technique called “roll subtraction,” where two sequences of images are taken of the star of interest before and after the instrument is rotated, respectively, so that the position of an exoplanet has rotated in the two image sequences. From there, by subtracting the images of one sequence from those of the other, and vice versa, we can effectively get rid of the light of the star and make images of its environment – planets and disk.”

The team then combined their measurements with previous observations made with ground instruments and compared them to planetary formation models. From this, they could deduce the quantity of accumulated gas and dust around the protoplanet during the observation period. The quality of the images also allowed them to highlight a spiral arm of gas and dust supplying the second confirmed candidate (PDS 70 c), as predicted by the models. Lastly, they detected a bright signal consistent with a protoplanet candidate enshrouded in dust.

“What makes this candidate so interesting is that it could be in 1:2:4 resonance with planets b and c, already confirmed in the system (i.e., its orbital period will be almost exactly two times and four times shorter than that of b and c, respectively),” said Christiaens. This is precisely what happens with three of Jupiter’s Galilean Moons (Ganymede, Europa, and Io), which are also in a 1:2:4 resonance. The possibility of a star system with three planets in this orbital relationship would be a gold mine for astronomers. “However, more observations are needed before this resonance can be confirmed,” Christiaens added.

The evolutionary sequence of protoplanetary disks with substructures, from the ALMA CAMPOS survey. These wide varieties of planetary disk structures are possible formation sites for young protoplanets. Image Credit: Hsieh et al. in prep.

In addition to demonstrating Webb’s capabilities, these findings could help inform our current understanding of how planetary systems form and evolve. This is one of the main objectives of the JWST: to use its advanced infrared optics to probe young star systems where planets are still in the process of forming. This has been a high priority for astronomers ever since Kepler began detecting exoplanets that defied widely accepted theories of how planetary systems form and evolve. In particular, the detection of many gas giants orbiting closely to their suns (“Hot-Jupiters”) contradicted theories that gas giants form in the outer reaches of star systems.

By observing young star systems at different stages of formation, astronomers hope to test various theories about how the Solar System came to be. As Christiaens summarized:

“The migration of planets is thought to play a crucial role in the evolution of planetary systems and helps explain the diversity of systems found to date via indirect methods. In many mature systems, planets have been found to resonate with each other, suggesting that this migration did indeed take place in the past. In our case, we observe a very young system, still in formation, where the 2 known giant planets seem to be in resonance and where the third potential planet, if confirmed, would also be with the other two. In the case of the Solar System, we suspect that the migration and resonance capture of the giant planets probably also took place a very long time ago, [which could] explain their current configuration (Great Tack hypothesis). Here we are potentially observing it live in another system!”

Further Reading: arXiv

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Improving a 1960s Plan to Explore the Giant Planets

March 18th, 2024

In the 1960s, NASA engineers developed a series of small lifting-body aircraft that could be dropped into the atmosphere of a giant planet, measuring the environment as they glided down. Although it would be a one-way trip to destruction, the form factor would allow a probe to glide around in different atmospheric layers, gathering data and transmitting it back to a parent satellite. An updated version of the 1960s design is being tested at NASA now, and a drop-test flight from a helicopter is scheduled for this month.

“We are looking to take an idea to flight and show that a lifting body aircraft can fly as a probe at this scale – that it can be stable, that components can be integrated into the probe, and that the aircraft can achieve some amount of lift,” said John Bodylski, the principal investigator at NASA’s Armstrong Flight Research Center in Edwards, California. Bodylski is working to prove that a lifting body aircraft design could meet the requirements for an atmospheric probe that could be used at giant planets, like Uranus or Jupiter.

Robert “Red” Jensen removes a major component from an aircraft mold for assembly of a prototype of an atmospheric probe as Justin Hall watches at NASA’s Armstrong Flight Research Center in Edwards, California. Credit: NASA/Steve Freeman

The idea behind the concept is that a lifting body aircraft relies on its unique blunt shape for lift, rather than wings. Bodylski and his team have designed two lifting body aircraft, both of which are about 70 cm (27 1/2 inches) long, and 60 cm (24 inches) wide. One is almost built and ready for flight.

NASA has a long history of doing flight tests with lifting bodies. From 1963 to 1975, NASA tested several designs to demonstrate the ability of pilots to maneuver and safely land a wingless vehicle. These vehicles included the M2-F1, M2-F2, HL-10, X-20, X-24A, and the X-24B. These lifting bodies were designed to validate the concept of flying a wingless vehicle back to Earth from space and landing it like an aircraft. The concept was influential in designing the Space Shuttle.

While the Space Shuttle and other human-carrying lifting body vehicles had inherent issues, even back in the 1960’s planetary scientists realized the concept could be more feasible for smaller uncrewed probes.

NASA says that current small atmospheric probes such as CubeSats, gather and transmit data for about 40 minutes and can take in approximately 10 data points before their parent satellite is out of range. Bodylski estimates this lifting body design could descend more rapidly and at a steeper angle, collecting the same information in 10 minutes, plus gather additional data for another 30 minutes from much deeper in a thick atmosphere.

The lifting body aircraft on Rogers Dry Lake, near what is now NASA’s Armstrong Flight Research Center in Edwards, California, include, from left, the X-24A, the M2-F3, and the HL-10. Credit: NASA

Lifting bodies have been in and out of vogue for decades. NASA actually had two designs for lifting bodies in the running as a precursor and later a successor to the Space Shuttle. The Dyna Soar was based on NASA’s X-20 lifting body and was designed to be launched by rocket into orbit, and the lifting body design would have allowed it to land like an airplane. Due to due to high costs, changing priorities for both the military and NASA – with the Apollo program just getting going — the Dyna Soar program was cancelled in December 1963, just before the first crewed test flight was scheduled for the following year.

