November 29, 1995
Astronomers have made the first unambiguous detection and image of
an elusive type of object known as a brown dwarf.
The evidence consists of an image from the 60-inch observatory on
Mt. Palomar, a spectrum from the 200-inch Hale telescope on Mt.
Palomar and a confirmatory image from NASA's Hubble Space Telescope.
The collaborative effort involved astronomers at the California
Institute of Technology, Pasadena, CA, and the Johns Hopkins
University, Baltimore, MD.
ABOUT THIS IMAGE:
These two false-color telescope images reveal the
faintest object ever seen around a star beyond our Sun,
and the first unambiguous detection of a brown dwarf.
The brown dwarf, called GL229B, orbits the red dwarf
star Gliese 229, located approximately 18 light-years
away in the constellation Lepus. The brown dwarf is
about 20-50 times the mass of Jupiter, but is so dense
it is about the same diameter as Jupiter (80,000 miles).
Brown dwarfs are a mysterious class of long-sought
objects that form the same way stars do, by condensing
out of a cloud of hydrogen gas. However, they do not
accumulate enough mass to sustain nuclear fusion at
their core, which make stars shine.
[left] - The brown dwarf (center) was first observed in
far red light October 27, 1994 using the adaptive optics
device and a 60-inch reflecting telescope on Palomar
Mountain in California. Another year was required to
confirm that the object was actually gravitationally
bound to the companion star. GL229B is at least four
billion miles from its companion star, roughly the
separation between the planet Pluto and our Sun. Even
though a cornograph on the detector masked most of the
light from the star, which is off the left edge of the
image, it is so bright relative to the brown dwarf the
glare floods the detector.
[right] - This image of the GL229B (center) was taken
with Hubble Space Telescope's Wide Field Planetary
Camera-2, infrared light, on November 17, 1995. The
Hubble observations will be used to accurately measure
the brown dwarf's distance from Earth, and yield
preliminary data on its orbital period, which may
eventually offer clues to the dwarf's origin. Though the
star Gliese 229 is off the edge of the image, it is so
bright it floods Hubble detector. The diagonal line is a
diffraction spike produced by the telescope's optical
Object Names: Gliese 229, GL229, Gliese 229b, GL229b,
Image Type: Astronomical
The brown dwarf, called Gliese 229B (GL229B), is a small companion
to the cool red star Gliese 229, located 19 light-years from Earth
in the constellation Lepus. Estimated to be 20 to 50 times the mass
of Jupiter, GL229B is too massive and hot to be classified as a
planet as we know it, but too small and cool to shine like a star.
At least 100,000 times dimmer than Earth's Sun, the brown dwarf is
the faintest object ever seen orbiting another star.
"This is the first time we have ever observed an object beyond our
solar system which possesses a spectrum that is astonishingly just
like that of a gas giant planet," said Shrinivas Kulkarni, a member
of the team from Caltech.
Kulkarni added, however, that "it looks like Jupiter, but that's
what you'd expect for a brown dwarf." The infrared spectroscopic
observations of GL229B, made with the 200-inch Hale telescope at
Palomar, show that the dwarf has the spectral fingerprint of the
planet Jupiter -- an abundance of methane. Methane is not seen in
ordinary stars, but it is present in Jupiter and other giant gaseous
planets in our solar system.
The Hubble data obtained and analyzed so far already show the object
is far dimmer, cooler (no more than 1,300 degrees Fahrenheit) and
less massive than previously reported brown dwarf candidates, which
are all near the theoretical limit (eight percent the mass of our
Sun) where a star has enough mass to sustain nuclear fusion.
Brown dwarfs are a mysterious class of long-sought object that forms
the same way stars do, that is, by condensing out of a cloud of
hydrogen gas. However, they do not accumulate enough mass to
generate the high temperatures needed to sustain nuclear fusion at
their core, which is the mechanism that makes stars shine. Instead
brown dwarfs shine the same way that gas giant planets like Jupiter
radiate energy, that is, through gravitational contraction. In fact,
the chemical composition of GL229B's atmosphere looks remarkably
like that of Jupiter.
The discovery is an important first step in the search for planetary
systems beyond the Solar System because it will help astronomers
distinguish between massive Jupiter-like planets and brown dwarfs
orbiting other stars. Advances in ground- and space-based astronomy
are allowing astronomers to further probe the "twilight zone"
between larger planets and small stars as they search for substellar
objects, and eventually, planetary systems.
Caltech astronomers Kulkarni, Tadashi Nakajima,
Keith Matthews, and
Ben Oppenheimer, and Johns Hopkins scientists Sam Durrance and
David Golimowski first discovered the object in October 1994. Follow-up
observations a year later were needed to confirm it is actually a
companion to Gliese 229. The discovery was made with a 60-inch
reflecting telescope at Palomar Observatory in southern California,
using an image-sharpening device called the Adaptive Optics
Coronagraph, designed and built at the Johns Hopkins University.
The same scientists teamed up with Chris Burrows of the Space
Telescope Science Institute to use Hubble's Wide Field Planetary
Camera-2 for follow-up observations on November 17. Another Hubble
observation six months from now will yield an exact distance to
The astronomers suspect that the brown dwarf developed during the
normal star-formation process as one of two members of a binary
"All our observations are consistent with brown dwarf
theory," Durrance said.
However, the astronomers say they cannot yet
fully rule out the possibility that the object formed out of dust
and gas in a circumstellar disk as a "super-planet."
Astronomers say the difference between planets and brown dwarfs is
based on how they formed. Planets in the Solar System are believed
to have formed out of a primeval disk of dust around the newborn Sun
because all the planets' orbits are nearly circular and lie almost
in the same plane. Brown dwarfs, like full-fledged stars, would have
fragmented and gravitationally collapsed out of a large cloud of
hydrogen but were not massive enough to sustain fusion reactions at
The orbit of GL229B could eventually provide clues to its origin. If
the orbit is nearly circular then it may have formed out of a dust
disk, where viscous forces in the dense disk would keep objects at
about the same distance from their parent star. If the dwarf formed
as a binary companion, its orbit probably would be far more
elliptical, as seen on most binary stars. The initial Hubble
observations will begin providing valuable data for eventually
calculating the brown dwarf's orbit.
However, the orbital motion is
so slow, it will take many decades of telescopic observations before
a true orbit can be calculated. GL229B is at least four billion
miles from its companion star, which is roughly the separation
between the planet Pluto and our Sun.
Astronomers have been trying to detect brown dwarfs for three
decades. Their lack of success is partly due to the fact that as
brown dwarfs age they become cooler, fainter, and more difficult to
see. An important strategy used by the researchers to search for
brown dwarfs was to view stars no older than a billion years.
Caltech's Nakajima reasoned that, although brown dwarfs of that age
would be much fainter than any known star, they would still be
bright enough to be spotted.
"Another reason brown dwarfs were not detected years ago is that
imaging technology really wasn't up to the task," Golimowski said.
With the advent of sophisticated light sensors and adaptive optics,
astronomers now have the powerful tools they need to resolve smaller
and dimmer objects near stars.
Hubble was used to look for the presence of other companion objects
as bright as the brown dwarf which might be as close to the star as
one billion miles. No additional objects were found, though it
doesn't rule out the possibility of Jupiter-sized or smaller planets
around the star, said the researchers.