CONTACTS:

Kevin Luhman, 1-814-863-4957, kluhman@astro.psu.edu

Barbara K. Kennedy (PIO), 1-814-863-4682, science@psu.edu

18 September 2006

Scientists Snap Images of First Brown Dwarf in Planetary System

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 Sun-like star. Such an
arrangement has never before been seen but might be common, the
scientists say, leading to solar systems with distorted planetary
orbits.

Kevin Luhman of Penn State University 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 ten 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 Penn State
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 our 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 ten 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 our Sun
that is orbited by a planet slightly smaller than Saturn.

The planet's orbit around the Sun-like 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 over ten times larger than Pluto's," said
Brian Patten of the Harvard-Smithsonian Center for Astrophysics
(CfA), 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 of CfA, a co-author.

The discoveries were made with Spitzer's infrared camera, built
primarily at NASA Goddard Space Flight Center in Greenbelt, Maryland.
The instrument's principal investigator is Giovanni Fazio. Other
team members include Massimo Marengo, Joseph Hora, Richard Ellis,
Michael Schuster, Sarah Sonnett, Elaine Winston, and Robert Gutermuth
of the CfA; 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, California, 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.

[ Christopher Wanjek / K L / B K K ]

IMAGES:

High-resolution images are on the web at

*http://www.science.psu.edu/alert/Luhman9-2006-2.htm*

and also at *http://www.spitzer.caltech.edu/Media*

Caption for figure with images of 2 stars and their brown dwarf companions:

Using infrared photographs obtained with NASA's Spitzer Space Telescope,
astronomers have discovered two very cold brown dwarfs orbiting the
stars HD 3651 (left) and HN Peg (right). These brown dwarfs have masses
of only 20 and 50 times the mass of Jupiter and have orbits that are more
than 10 times larger than Pluto's orbit. HD 3651 and HN Peg are in the
Sun's neighborhood of the Galaxy, with distances of only 36 and 60 light
years from the Sun.

Credit: NASA / JPL-Caltech / K. Luhman (Penn State University) / B. Patten
(Harvard-Smithsonian)