Later, in 1996 NASA selected Lockheed Martin to build and fly the X-33 test vehicle to demonstrate advanced technologies for a new reusable spaceplane vehicle to succeed the Space Shuttle. Called VentureStar, it would have been a single-stage-to-orbit vehicle. However, NASA cancelled the project in 2001 before any test flights were carried out after some technical problems proved too difficult to solve.

More info on Bodylski’s project.

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Finally, an Explanation for the “String of Pearls” in Supernova 1987A

March 18th, 2024

Not long after the explosion of Supernova 1987a, astronomers were abuzz with predictions about how it might look in a few years. They suggested a pulsar would show up soon and many said that the expanding gas cloud would encounter earlier material ejected from the star. The collision would light up the region around the event and sparkle like diamonds.

Today, astronomers look at the site of the stellar catastrophe and see an expanding, glowing ring of light. Over the years, its shape has changed to a clumpy-looking string of pearls. What’s happening to affect its appearance? The answer lies in something called the “Crow Instability.” We see this aerodynamical process when vortexes off the wingtips of airplanes interact with the contrails from their engines. The instability breaks up the contrail into a set of vortex “rings”.

University of Michigan graduate student Michael Wadas says this type of instability could explain why Supernova 1987a formed a string of pearls. “The fascinating part about this is that the same mechanism that breaks up airplane wakes could be in play here,” said Wadas, who is now doing post-graduate work at CalTech. If that’s true, it will go a long way toward explaining why those ghostly pearls exist.

The expanding ring-shaped remnant of SN 1987A and its interaction with its surroundings, seen in X-ray and visible light. The star that became SN 1987a expelled concentric rings of material during its red and blue supergiant phases, and the shockwave from the supernova lit them up. Image: Public Domain, https://commons.wikimedia.org/w/index.php?curid=278848
The expanding ring-shaped remnant of SN 1987A and its interaction with its surroundings, seen in X-ray and visible light. The star that became SN 1987a expelled concentric rings of material during its red and blue supergiant phases, and the shockwave from the supernova lit them up. Image: Public Domain, https://commons.wikimedia.org/w/index.php?curid=278848

About 1978a and its String of Pearls

Light and neutrinos from Supernova 1987a reached Earth on February 23, 1987. The original star, Sanduleak -69 202, lay about 168,000 light-years away in the Large Magellanic Cloud. It exploded as Type II, the first one in modern times to show astronomers the details of a core-collapse supernova. Since then, astronomers watched as a ring of ejected material and a shockwave from the explosion itself spread to space. It slammed into the material shed earlier in the star’s life. It does have a neutron star in the center. Astronomers detected it in 2019 and observed it using X-ray and gamma-ray observatories.

Several months after the explosion, astronomers used the Hubble Space Telescope to image bright rings surrounding the explosion site. That material came from the stellar wind of the progenitor star. Ultraviolet light from the explosion ionized the gases in the cloud. The inner ring lay about 2/3 of a light-year from the original star. The expanding ejecta from the supernova eventually collided with it in 2001. That heated it further. The shockwave has now expanded beyond the rings, leaving behind pockets of warm dust and glowing clouds of gas. The turbulence of that shockwave and the damage it did to regions of the inner ring is created the “pearls”.

Competing Theories for the String

So, what physics underlies the appearance of the pearls? Astronomers have tried to explain the string using something called a Rayleigh-Taylor instability. That occurs when two fluids (or plasmas) of different densities interact with each other. Think of oil and water trying to mix, or a heavy pyroclastic flow streaming out of a volcano. The interaction forms interesting and predictable shapes in the fluids. For 1978a, the denser “fluid” is the material ejected during the supernova explosion. It is colliding with a less dense cloud of material ejected earlier that has spread out to space. However, there are issues with using the Rayleigh-Taylor instability to explain what we see at the supernova site.

A simulation shows the shape of the gas cloud on the left and the vortices, or regions of rapidly rotating flow, on the right. Each ring represents a later time in the evolution of the cloud. The gas cloud starts as an even ring with no rotation. It becomes a lumpy ring as the vortices develop. Eventually, the gas breaks up into distinct clumps. Credit: Michael Wadas, Scientific Computing and Flow Laboratory

“The Rayleigh-Taylor instability could tell you that there might be clumps, but it would be very difficult to pull a number out of it,” said Wadas, who suggested the Crow Instability in a paper just published in Physical Review Letters. Jet contrails are a better comparison because the wingtip vortices break up the long smooth line of a jet contrail. The vortices flow into each other, leaving gaps that can be predicted.

To explore that idea, Wadas and his colleagues simulated the way winds push a model cloud outward while also dragging on its surface. The top and bottom of the cloud got pushed out faster than the middle. That caused it to curl in on itself, triggering a Crow Instability that broke the cloud apart into 32 even clumps similar to the string of pearls at 1987a (which has 30-40 clumps). That predictable number of clumps is why the team suggested the Crow Instability as a formation agent for the string. They also think it could help predict the formation of more beaded rings around the explosion site or when dust around a star coalesces to form planets. Recent JWST infrared images seem to show even more clumps that have appeared in the ring, and it will be interesting to see if more of them appear in the future.

