Planets Circling Other Planets?
Moons circle planets, and planets circle stars. Now, astronomers
have learned that planets may also circle celestial bodies almost
as small as planets.
NASA's Spitzer Space Telescope has spotted a dusty disc of
planet-building material around an extraordinarily low-mass brown
dwarf, or "failed star." The brown dwarf, called OTS 44, is only 15
times the mass of Jupiter. Previously, the smallest brown dwarf
known to host a planet-forming disc was 25 to 30 times more massive
The finding will ultimately help astronomers better understand
how and where planets – including rocky ones resembling our
own – form.
"There may be a host of miniature solar systems out there, in
which planets orbit brown dwarfs," said Dr. Kevin Luhman, lead
author of the new study from the Harvard-Smithsonian Center for
Astrophysics, Cambridge, Mass. "This leads to all sorts of new
questions, like 'Could life exist on such planets?' or 'What do you
call a planet circling a planet-sized body? A moon or a
Brown dwarfs are something of misfits in the astronomy world.
These cool orbs of gas have been called both failed stars and super
planets. Like planets, they lack the mass to ignite and produce
starlight. Like stars, they are often found alone in space, with no
parent body to orbit.
"In this case, we are seeing the ingredients for planets around
a brown dwarf near the dividing line between planets and stars.
This raises the tantalizing possibility of planet formation around
objects that themselves have planetary masses," said Dr. Giovanni
Fazio, an astronomer at the Harvard Smithsonian Center for
Astrophysics and a co-author of the new study.
The results were presented today at the Planet Formation and
Detection meeting at the Aspen Center for Physics, Aspen, CO, and
will be published in the Feb. 10th issue of The Astrophysical
Planet-forming, or protoplanetary, discs are the precursors to
planets. Astronomers speculate that the disc circling OTS 44 has
enough mass to make a small gas giant planet and a few Earth-sized,
rocky ones. This begs the question: Could a habitable planet like
Earth sustain life around a brown dwarf?
"If life did exist in this system, it would have to constantly
adjust to the dwindling temperatures of a brown dwarf," said
Luhman. "For liquid water to be present, the planet would have to
be much closer to the brown dwarf than Earth is to our Sun."
"It's exciting to speculate about the possibilities for life in
such as system, of course at this point we are only beginning to
understand the unusual circumstances under which planets arise," he
Brown dwarfs are rare and difficult to study due to their dim
light. Though astronomers recently reported what may be the
first-ever image of a planet around a brown dwarf called 2M1207,
not much is understood about the planet-formation process around
these odd balls of gas. Less is understood about low-mass brown
dwarfs, of which only a handful are known.
OTS 44 was first discovered about six months ago by Luhman and
his colleagues using the Gemini Observatory in Chile. The object is
located 500 light-years away in the Chamaeleon constellation.
Later, the team used Spitzer's highly sensitive infrared eyes to
see the dim glow of OTS 44's dusty disc. These observations took
only 20 seconds. Longer searches with Spitzer could reveal discs
around brown dwarfs below10 Jupiter masses.
Other authors of this study include Dr. Paola D'Alessia of the
Universidad Nacional Autonoma de Mexico; and Drs. Nuria Calvet,
Lori Allen, Lee Hartmann, Thomas Megeath and Philip Myers of the
Harvard-Smithsonian Center for Astrophysics.