Planetary Systems Without Stars?

When a planetary system forms, is it possible for none of the bodies to have sufficient mass to become a star? If so, are there ways of predicting how many such systems might exist, and thus, what fraction of the mass of the universe they might represent? How could such starless systems be detected?

This was a question asked on the CompuServe Astronomy Forum. Here are some of the answers from knowledgeable people who are admittedly non-experts on this particular subject. But it's worth considering...

James Tappin, University of Birmingham:

This is more or less guesswork from a Solar-wind specialist. I would think that it would be very difficult to get a small enough gas cloud to collapse under its own gravity (it shouldn't be that hard to calculate but I don't have the numbers on hand right now). However I suspect that a system could be disrupted to produce a free planet-moons system. For example, it might be possible to disrupt the solar system so that Jupiter kept some of its moons (or maybe Pluto-Charon). Probably some real planetary dynamicist will say I've got all this wrong, but it's my three-ha'pworth for starters.

Ethan Bradford, University of Washington:

In fact this is an active research question. It bears on whether the missing mass in the galaxy can be baryonic matter in failed stars (known as "brown dwarfs" in the astronomical parlance). The number of stars formed at a given mass is known as the "mass function"; since no brown dwarfs have been measured, it must either be extrapolated from observations or derived from theory and both are very uncertain. Brown dwarfs might be detected in the infrared, but they are more likely to be detected (or have already been detected) in the "MACHO" searches that look for gravitational lensing of bright stars in the Magellanic Clouds or the galactic bulge.

Dave Clements, Oxford University

This is almost the same concept as the sub-stellar brown dwarfs that many people are hoping to discover. A brown dwarf is a 'failed' star, 0.1 solar mass or less in size, that is unable to generate densities in its core sufficient to trigger Hydrogen fusion. They will be warm, around 300oK in temperature, and so may be detected by infrared observations or optically as very red stellar objects. Since these objects will be baryonic (i.e.: made up of neutrons and protons like the rest of us) their total contribution to the mass of the universe cannot be beyond the baryonic mass limit inferred from primordial nucleosynthesis (i.e.: about one percent of what is needed to close the universe).

The question of planets around such objects is a tougher one, since we don't know much about brown dwarfs or planets around normal stars yet, but I guess it's possible...

Published in the January 1997 issue of the NightTimes