For More Information

Explaining a Supernova’s “String of Pearls”
Hydrodynamic Mechanism for Clumping along the Equatorial Rings of SN1987A and Other Stars

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NASA is Working on Zero-Boil Off Tanks for Space Exploration

March 18th, 2024

No matter what mode of transportation you take for a long trip, at some point, you’ll have to refuel. For cars, this could be a simple trip to a gas station, while planes, trains, and ships have more specialized refueling services at their depots or ports. However, for spacecraft, there is currently no refueling infrastructure whatsoever. And since the fuel spacecraft use must be stored cryogenically, and the tanks the fuel is stored in are constantly subjected to the thermal radiation from the Sun, keeping enough fuel in a tank for a trip to Mars with astronauts is currently infeasible. Luckily, NASA is currently working on it and recently released a detailed look at some of that work on a blog on their website.

The problem definition is very clear – cryogenic hydrogen and oxygen are used as fuel on most spacecraft missions. Once in space, the tanks the fuel is stored in heat up due to the constant solar radiation they’re subjected to. Since there’s no air, there’s no way to radiate out that heat, so eventually, it can get through even the most sophisticated passive thermal insulation system. When it does, the fuel starts to boil, and mission planners typically have chosen to eject the vaporous fuel out into space rather than leaving it as a potential medium to heat the rest of the fuel faster.

This resultant fuel lost to this sublimation can cost as much as half of the cryogenic fuel needed for a 3-year mission to Mars – in just a single year. In short, crewed trips to Mars are impossible using the current fuel storage technology in space. However, there are alternatives, known as Zero Boil-Off (ZBO) or Reduced Boil-Off (RBO) systems. These advanced tanks use a combination of “active” processes to maintain tank pressure and not allow too much loss of fuel during long space flights.

Fraser makes an argument for why refueling is so critical.

An “active” process must be actively controlled and typically requires some sort of power input. In particular, ZBO systems rely on two technology ideas – a jet mixing of the propellant and a droplet injection technology. Let’s take a look at the mixing technology first.

In this example, part of the fuel would be forcibly mixed back into the vapor space in a particular way that would allow it to control the phase changes of the vapor/fuel interface. In essence, it would stop the fuel from sublimating into a vapor in the first place. Similarly, a droplet injection system would use a novel type of spray bar to inject fuel droplets into the vapor area, causing it to condense and remove some of the pressure from the system.

To add another layer of complexity to these already complicated fluid dynamics systems, this all must be done in microgravity, where things like droplet formation and liquid mixing don’t always happen the same way as they do on Earth. So, NASA decided to do what it does best and run some experiments – in this case on the ISS.

Image of the ZBOT-1 experiment being installed on the ISS by astronaut Joseph Acaba.
Credit – NASA

Back in 2017, NASA started the ZBOT-1 Experiment on the ISS. It was intended to quantify how the jet mixing would behave in microgravity, and the result of some 30+ tests was that we still understand very little about how these systems work in microgravity. While how they were is different than what most fluid engineers are used to, they are still acting according to physical laws, so more experiments would help narrow down the models that tank designers can use to understand how these ZBO systems might best be used.

Two other experiments are focused on furthering that understanding – one called the ZBOT-NC Experiment, is due to be launched to the ISS in 2025. It will study the effects of microgravity on “non-condensable gases,” which can be used to control the pressure inside the fuel tank. Data from its observations can also be fed into the CFD models, allowing scientists to understand better how the model differs from reality in microgravity.

The final test in the series will focus on droplet phase changes. Known as the ZBOT-DP test, this is the most ambitious of the three, as it tests a technology that has never been used in microgravity at all before. It will focus on understanding how droplets interact with their surroundings, including superheated tank walls, in microgravity environments. They could eventually lead to a fully functional droplet system and an active control system to ensure no tank boil-off happens.

The idea of in-space refueling has been around for a long time, as this VideoFromSpace feature shows.
Credit – VideosFromSpace YouTube Channel / NASA Technology

That’s still a long way off those, with no planned date for the ZBOT-DP test. Given the importance of this technology to missions like the crewed Artemis mission planned in the next few years, it seems that the successful completion of these experiments and the design and testing of a fully ZBO fuel tank should be very high on NASA’s priority list. While the agency’s already supporting it, let’s hope that the researchers involved can prove their ideas before they’re needed for a real human mission.

Learn More:
NASA – Zero-Boil-Off Tank Experiments to Enable Long-Duration Space Exploration
UT – Why Build Big Rockets at All? It’s Time for Orbital Refueling
UT – There’s Now a Gas Station… In Space!
UT – Robotics Refueling Research Scores Huge Leap at Space Station

Lead Image:
The Gateway space station—humanity’s first space station around the Moon—will be capable of being refueled in space.
Credits: NASA

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Webb Reveals Secrets of Neptune’s Evolution

March 18th, 2024

A twinset of icy asteroids called Mors-Somnus is giving planetary scientists some clues about the origin and evolution of objects in the Kuiper Belt. JWST studied them during its first cycle of observations and revealed details about their surfaces, which gives hints at their origins. That information may also end up explaining how Neptune got to be the way it is today.

The Mors-Somnus binary is part of a collection of objects beyond Neptune. They’re called, aptly enough, “Trans-Neptunian Objects” or TNOs, for short. About 3,000 are numbered and known, and many more aren’t yet surveyed. They all lie beyond the orbit of Neptune and are divided into various classes. There are the classical Kuiper Belt Objects (KBOs) and scattered disc objects. Within those two classes, there are resonant TNOs—which move in resonance with Neptune and extreme TNOs, which orbit far beyond Neptune (around 30 AU). Then there are objects in orbits similar to Pluto’s, called “plutinos”. Mors-Somnus is also a Plutino.

