Extrasolar planet
Planet Fomalhaut b (inset against Fomal-
haut’s interplanetary dust cloud) imaged by
the Hubble Space Telescope’s coronagraph
(NASA photo)
HR 8799 (center blob) with infrared images
of planets HR 8799d (bottom), HR 8799c (up-
per right), and HR 8799b (upper left)
An extrasolar planet, or exoplanet, is a
planet beyond our Solar System, orbiting a
star other than our Sun. As of May 2009, 347
exoplanets are listed in the Extrasolar Plan-
ets Encyclopaedia.[1] The vast majority have
been detected through radial velocity obser-
vations and other indirect methods rather
than actual imaging.[1] Most announced exo-
planets are massive gas giant planets
thought to resemble Jupiter, but this is a se-
lection effect due to limitations in detection
technology. Projections based on recent de-
tections of much smaller worlds suggest that
lightweight, rocky planets will eventually be
found to outnumber extrasolar gas giants.[2]
Extrasolar planets became a subject of sci-
entific investigation in the mid-19th century.
Many astronomers supposed that such plan-
ets existed, but they had no way of knowing
how common they were or how similar they
might be to the planets of the Solar System.
The first confirmed radial velocity detection
was made in 1995, revealing a gas giant plan-
et in a four-day orbit around the nearby G-
type star 51 Pegasi. The frequency of detec-
tions has tended to increase on an annual
basis since then.[1] It is estimated that at
least 10% of sun-like stars have planets, and
the true proportion may be much higher.[3]
The discovery of extrasolar planets sharpens
the question of whether some might support
extraterrestrial life.[4]
Currently Gliese 581 d, the fourth planet
of the red dwarf star Gliese 581 (approxim-
ately 20 light years from Earth), appears to
be the best example yet discovered of a pos-
sible terrestrial exoplanet that orbits within
the habitable zone surrounding its star. Al-
though initial measurements suggested that
Gliese 581 d resided outside the so-called
"Goldilocks Zon