A Brown Dwarf Closer than Centauri?

by Paul Gilster on January 6, 2009

If, as we have often speculated in these pages, there is a brown dwarf closer to us than the Centauri stars, it may well be the WISE mission that finds it. The Wide-field Infrared Survey Explorer is a 40 cm telescope cooled below 17 K (-430 Fahrenheit) that will image the entire sky in four infrared wavelengths. If we’re looking for nearby brown dwarfs, an all-sky survey like this is the way to go, because such stars should be distributed uniformly in the space around us.

According to information Amanda Mainzer (JPL) presented yesterday at the American Astronomical Society meeting in Long Beach (CA), brown dwarfs are now thought to make up two-thirds of the stars in our stellar neighborhood, most of them as yet undetected. One of them might well be closer than the 4.3 light years that separate us from Alpha Centauri. And WISE should be up to the challenge of finding it, being able to detect cool brown dwarfs (down to 200 K) at Centauri distance and objects down to Jupiter-mass if closer than one light year.

We’re at the bridge between planets and stars here. L and T-class brown dwarfs include objects with temperatures down to 600 K (620 F), but WISE should be able to find numerous brown dwarfs that are cooler still, objects that may lead us to define a new spectral class. Current estimates are that the mission will uncover up to a thousand new brown dwarfs — we’ll see how close some of them may be when the mission goes to work after a launch scheduled for November of this year.

More from the brown dwarf front: The Hubble Space Telescope has been used in a study of 233 multiple star systems, which turned up only two brown dwarfs as companions to normal stars. Sergio Dieterich (Georgia State University) reported on this one yesterday at AAS, noting that the lack of brown dwarfs around solar-type stars seems to be telling us something:

“We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit. Especially when we consider the fact that brown dwarfs binaries do exist, the fact that there are very few binaries whose components lie on different sides of the hydrogen burning limit is significant.”

This work is part of the RECONS (Research Consortium on Nearby Stars) survey, which looks for the Sun’s neighbors within 10 parsecs (32.6 light years) of Earth. The current total of brown dwarfs in this range of space is twelve, compared with 239 identified red dwarf stars. Now the Hubble work shows that brown dwarfs don’t co-exist readily even with these small stars, an indication that mass is not the operative element in keeping these stellar types apart. “If mass ratio was the driving factor,” says Dietrich, “we would expect to find more brown dwarfs around small red stars than around solar type stars.”

AAS will also be the scene of another brown dwarf report, this one by Micaela Stumpf (Max-Planck-Institute for Astronomy, Heidelberg), which likewise implies that brown dwarfs do not tend to be found in the company of larger stars. Stumpf also reports that the brown dwarf binary Kelu-1 AB may actually be a triple system, accounting for the puzzling discrepancy between the masses of the two known dwarfs and the total mass of the system, which had been estimated on the basis of orbital dynamics. If confirmed, Kelu-1 AB may turn out to be the first known triple brown dwarf system. We have high hopes for WISE, but the deep infrared searches of the next decade may be what it takes to unravel the true size and character of the brown dwarf population.

On Kelu-1 AB, see Stumpf, “Kelu-1 AB – A possible brown dwarf triple system,” submitted to Astronomy & Astrophysics and available online.

Hans Bausewein January 6, 2009 at 14:48

Has anyone done the math for the possibility of an unknown solar system planet in a very wide orbit?

I think it must be quite large to be still hot enough after 5 billion years of cooling to be detectable by WISE.

On the other hand it cannot be so large or nearby that it would have been found by other instruments or by the disturbance of the solar system, it would cause.

In the end, knowing that it cannot exist, is also a result.

Adam January 8, 2009 at 18:06

Daniel Whitmire and a colleague have computed a super-Jovian in the Oort cloud might be flinging comets inwards. I’m not sure what the current estimated distance for the object might be, but it’s a possible find if brown dwarfs can be detected down to the 200 K temperature limit by WISE within a light-year.

Brown dwarfs seem to be rather rare. The big puzzle is why stars are made in the observed mass-range, and brown dwarfs are made in far fewer numbers. Perhaps planet-scale “brown dwarfs” are more common and present in abundance between the stars? WISE might tell us.

Paul F. Dietz January 9, 2009 at 17:47

The Pan-STARRS system will be able to detect Jupiter-like planets out to about 2000 AU. It works on reflected light, not IR emission, so it’s more limited in range than a system that detects internal radiated heat (since the latter signal will decline as r^-2 rather than r^-4). However, it will also be able to detect smaller, colder planets (Neptunes, Earths, Marses) out to hundreds of AU.

An earth-sized object in the Kuiper belt a few hundred AU out might be economically significant in the future, if it is large enough and its exopause cold enough to retain helium in its atmosphere. It might be the most accessible source of 3He in the solar system.

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