The orbit of Mors-Somnus with respect to Neptune in the outer Solar System. Courtesy JPL.
The orbit of Mors-Somnus with respect to Neptune in the outer Solar System. Courtesy JPL.

Neptune and Beyond

Why is there such a varied bunch of objects “out there”? Where did they originate and how have they changed over time? One way to answer those questions is to study the surface properties of Kuiper Belt Objects and, in particular, icy rocks like Mors-Somnus. One way to do that is to take spectra of their surfaces. The data reveals information about the surface compositions of these objects. That, in turn, tells scientists something about the environments in which they formed and those they’ve experienced over time.

Neptune itself likely formed closer to the Sun but then migrated to the outer Solar System (along with Jupiter, Saturn, and Uranus). At the same time, a huge dense disk of rocky and icy planetesimals and asteroids populated space out to about 35 AU. As the giant planets migrated to more distant orbits, they preferentially scattered those smaller bodies. These icy asteroids and cometary bodies settled into the Kuiper Belt, scattered disk, and the Oort Cloud. How that activity progressed and where those icy bodies came from in the first place are questions planetary scientists are working to answer.

More About Mors-Somnus and Neptune

This is where Mors-Somnus comes in handy. The pair is a good example of a “cold classical” TNO. It was studied by JWST as part of a program called Discovering the Surface Compositions of Trans-Neptunian Objects (DiSCO-TNOs) led by Ana Carolina de Souza Feliciano and Noemí Pinilla-Alonso at the University of Central Florida. The project identifies the unique spectral properties of these small celestial bodies beyond Neptune, something that hasn’t been done before now.

An artist’s conception of Mors-Somnus, a binary duo comprised of a pair of icy asteroids bound by gravity, is shown. These lie just beyond the orbit of Neptune. JWST was used to analyze their surface compositions for the first time. Image credit: Angela Ramirez, UCF
An artist’s conception of Mors-Somnus, a binary duo — a pair of icy asteroids bound by gravity, is shown. These lie just beyond the orbit of Neptune. JWST was used to analyze their surface compositions for the first time. Image credit: Angela Ramirez, UCF

The Mors-Somnus is a member of the same dynamical group as other nearby TNOs and they share spectroscopic characteristics with other cold-classical group objects. This means they probably all formed at about the same time. They probably originated beyond 30 astronomical units from the Sun. Trans-Neptunian binaries such as Mors-Somnus provide a unique way to look at the formation and evolution of planetesimals in that region of space.

Studying the composition of small celestial bodies such as Mors-Somnus gives us precious information about where we came from, Pinilla-Alonso said. “We are studying how the actual chemistry and physics of the TNOs reflect the distribution of molecules based on carbon, oxygen, nitrogen, and hydrogen in the cloud that gave birth to the planets, their moons, and the small bodies,” she says. “These molecules were also the origin of life and water on Earth.”

The Importance of Objects Beyond Neptune

The chemical and physical properties of TNOs offer a treasure trove of information about what conditions were like in the early Solar System. They likely contain pristine materials that existed in the protoplanetary disk from which our Solar System formed, including primitive ices. Those ices don’t change due to solar heating (since the Sun is so far away), but they can be darkened by ultraviolet radiation over time, as planetary scientists have seen at Pluto and other icy worlds. And, those bodies can get transported from their birth regions to other parts of the solar system. If their surfaces don’t change much, then scientists can used spectral studies to trace where groups of objects originated.

The TNO region also contains what scientists call a “dynamical structure”. That is, its distribution of objects by various characteristics, including their orbits and motions over time. Objects and events can change the dynamical structure. For example, the dynamical structure of the trans-Neptunian region bears the traces of planetary migration that occurred in the first billion years of the Solar System’s existence. The TNOs, and in particular, binaries like Mors-Somnus were affected by such migrations.

Migration and Neptune

It’s very likely that this binary pair originally formed well beyond the orbit of Neptune. The researchers found similar spectroscopic characteristics between Mors and Somnus and the cold-classical group. It’s compositional evidence that this binary pair formed well beyond 30 astronomical units (nearly 2.7 billion miles away). Then, they moved to their present positions under the gravitational influence of other planetary migrations.

A model of possible migration paths in the outer solar system due to giant planet migrations. Model: R. Gomes, image by Morbidelli and Levison.
A model of possible migration paths in the outer solar system due to giant planet migrations. Model: R. Gomes, image by Morbidelli and Levison.

Thanks to gravitational perturbations from Neptune, Mors-Somnus and its neighbors moved closer to the planet. They now orbit in resonance with the planet. All these objects are potential tracers for Neptune’s migration path before it settled into its final orbit, the researchers say.

Binaries separated by distance, as Mors-Somnus is, rarely survive outside of areas bound by gravity, where they are sheltered by other KBOs. To survive migration, they require a slow transportation process toward their destination. The migration of Neptune to its final orbit offered such a leisurely opportunity.

Using JWST to study the surface characteristics of smaller distant worlds is a great accomplishment, according to co-author Pinilla-Alonso. The telescope has studied larger worlds out there, but this is the first time it’s focused on such tiny members of the outer Solar System. “For the first time, we can not only resolve images of systems with multiple components like the Hubble Space Telescope did, but we can also study their composition with a level of detail that only Webb can provide. We can now investigate the formation process of these binaries like never before.”

