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Posted by baalke on September 19, 2006, 1:47 pm
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http://live.psu.edu/story/19570
Scientists snap images of first brown dwarf in planetary system
Pennsylvania State University
September 18, 2006
University Park, Pa. -- Scientists using NASA's Spitzer Space Telescope
have discovered and directly imaged a small brown dwarf star, 50 times
the mass of Jupiter, orbiting with a planet around a sunlike star. Such
an arrangement has never before been seen but might be common, the
scientists said, leading to solar systems with distorted planetary
orbits.
Kevin Luhman of Penn State is the lead author on a report describing
this discovery, which will be published in The Astrophysical Journal.
The discovery concerns a class of the coldest brown dwarfs, called T
dwarfs.
"Over the last 10 years, astronomers have been extremely successful in
finding planets close to their host stars using indirect detection
methods," said Luhman, an assistant professor in the Department of
Astronomy and Astrophysics. "Because of its infrared capabilities,
Spitzer is well suited for directly detecting cool T dwarfs, and
perhaps
even large planets, in the outer parts of planetary systems."
Luhman's team also discovered a second brown dwarf that is smaller yet,
about 20 times the mass of Jupiter, orbiting another star. This smaller
object could be the youngest T dwarf known, offering scientists a
snapshot of early brown-dwarf development. The two T dwarfs are the
first to be imaged by Spitzer. Shortly after these companions were
found, Spitzer also discovered a T dwarf that is floating through space
by itelf rather than orbiting a star. The team that discovered that T
dwarf is led by Daniel Stern at NASA's Jet Propulsion Laboratory.
Brown dwarfs are small stars that are not massive enough to burn
hydrogen, like the sun does. Their cores are not hot enough to trigger
such nuclear fusion. As a result, their surface temperature is only a
few thousands of degrees when young, cooling considerably to about the
temperature of a planet as they age. Consequently, they are dim and
hard
to identify and, as a result, the first unambiguous identification came
only about 10 years ago.
The more massive of the two newly discovered T dwarfs is called HD 3651
B, located in the constellation Pisces. This object is in a solar
system
containing a star slightly less massive than the sun that is orbited by
a planet slightly smaller than Saturn.
The planet's orbit around the sunlike star is highly elliptical, which
had suggested that the gravity of some unseen object farther away from
the star was pulling the planet outward. Sure enough, it was a T dwarf.
Many extrasolar planets have been discovered with highly elliptical
orbits. The Spitzer discovery is the first evidence to support the
theory that small companions such as T dwarfs can hide in such solar
systems and can cause the orbits of planets to be extreme.
"The orbit of the planet in this system is similar to Mercury's, but
the
T dwarf has an orbit more thanr 10 times larger than Pluto's," said
Brian Patten of the Harvard-Smithsonian Center for Astrophysics, a
co-author. "Although HD 3651 B would be just beyond naked-eye
visibility
to an intrepid astronomer living on this system's planet, the T dwarf
makes its presence known through gravity."
The other T dwarf is called HN Peg B in the constellation Pegasus.
Whereas most brown dwarfs are billions of years old, HN Peg B is
relatively young, only about 300 million years old. The scientists
determined its age by carefully studying the companion star, which was
formed at the same time from the same gas cloud. The system also
contains a previously discovered disk of dust and rocks.
"Detectable debris disks and T dwarf companions are fairly rare, so the
presence of both around the same star makes this a particularly
exciting
star system," said Giovanni Fazio, also of the center, a co-author.
The discoveries were made with Spitzer's infrared camera, built
primarily at NASA Goddard Space Flight Center in Greenbelt, Md. The
instrument's principal investigator is Fazio. Other team members
include
Massimo Marengo, Joseph Hora, Richard Ellis, Michael Schuster, Sarah
Sonnett, Elaine Winston and Robert Gutermuth of the Center for
Astrophysics; John Stauffer of Caltech; Tom Megeath of the University
of
Toledo; Dana Backman of the SOFIA/SETI Institute; Tod Henry of Georgia
State University; and Michael Werner of NASA Jet Propulsion Laboratory.
The Jet Propulsion Laboratory, in Pasadena, Calif., manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate,
Washington. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology in Pasadena.
Contact
Barbara Kennedy
science@psu.edu
www.science.psu.edu
814-863-4682
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