25 Light-Years Away, Seven Years Of Persistent Research Pays
Off
NASA's Hubble Space Telescope has taken the first visible-light
snapshot of a planet circling another star. The team of astronomers
who made the discovery includes researchers at NASA's Jet
Propulsion Laboratory, Pasadena, CA.
Estimated to be no more than three times Jupiter's mass, the
planet, called Fomalhaut b, orbits the bright southern star
Fomalhaut, located 25 light-years away in the constellation Piscis
Australis, or the "Southern Fish."
Fomalhaut has been a candidate for planet hunting ever since an
excess of dust was discovered around the star in the early 1980s by
NASA's Infrared Astronomy Satellite.
In 2004, the coronagraph in the High Resolution Camera on
Hubble's Advanced Camera for Surveys produced the first-ever
resolved visible-light image of the region around Fomalhaut. It
clearly showed a ring of protoplanetary debris approximately 21.5
billion miles (34.6 billion kilometers) across and having a sharp
inner edge.
This large debris disk is similar to
the Kuiper Belt, which encircles our solar system and contains a
range of icy bodies from dust grains to objects the size of dwarf
planets, such as Pluto.
Hubble astronomer Paul Kalas, of the University of California at
Berkeley, and team members proposed in 2005 that the ring was being
gravitationally modified by a planet lying between the star and the
ring's inner edge.
Circumstantial evidence came from Hubble's confirmation that the
ring is offset from the center of the star. The sharp inner edge of
the ring is also consistent with the presence of a planet that
gravitationally "shepherds" ring particles. Independent researchers
have subsequently reached similar conclusions.
Now, Hubble has actually photographed a point source of light
lying 1.8 billion miles (2.9 billion kilometers) inside the ring's
inner edge. The results are being reported in the November 14 issue
of Science magazine.
"Our Hubble observations were incredibly demanding. Fomalhaut b
is 1 billion times fainter than the star. We began this program in
2001, and our persistence finally paid off," Kalas said.
"Fomalhaut is the gift that keeps on giving. Following the
unexpected discovery of its dust ring, we have now found an
exoplanet at a location suggested by analysis of the dust ring's
shape. The lesson for exoplanet hunters is 'follow the dust,'" said
team member Mark Clampin of NASA's Goddard Space Flight Center in
Greenbelt, MD.
Observations taken 21 months apart by Hubble's Advanced Camera
for Surveys' coronagraph show that the object is moving along a
path around the star, and is therefore gravitationally bound to it.
The planet is 10.7 billion miles (17.2 billion kilometers) from the
star, or about 10 times the distance of the planet Saturn from our
sun.
The planet Fomalhaut b is brighter than expected for an object
of three Jupiter masses. One possibility is that it has a
Saturn-like ring of ice and dust reflecting starlight. The ring
might eventually coalesce to form moons. The ring's estimated size
is comparable to the region around Jupiter and its four largest
orbiting satellites.
Kalas and his team first used Hubble to photograph Fomalhaut in
2004, and made the unexpected discovery of its debris disk, which
scatters Fomalhaut's starlight. At the time they noted a few bright
sources in the image as planet candidates. A follow-up image in
2006 showed that one of the objects is moving through space with
Fomalhaut, but changed position relative to the ring since the 2004
exposure. The amount of displacement between the two
exposures is exactly as predicted and corresponds to an
872-year-long orbit as calculated from Kepler's laws of planetary
motion.
Future observations will attempt to see the planet in infrared
light and will look for evidence of water vapor clouds in the
atmosphere. This would yield clues to the evolution of a
comparatively newborn 100-million-year-old planet. Astrometric
measurements of the planet's orbit will provide enough precision to
yield an accurate mass.
NASA's James Webb Space Telescope, scheduled to launch in 2013,
will be able to make coronagraphic observations of Fomalhaut in
near- and mid-infrared wavelengths. Webb will be able to hunt for
other planets in the system and probe the region interior to the
dust ring for structures such as an inner asteroid belt.