For More Information

UCF Scientists Use James Webb Space Telescope to Uncover Clues About Neptune’s Evolution
Spectroscopy of the Binary TNO Mors–Somnus with the JWST and Its Relationship to the Cold Classical and Plutino Subpopulations Observed in the DiSCo-TNO Project

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Little Red Dots in Webb Photos Turned Out to Be Quasars

March 16th, 2024

In its first year of operation, the James Webb Space Telescope (JWST) made some profound discoveries. These included providing the sharpest views of iconic cosmic structures (like the Pillars of Creation), transmission spectra from exoplanet atmospheres, and breathtaking views of Jupiter, its largest moons, Saturn’s rings, its largest moon Titan, and Enceladus’ plumes. But Webb also made an unexpected find during its first year of observation that may prove to be a breakthrough: a series of little red dots in a tiny region of the night sky.

These little red dots were observed as part of Webb’s Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) and the First Reionization Epoch Spectroscopically Complete Observations (FRESCO) surveys. According to a new analysis by an international team of astrophysicists, these dots are galactic nuclei containing the precursors of Supermassive Black Holes (SMBHs) that existed during the early Universe. The existence of these black holes shortly after the Big Bang could change our understanding of how the first SMBHs in our Universe formed.

The research was led by Jorryt Matthee, an Assistant Professor in astrophysics at the Institute of Science and Technology Austria (ISTA) and ETH Zürich. He was joined by researchers from the MIT Kavli Institute for Astrophysics and Space Research, the Cosmic Dawn Center (DAWN), the National Astronomical Observatory of Japan (NAOJ), the Niels Bohr Institute, the Max Planck Institute for Astronomy (MPIA), the Centro de Astrobiología (CAB), and multiple universities and observatories. Their findings were published in a study recently published in The Astrophysical Journal.

This image shows the region of the sky in which the record-breaking quasar J0529-4351 was observed by the ESO’s Very Large Telescope (VLT) in Chile. Credit: ESO

Scientists have known for some time that Supermassive Black Holes reside at the center of most massive galaxies. And whereas some are relatively dormant, like the SMBH located in the center of the Milky Way (Sagittarius A*), others are extremely active and are growing at the rate of several Solar masses a year. These fast-growing black holes power particularly luminous Active Galactic Nuclei (AGNs) – or quasars – which become so bright they temporarily outshine all the stars in their disk, the brightest of which are known as quasars.

Quasars are among the brightest objects known to astronomers and can be seen at the very edge of our expanding Universe. In recent years, though, astronomers have spotted several quasars and SMBHs in the early Universe that are larger than cosmological models predict. As Matthee explained in a recent ISTA press release:

“One issue with quasars is that some of them seem to be overly massive, too massive given the age of the Universe at which the quasars are observed. We call them the ‘problematic quasars.’ If we consider that quasars originate from the explosions of massive stars–and that we know their maximum growth rate from the general laws of physics, some of them look like they have grown faster than is possible. It’s like looking at a five-year-old child that is two meters tall. Something doesn’t add up.”

Mathee and his team identified the population of little red dots while studying images taken during the EIGER and FRESCO surveys, a large and medium first-year JWST campaign in which Mathee was involved. The EIGER campaign was specifically designed to search for rare blue supermassive quasars and their environments, and not for quasars in the early Universe. However, Webb‘s Near Infrared Camera (NIRCam) can acquire emissions spectra from all objects in the known Universe. These objects had been previously observed by Hubble and mistaken for regular galaxies.

JWST's near-infrared view of the star-forming region NGC 604 in the Triangulum galaxy. Credit: NASA, ESA, CSA, STScI
JWST’s near-infrared view of the star-forming region NGC 604 in the Triangulum galaxy. Credit: NASA, ESA, CSA, STScI

But thanks to the NIRCam’s resolution, the ISTA-led team identified them as SMBHs almost by accident. According to Mathee, this accidental discovery could have profound implications for astronomy and cosmology:

“Without having been developed for this specific purpose, the JWST helped us determine that faint little red dots–found very far away in the Universe’s distant past–are small versions of extremely massive black holes. These special objects could change the way we think about the genesis of black holes. The present findings could bring us one step closer to answering one of the greatest dilemmas in astronomy: According to the current models, some supermassive black holes in the early Universe have simply grown ‘too fast’. Then how did they form?”

The team was able to make the distinction between galaxies and small quasars thanks to NIRCam’s detection of deep-red emission lines (aka. H? spectral lines) that are produced when hydrogen atoms are heated. They also found that the lines they observed had a wide-line profile, which they used to trace the motion of the hot hydrogen gas. “The wider the base of the H? lines, the higher the gas velocity,” said Mathee. “Thus, these spectra tell us that we are looking at a very small gas cloud that moves extremely rapidly and orbits something very massive like an SMBH.”

Just as important were the redshift values they obtained for these SMBGs (Z= 4.2-5.5), which indicate these objects existed more than 12 billion years ago – roughly 1 billion years after the Big Bang. Furthermore, they observed that these SMBHs were not overly massive like those visible in nearby galaxies today. As Mathee indicated:

“While the ‘problematic quasars’ are blue, extremely bright, and reach billions of times the mass of the Sun, the little red dots are more like ‘baby quasars.’ Their masses lie between ten and a hundred million solar masses. Also, they appear red because they are dusty. The dust obscures the black holes and reddens the colors.”

