WD 0806-661

Kevin Luhman, PSU, SSC, JPL / CalTech, NASA


Larger infrared image animation.


A substellar object with six to nine Jupiter-masses
has been found to share the same proper motion
as the white dwarf WD 0806-661 (more).

If confirmed to be gravitationally bound, the two substellar objects have a separation of around 2,500 AUs (130") and would have an orbital period of years if the system is around 1.5 billion years old (Luhman et al, 2011b; and Luhman et al, 2011a). WD 0806-661 "b" is extremely cool, possibly cooler than UGPS-0722-05, but its estimated mass is so low that it lies in the realm of Jupiter-class planets -- possibly cooler than CFBDSIR J1458+1013 b (Liu et al, 2011), another potential Y-dwarf discovered in 2010 (Luhman et al, 2011b; Ian O'Neill, Discover, March 12, 2011; and staff, New Scientist, March 12, 2011).

Francis Reddy, GSFC, NASA


Large and jumbo illustrations.


The apparent brown dwarf is separated
from the white dwarf WD 0806-661 by
some 2,500 AUs (more).

White Dwarf 0806-661

WD 0806-661 is a white dwarf stellar remnant of spectral class DQ (formerly DC₂), which exhibits strong atomic or molecular carbon lines (Luhman et al, 2011b). Its progenitor star may have been a spectral class A star with 2.1 +/-0.3 Solar-masses (Luhman et al, 2011a), which blew off its outer gas layers of hydrogen and helium and is now around 1.5 +0.5/-0.3 billion years ago (Luhman et al, 2011). WD 0806-661 has about 0.62 +/- 0.03 of Sol's mass (Luhman et al, 2011b), but with only around a few percent of its diameter (around the size of a planet like Earth), and a similarly small share of its brightness. White dwarfs are incredibly dense objects because they squeeze a stellar mass into a planetary volume. While tiny compared to main sequence stars, white dwarf stars are actually intensely hot, but without the internal heat of fusion to keep them burning, they gradually cool and fade away. Some alternative names and useful designation for WD 0806-661 are: GJ 3483, L 97-3, LTT 3059, and 2MASS J08065373-6618167.

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

The observed number of white dwarfs in the Solar neighborhood comprises only about one percent of its dynamical mass density (Liebert et al, 1988), but most white dwarf companions in binary systems with bright, main sequence stars have not been detected probably due to their relative faintness and because of the glare from the primary star (James Liebert, 1980). A recent survey of 146 DZ white dwarfs and their "externatlly polluted atmospheres" suggests that at least 3.5 percent of white dwarfs may harbor the remnants of terrestrial planetary systems, while another study of 129 white dwarfs within 20 parsecs (65.2 light-years) of the Solar System suggests that at least one fifth (20 percent) have "photospheric metals" that may have come from accretion of circumstallar debris disks which may have eventually produced planets or asteroid-like bodies (Farihi et al, 2010; and Sion et al, 2009). All the known white dwarfs within 20 parsecs (65.2 light-years) of the Solar System appear to be members of the thin disk, as none exhibit kinematic properties of the halo or extended thick disk Sion et al, 2009).

Brown Dwarf or Planet "b"

Janella Williams



Large and jumbo illustrations.



Astronomers suspect that the apparent
was once closer to its primary stellar
companion before it lost mass to become
a white dwarf stellar remnant (more).

Substellar object "b" has an estimated mass of only around seven Jupiter-masses (Luhman et al, 2011b). It appears to have an effective temperature of roughly 300 Kelvins (27 Celsius or 86 Fahrenheit) and so may be a member of the proposed "Y" spectral class for brown dwarfs (Kirkpatrick et al, 1999). Despite its current separation from its primary of roughly 2,500 AUs which would suggest that the object formed as a brown dwarf separate from any planets orbiting the white dwarf primary, it is possible that the object originally formed closer to the primary within its circumstellar gas and dust disk but drifter into a farther orbit when the progenitor star blew off around three-fourths of its mass in a planetary nebula. (More from Luhman et al, 2011a; PSU news release; NASA / JPL news release; and Jason Major, Universe Today, October 21, 2011).

Robert Hurt, IPAC, NASA

Larger illustration: Sol; M,L,T dwarfs;
and Jupiter.

At one billion years in age, large brown dwarfs
are reddish like the smallest M-type stars, but
cooler, dimmer T- and hypothesize Y-dwarfs are
bluer and so should be more magenta in hue (more).

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© Anglo-Australian Telescope Board
(Image by Chris Tinney)





Wide field image with satellite trail.





"True-color" image of the brown
dwarf binary DENIS 1228-1547

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover the other nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter-masses would be defined as planets.

WISE, UC-Boulder,
CalTech, JPL, NASA




Larger composite image.




Large planets and brown dwarfs that have multiples
of Jupiter's mass do not increase in diameter at
the same rate due to gravitational compression,
but they may become redder as they get hotter
and radiate more visible as well as infrared light.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.