Aspen, CO— Using the Spitzer Space Telescope, a team of astronomers led by
Kevin Luhman (Harvard-Smithsonian Center for Astrophysics) has discovered
a protoplanetary disk around a surprisingly low-mass brown dwarf. This
remarkable finding raises the possibility of planet formation around
objects that themselves have planetary masses. Moreover, the presence of a
disk suggests that terrestrial planets could form and thrive orbiting an
object too small to shine via nuclear fusion.

“It’s an exciting possibility—one that hasn’t been explored extensively
because this is the first evidence for the building blocks of planets
around such a small object,” said Luhman.

The team’s findings were presented today in a press conference at the
Planet Formation and Detection meeting in Aspen, Colorado, and will be
published in the Feb. 10th issue of The Astrophysical Journal Letters.

The brown dwarf in question, OTS 44, is located approximately 500
light-years away in the southern constellation Chamaeleon. OTS 44 weighs
in at around 15 Jupiter masses, placing it near the dividing line between
brown dwarfs (generally defined as objects of 15-70 Jupiter masses) and
planets. At a temperature of 3,600˚ F (2300 Kelvin), OTS 44 is the
coolest and least massive brown dwarf known to have a circumstellar disk.

Although the team cannot measure the total mass of the disk, it likely
contains enough matter to make one small gas giant or several Earth-sized
planets. “This brown dwarf and its disk could eventually evolve into a
miniature version of our solar system,” said Luhman.

Due to the brown dwarf’s low temperature, an Earth-sized world would have
to orbit much closer to the brown dwarf than the Earth from the Sun in
order to be as warm as Earth. Theorists estimate that liquid water could
exist on the surface of a planet about 1 to 4 million miles from the brown
dwarf. The disk of OTS 44 extends beyond both sides of this “habitable
zone.”

Without nuclear fusion to sustain it, the brown dwarf will gradually cool
and dim. If an Earth-sized world forms near the brown dwarf, it will be
scorching at first, then grow cooler and more hospitable over time. Since
the brown dwarf cools more slowly as it gets older, such a planet could
remain in the habitable zone for an extended time, raising the intriguing
possibility that life might evolve.

“That is pure speculation, of course. But finding a circumstellar disk
around such a small brown dwarf certainly widens the possibilities for
planet formation,” said Luhman.

The researchers plan to search for similar disks around other nearby brown
dwarfs. Spitzer revealed the disk of OTS 44 in only 20 seconds of
observing time. Further searches may locate similar disks around even
smaller central objects of 10 Jupiter masses or less.

The team detected OTS 44’s circumstellar disk using Spitzer’s Infrared
Array Camera, or IRAC. IRAC data showed an excess of infrared emission at
long wavelengths—the signature of a dusty disk that absorbs radiation from
the brown dwarf, heats up, and re-radiates the energy in the infrared.

Other members of the discovery team are Paola D’Alessio (UNAM); Nuria
Calvet, Lori Allen, Lee Hartmann, Thomas Megeath, Philip Myers, and
Giovanni Fazio (Harvard-Smithsonian Center for Astrophysics)

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.