Long exposures made with the Hubble Space Telescope show brilliant quasars flaring in the hearts of six distant galaxies. Credit: NASA/ESA

Eventually, the outflow of hydrogen gas will puncture the clouds of dust and gas that surround and obscure massive black holes (“dust cocoon”), and these smaller SMBHs will evolve into much larger ones. Thus, Mathee and his team hypothesized that the little red dots are small, red versions of giant blue SMBHs in the phase that predates the “problematic quasars.” Through follow-up observations, astronomers can conduct detailed studies of these baby SMBHs, which could lead to a better understanding of how problematic quasars come to exist.

“Black holes and SMBHs are possibly the most interesting things in the Universe. It’s hard to explain why they are there, but they are there,” Mathee concluded. “We hope that this work will help us lift one of the biggest veils of mystery about the Universe.”

Further Reading: ISTA, The Astrophysical Journal

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The Maximum Mass of a Neutron Star is 2.25 Solar Masses

March 16th, 2024

When stars grow old and die, their mass determines their ultimate fate. Many supermassive stars have futures as neutron stars. But, the question is, how massive can their neutron stars get? That’s one that Professor Fan Yizhong and his team at Purple Mountain Observatory in China set out to answer.

It turns out that a non-rotating neutron star can’t be much more than 2.25 solar masses. If it was more massive, it would face a much more dire fate: to become a black hole. To figure this out, the team at Purple Mountain looked into what’s called the Oppenheimer limit. That’s the critical gravitational mass (abbreviated MTOV) of a massive object. If a neutron star stays below that Oppenheimer limit, it will remain in that state. If it grows more massive, then it collapses into a black hole.

A composite image of the Crab Nebula features X-rays from Chandra (blue and white), optical data from Hubble (purple), and infrared data from Spitzer (pink). The Crab Nebula is powered by a quickly spinning, highly magnetized neutron star called a pulsar, which was formed when a massive star ran out of its nuclear fuel and collapsed. Scientists now want to know how much mass characterizes a neutron star as opposed to a black hole.
A composite image of the Crab Nebula features X-rays from Chandra (blue and white), optical data from Hubble (purple), and infrared data from Spitzer (pink). The Crab Nebula is powered by a quickly spinning, highly magnetized neutron star called a pulsar, which was formed when a massive star ran out of its nuclear fuel and collapsed. Scientists now want to know how much mass characterizes a neutron star as opposed to a black hole.

Understanding the Physics of a Neutron Star

So, why determine the upper mass of a neutron star? The Oppenheimer limit for these objects has some implications for both astrophysics and nuclear physics. Essentially, it indicates that compact objects with masses greater than 2.25 solar masses are probably what scientists term the “lightest” black holes. Those objects would likely exist in a range of 2.5 to 3 solar masses.

The whole thing is rooted in the way that stars age. Everything depends on their starting mass. So, for example, our Sun is a lower-mass yellow dwarf and it will take more than 10 billion years to go through its whole life cycle. It’s about 4.5 billion years old now. As it ages, it will consume heavier elements in its core, which will heat it up. That drives expansion, which means the Sun will become a red giant and cast off its outer layers beginning in about five billion years. Eventually, it will shrink to become a white dwarf. That tiny object will contain less than the mass of the Sun, although some white dwarfs can be slightly more massive.

How a Neutron Star Forms

Stars much more massive than the Sun go through the same cycle, but they end their lives in supernova explosions. What’s left becomes a black hole. Or, if there’s not quite enough mass left after the explosion, the remnant becomes a neutron star. So, that means there’s a delicate line between it and a black hole. That line is the Oppenheimer limit.

X-ray image of the Tycho supernova, also known as SN 1572, located between 8,000 and 9,800 light-years from Earth. Its core collapse could result in a neutron star or a black hole, depending on final mass. (Credit: X-ray: NASA/CXC/RIKEN & GSFC/T. Sato et al; Optical: DSS)
X-ray image of the Tycho supernova, also known as SN 1572, located between 8,000 and 9,800 light-years from Earth. Its core collapse could result in a neutron star or a black hole, depending on final mass. (Credit: X-ray: NASA/CXC/RIKEN & GSFC/T. Sato et al; Optical: DSS)

Stars between 8 and 25 solar masses produce neutron stars. Something called “neutron degeneracy pressure” holds those odd remnants together. The leftover core of the star compresses after the supernova explosion. But, neutrons and protons in atomic nuclei in the core get pushed tightly together and they can’t be compressed any more. So, the system goes into a weird equilibrium. At that point, the resulting neutron star is approaching the Oppenheimer limit. If the object gains (or has) any more mass, that puts it over the limit. The result is a black hole.

Refining the Oppenheimer Limit for Neutron Stars

Professor Fan’s team worked to find a more precise value for the Oppenheimer Limit. To do this, they gathered data from such observations as those made by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO gravitational wave detector, as well as an instrument aboard the International Space Station called The Neutron Star Interior Composition Explorer Mission (NICER). These and other missions detect the effects of neutron star collisions and neutron star-black hole encounters. NICER, in particular, studies the timing of x-ray emissions at neutron stars and works to answer the question: How big is a neutron star? By knowing the size and mass of neutron stars, astronomers can gain a further understanding of their formation and the exotic matter they contain.

The team incorporated information about the maximum mass cutoff (i.e. what’s the highest level of mass a neutron star can have) inferred from the distribution of these objects. They used models of the equation of state in their work. The equation of state basically looks at the state of matter in the neutron star (and black hole) and the models describe the parameters under which it exists (including pressure, volume, and temperature). The result of their work gives not only an upper bound to the mass of the neutron star (~2.5 solar masses) but also reveals that such a neutron star would have a radius of around 11.9 kilometers.

