Brown Dwarfs May Have Planets

by Paul Gilster on October 22, 2005

If brown dwarfs, those ‘failed stars’ that never make it to the stage of full nuclear burning, can have planets around them, then the speculations of Karl Schroeder’s novel Permanence (New York: Tor Books, 2002) may be closer to reality than Centauri Dreams once thought. Schroeder imagines human colonies, artificially sustained through extraordinary technologies, on planets surrounding a variety of brown dwarf stars, an entire civilization of humans living in the spaces between the ‘lit’ stars we see in the night sky.

Now the Spitzer Space Telescope has found the signs of early planet formation around six young brown dwarfs located some 520 light years away in the Chameleon constellation. Ranging in size from between 40 to 70 times the mass of Jupiter, the brown dwarfs are between 1 and 3 million years old. And five of them have disks made up of dust particles that are clearly sticking together, in what looks suspiciously like the early stages of planet formation. The astronomers doing this work found relatively large dust grains in their data and small crystals of the mineral olivine. From a Jet Propulsion Laboratory news release:

“We are seeing processed particles that are linking up and growing in size,” said Dr. Ilaria Pascucci, a co-author also of the University of Arizona. “This is exciting because we weren’t sure if the disks of such cool objects would behave the same way that stellar disks do.”

Centauri Dreams‘ take: another key sign of planetary formation is a pronounced flattening of the observed disks. Planet formation seems to occur everywhere we look, and if the observational evidence continues to mount, we may have to face the fact that the number of planets in the galaxy is far higher than previously thought.

The paper on this work is Apai, Pascucci, Bouwman, et al., “The Onset of Planet Formation in Brown Dwarf Disks,” published online in Science Express (an abstract is here, but you’ll need AAAS membership for full access). If you’re not familiar with the service, Science Express offers electronic publication of selected papers that are to appear in Science in the next few weeks. It’s clearly an attempt to leverage the online preprint phenomenon made so conspicuous by arXiv, and is a welcome addition to our toolkit.


ljk February 12, 2007 at 23:26

Astrophysics, abstract

From: Daniel Apai Dr [view email]

Date: Sat, 10 Feb 2007 21:00:08 GMT (37kb)

Disks around Brown Dwarfs and Cool Stars

Authors: Daniel Apai, Kevin Luhman, Michael Liu

Comments: Summary of the Cool Stars 14 Splinter Session “Disks around Brown Dwarfs and Cool Stars”. 8 pages, 1 figure

We review the current picture of disks around cool stars and brown dwarfs, including disk fractions, mass estimates, disk structure and dispersal, accretion, dust composition, and the debris disk phase. We discuss these in the framework of recent planet formation models.

ljk February 19, 2007 at 0:58

Astrophysics, abstract

From: Basmah Riaz [view email]

Date: Fri, 16 Feb 2007 17:37:51 GMT (62kb)

Characterizing the disk around the TW Hydrae Association brown dwarf 2MASSW J1207334-393254

Authors: Basmah Riaz, John E. Gizis

Comments: 19 pages, 4 figures, accepted in ApJ

We present detailed modeling of the disk around the TW Hydrae Association (TWA) brown dwarf 2MASSW J1207334-393254 (2M1207), using {\it Spitzer} observations from 3.6 to 24 $\micron$. The spectral energy distribution (SED) does not show a high amount of flaring. We have obtained a good fit using a flat disk of mass between $10^{-4}$ and $10^{-6}$ $M_{\sun}$, $\dot{M}$ $\la10^{-11} M_{\sun}$/yr and a large inclination angle between 60$\degr$ and 70$\degr$. We have used three different grain models to fit the 10 $\micron$ Si emission feature, and have found the results to be consistent with ISM-like dust. In comparison with other TWA members, this suggests lesser dust processing for 2M1207 which could be explained by mechanisms such as aggregate fragmentation and/or turbulent mixing. We have found a good fit using an inner disk radius equal to the dust sublimation radius, which indicates the absence of an inner hole in the disk. This suggests the presence of a small K-$L^{\prime}$ excess, similar to the observed K-[3.6] excess.

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