Star Bright and the Sub-Brown Dwarfs: Part II

Our questions about sub-brown dwarfs and extrasolar planets are hampered by our ability to observe them. For now, they are mere shadows, teasing us with their presence but denying us a good solid look.

Most brown dwarfs are located by infrared instruments, which detect the heat generated by the faded star. Infrared instruments can also detect a young sub-brown dwarf during the first 20 million years of its life. In these early years, a sub-brown dwarf has an atmospheric temperature of 1,500 to 2,000 K (1,227 to 1,727 degrees C; 2,240 to 3,140 F), giving it a thermal signature similar to a very cool star. Some astronomers claim to have detected sub-brown dwarfs in this way in young star-forming regions such as Sigma Orionis. But most sub-brown dwarfs will be too old and too cold to be detected by infrared telescopes.

Astronomers have not yet obtained a direct image of a planetary-mass object in orbit around a star. Most extrasolar planets have been found because of the gravitational influence they exert: as a planet orbits a star, the planet will pull at the star from different sides. As a star is pulled away from us, the starlight is Doppler-stretched to longer red wavelengths. When the star is pulled toward us, the starlight is scrunched toward shorter blue wavelengths. Astronomers look at this light shift to determine characteristics of the orbiting planet, such as the minimum possible mass of the planet and its distance from the star.

As sub-brown dwarfs drift around in space – and studies suggest there are a great deal of them out there – it is possible that they will eventually become gravitationally bound to a more massive star. If this happens, the sub-brown dwarf will gravitationally act on a star the same way a planet does. If the sub-brown dwarf is too cold to be recognized by infrared instruments, how could we tell that this is a binary star system and not a gas giant planet orbiting a star?

Alan Boss, who is the chairperson for the IAU’s Working Group on Extrasolar Planets, says the captured sub-brown dwarf would be interpreted to be a planet. The only way to tell that such an object is a sub-brown dwarf is to know that it formed like a star.

He says that even more massive objects, crossing over into brown dwarf dimensions, also would be defined as planets if they are found orbiting stars.

“If it turns out that companions to solar-type stars tend to have maximum masses of 15 Jupiter masses, I would call them all planets,” says Boss. “I don’t think many sub-brown dwarfs are likely to be captured around solar-type stars, so if a 15 Jupiter mass object was found in orbit around a solar-type, I would expect it to have formed there, not to have been captured.”

According to Boss, capture isn’t likely, and sub-brown dwarfs can’t originally form close to a sun-like star to create a binary system, either. Boss says that the circumstances of a star’s birth would prevent this from happening. Any sub-brown dwarf that orbited around a more massive star in its earliest years would gather a lot of mass, and would end up being much more massive itself. What’s more likely, he says, is that after sub-brown dwarfs form as members of multiple star systems, before they can gather any more mass they are ejected outward to becomes free-floaters in regions of recent star formation. In this light, any low-mass companion to a sun-like star would have to be born as a planet.

There are, however, a few sun-like stars that have brown dwarf companions with close orbits. This is rare — about 80 percent of brown dwarfs are solitary, and when they do form binaries with solar-like stars, they tend to do so at very large distances, such as 100 AU (100 times the distance between the Earth and the sun). Extrasolar planets, on the other hand, often orbit very close — within 5 AU or less — of a sun-like star. Astronomers refer to the paucity of close-orbit brown dwarfs as “the brown dwarf desert.”

But could this be a desert of our own making? Stars can swap in and out of binary systems, so even if a brown dwarf could not form close to a sun-like star, one could end up there later. This may have been what happened with the close-orbit brown dwarfs. Yet a star that is hotter than its companion could strip away mass. Perhaps brown dwarfs are more common at great distances to sun-like stars because when they are further in, they tend to lose mass and “turn into” extrasolar planets. Yet according to Iain Neill Reid of the Space Telescope Science Institute, that’s not likely for even the less massive sub-brown dwarfs.

“A sub-brown dwarf orbiting a (sun-like) star would not lose enough mass to transform into a gas giant planet,” says Reid. “Even with a close orbit, the gas evaporation rate isn’t so high that the initial masses of planets were much higher than they are at present.”

Astrobiologists looking for life in the universe will need to be able to distinguish between brown dwarfs, sub-brown dwarfs and extrasolar planets. While life as we know it couldn’t exist on a gas giant planet, not to mention a brown dwarf, such objects influence the terrestrial planets where life is most likely to be found.

According to Boss, if solar systems can contain sub-brown dwarfs, they would be very different from systems that have gas giant planets like Jupiter and Saturn.

“The difference is in the evolutionary history,” says Boss. “The sub-brown dwarf would have been there from the very beginning, while the planet would have formed somewhat more recently. We are still trying to learn what the implications are of such a scenario for the formation of terrestrial planets and other features of our solar system.”

Boss says that comets in a system with a sub-brown dwarf might have a hard time forming and being ejected to an Oort Cloud-like region. Since comets are thought to be important delivery systems for water and organic chemicals, a sub-brown dwarf could mean the difference between a solar system that has life and one that does not.

What’s Next

Since the discovery of brown dwarfs less than a decade ago, astronomers have come to think that they might be more numerous than the visible stars in the sky.

“In broad terms, 80 percent of the nearby stars are (red) dwarfs, 10 percent are solar-type stars, and 10 percent are more massive,” says Reid. “There are probably around the same number of brown dwarfs as stars within the immediate solar neighborhood.”

The true abundance of brown dwarfs, sub-brown dwarfs and extrasolar planets is not known, and large areas of the sky still need to be explored. Most of the brown dwarfs have been located by the Two Micron All Sky Survey (2MASS), although the Sloan Digital Sky Survey (SDSS) and the planet-finding Doppler technique also have been used to find brown dwarfs. Reid is completing a census of low-mass stars and brown dwarfs in the immediate solar neighborhood.

“In collaboration with several others, notably Kelle Cruz, a graduate student at the University of Pennsylvania, we’re using 2MASS to search for objects with near-infrared colors and magnitudes consistent with very low-mass dwarfs,” says Reid. “Our goal is to better define the mass distribution of those objects, and get an idea of how many are lurking unseen near the sun.”
Boss, meanwhile, says that he is trying to learn how sheet-like clouds collapse and fragment into proto-stars.