It’s interesting to see the precision in these measurements and models, based on actual data from multi-messenger observations of gravitational waves and soft X-ray emissions. Fan and the team suggest in the paper they published about their work that the objects with masses between 2.5 and 3 solar masses (detected by second-generation gravitational wave detectors) are most likely the lightest black holes.

Further Implications

The work also has some pretty interesting implications for cosmology, in particular the Hubble Constant. That’s the value assigned to the rate at which the Universe is expanding. It lies somewhere around 70 kilometers per second per megaparsec (plus or minus 2.2 km/sec/Mpc). The numbers depend on which methods astronomers use to calculate them.

The Fan team’s work suggests that the mass cutoff for neutron stars detected by gravitational waves should align with MTOV. That does not change with redshift. The Oppenheimer Limit mass cutoff is associated with both the redshifted mass of the object and its redshift. That’s predicted by the cosmological model and luminosity distance. This provides a new method to test the underlying cosmological model of the Universe. The current model begins with the Big Bang, inflation, and expansion. It also includes the distribution of all the matter (including dark and baryonic matter), and in corporates the contribution of dark energy.

For More Information

Maximum mass of non-rotating neutron star precisely inferred to be 2.25 solar masses
Maximum gravitational mass MTOV = 2.25 +0.08/-0.07 Ms inferred at about 3% precision with multimessenger data of neutron stars
ArXiv Preprint

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Could Earth Life Survive on a Red Dwarf Planet?

March 15th, 2024

Even though exoplanet science has advanced significantly in the last decade or two, we’re still in an unfortunate situation. Scientists can only make educated guesses about which exoplanets may be habitable. Even the closest exoplanet is four light-years away, and though four is a small integer, the distance is enormous.

That doesn’t stop scientists from trying to piece things together, though.

One of the most consequential questions in exoplanet science and habitability concerns red dwarfs. Red dwarfs are plentiful, and research shows that they host multitudes of planets. While gas giants like Jupiter are comparatively rare around red dwarfs, other planets are not. Observational data shows that about 40% of red dwarfs host super-Earth planets in their habitable zones.

Red dwarfs have a few things going for them when it comes to exoplanet habitability. These low-mass stars have extremely long lifespans, meaning the energy output is stable for long periods of time. As far as we can tell, that’s a benefit for potential habitability and the evolution of complex life. Stability gives life a chance to respond to changes and persist in their niches.

But red dwarfs have a dark side, too: flaring. All stars flare to some degree, even our Sun. But the Sun’s flaring is not even in the same league as red dwarf flaring. Red dwarfs can flare so powerfully that they can double their brightness in a very short period of time. Is there any way life could survive on red dwarf planets?

This is an artist's concept of a red dwarf star undergoing a powerful eruption, called a stellar flare. A hypothetical planet is in the foreground. Credit: NASA/ESA/G. Bacon (STScI)
This is an artist’s concept of a red dwarf star undergoing a powerful eruption called a stellar flare. A hypothetical planet is in the foreground. Credit: NASA/ESA/G. Bacon (STScI)

New research from scientists in Portugal and Germany examines that question. To test the idea of red dwarf exoplanet habitability, the researchers used a common type of mould and subjected it to simulated red dwarf radiation, protected only by a simulated Martian atmosphere.

The research is “How habitable are M-dwarf Exoplanets? Modelling surface conditions and exploring the role of melanins in the survival of Aspergillus niger spores under exoplanet-like radiation.” The lead author is Afonso Mota, an astrobiologist at the Aerospace Microbiology Research Group in the Institute of Aerospace Medicine at the German Aerospace Center (DLR.) The paper has been submitted to the journal Astrobiology and is currently in pre-print.

Aspergillus niger is ubiquitous in soil and is commonly known for the black mould it can cause on some fruits and vegetables. It’s also a prolific producer of melanin. Melanin absorbs light very efficiently, and in humans, melanin is produced by exposure to UV radiation and darkens the skin. Melanins are widespread in nature, and extremophiles use them to protect themselves. Melanin can dissipate up to 99.9% of absorbed UV. Scientists think that the appearance of melanins may have played a critical role in the development of life on Earth by protecting organisms from the Sun’s harmful radiation.

A scanning electron microscope of freeze-dried Aspergillus niger. Image Credit: By Mogana Das Murtey and Patchamuthu Ramasamy - [1], CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=52254793
A scanning electron microscope of freeze-dried Aspergillus niger. Image Credit: By Mogana Das Murtey and Patchamuthu Ramasamy – [1], CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=52254793

In essence, this research asks a pretty simple question. Can Aspergillus niger’s melanin help it survive red dwarf flaring when protected by a thin atmosphere like Mars’?

Proxima Centauri and TRAPPIST-1 are both well-known red dwarfs in exoplanet science because they host rocky exoplanets in their habitable zones. This study zeroes in on Proxima Centauri b (PCb hereafter) and TRAPPIST-1 e (T1e hereafter.) They’re both likely to have temperatures that allow liquid water to exist on their surfaces, given the right atmospheric properties. Both PCb and T1e likely have tolerable radiation environments, as well.

This figure from the research shows the Top of Atmosphere UV and X-ray radiation on Proxima Centauri and TRAPPIST-1 exoplanets. Image Credit: Mota et al. 2024.
This figure from the research shows the Top of Atmosphere UV and X-ray radiation on Proxima Centauri and TRAPPIST-1 exoplanets. Image Credit: Mota et al. 2024.

It’s impossible to model the surface conditions of these planets perfectly, but researchers can get close by using what’s called the equilibrium temperature. Measuring stellar flaring is easier because it can be observed accurately from great distances. Melanin production in A. niger is likewise well understood. By working with all three factors, the researchers were able to model how the mould would fare on the surface of a habitable zone planet around a red dwarf.

“In the context of astrobiology, and particularly astromycology, the study of extremotolerant fungi has proven critical to better understanding the limits of life and habitability,” the authors write. “Aspergillus niger, an extremotolerant filamentous fungus, has been frequently used as a model organism for studying fungal survival in extreme environments, growing in a wide range of conditions.”

A. niger’s spores have a complex and dense coating of melanin that protects them from UV and X-ray radiation. They’ve been found in the International Space Station, a testament to their ability to withstand some of the hazards in space. Though they’re terrestrial, scientists can use them to study the potential habitability of exoplanets.

In this work, the researchers tested the survivability of A. niger spores in simulated surface conditions of PCb and T1c, where the red dwarf stars bathe the planetary surfaces in powerful UV and X-ray radiation.

The researchers tested different types of A. niger spores in different solutions. One was a wild strain, one was a mutant strain modified to produce and excrete pyomelanin, one of the melanins of particular interest to scientists, and the third was a melanin-deficient strain. The spores were suspended in either saline solutions, melanin-rich solutions, or a control solution for a period of time while being exposed to different amounts of both X-ray and UV radiation.

After exposure, the three types of A. niger spores were tested for their survivability and viability.

The results show that A. niger would be able to survive the intense radiation environments that can sterilize the surfaces of red dwarf exoplanets. Not if directly exposed, but if under only a few millimetres of soil or water. “If unattenuated, X-rays from flares would most likely sterilize the surface of all studied exoplanets. However, microorganisms suited to survive under the surface would be unaffected by most exogenous radiation sources under a few millimetres of soil or water,” the researchers explain.

This figure from the research shows the estimated subsurface X-ray absorbed dose throughout a thin layer of soil (orange) or water (blue). Water has a lower capacity for attenuating these high-energy photons, so a thicker water layer is needed to reduce the same dose compared to soil. The three dashed lines represent the LD90 (Lethal dose for 90% of a population) values for E. coli, A. niger, and D. radiodurans. E. coli is a common bacterium, and D. radiodurans is a radiation-resistant extremophile. Image Credit: Mota et al. 2024.
This figure from the research shows the estimated subsurface X-ray absorbed dose throughout a thin layer of soil (orange) or water (blue). Water has a lower capacity for attenuating these high-energy photons, so a thicker water layer is needed to reduce the same dose compared to soil. The three dashed lines represent the LD90 (Lethal dose for 90% of a population) values for E. coli, A. niger, and D. radiodurans. E. coli is a common bacterium, and D. radiodurans is a radiation-resistant extremophile. Image Credit: Mota et al. 2024.

What the study comes down to is melanin. The more melanin there is, the higher the survival rate for A. niger.

“The experiments performed in this study corroborate the multifunctional purpose of melanin since A. niger MA93.1 spores germinated faster and more efficiently in a melanin-rich extract when compared to the two control solutions,” the authors write. A. niger MA93.1 is the mutant strain modified to produce and excrete melanin.

These figures from the research show the protective power of melanin when A. niger is exposed to UV-C radiation (left) and X-ray radiation (right.) A. niger in melanin solution showed better outgrowth after radiation exposure than either the saline solution or the control solution. The solid lines represent non-irradiated A. niger, and the dashed lines represented non-irradiated A. niger control samples. Image Credit: Mota et al. 2024.
These figures from the research show the protective power of melanin when A. niger is exposed to UV-C radiation (left) and X-ray radiation (right.) A. niger in melanin solution showed better outgrowth after radiation exposure than either the saline solution or the control solution. The solid lines represent non-irradiated A. niger, and the dashed lines represented non-irradiated A. niger control samples. Image Credit: Mota et al. 2024.

For the exoplanets T1e and PCb, the research is promising for those of us hoping for habitability on other planets. When it comes to UV-C radiation, a significant fraction of spores from samples containing melanin could survive the superflares striking PCb and T1e, even with very little atmospheric shielding. Exposure to X-rays was similar.

While we all like to imagine complex life elsewhere in the Universe, we’re more likely to stumble on worlds nothing like Earth. If we find life, it’ll probably be simple organisms that are finding a way to survive in what we would consider marginal or extreme environments. Since red dwarfs are so common, that’s likely where we’ll find this life.

This study bolsters that idea.

“Furthermore,” the authors write in their conclusion, “results from this work showed how A. niger, like other extremotolerant and extremophilic organisms, would be able to survive harsh radiation conditions on the surface of some M-dwarf exoplanets.”

The melanin plays a critical role in their potential survival, the authors conclude. “Additionally, melanin-rich solutions were shown to be highly beneficial to the survival and germination of A. niger spores, particularly when treated with high doses of UV and X-ray radiation.”

There’s an ongoing scientific discussion around red dwarf exoplanet habitability, with flaring playing a prominent role. But this research shows maybe it’s too soon to write red dwarfs off while also shedding light on how life on Earth may have got going.

“These results offer an insight into how lifeforms may endure harmful events and conditions prevalent on exoplanets and how melanin may have had a role in the origin and evolution of life on Earth and perhaps on other worlds.”

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