A Sun-like star around which has been
discovered a planet, 51 Pegasi b, with about half the mass of Jupiter.
With the exception of some pulsar planets,
51 Pegasi b was the first extrasolar planet to be found. It is the prototype for a new and previously unsuspected class
of planet known as epistellar Jovians,
or "hot Jupiters," with remarkably small orbits. The existence
of giant planets very close to their host stars has led to the suggestion
that inward orbital migration is a
common feature of young planetary systems.
51 Pegasi b orbits its host star at less than one eighth the distance of Mercury from the Sun so that, assuming it
has a radius of 1.2 to 1.4 times that of Jupiter, it must have a surface
temperature of around 1,000°C. Even so, calculations show that it should
still be able to retain a massive atmosphere of predominantly light gases.
51 Pegasi ranked in the top five stars most likely to support life in a
shortlist drawn up in 2006 by astrobiologist Margaret Turnbull,
one of the authors of HabCat (Catalog of
Nearby Habitable Systems).
In 2001, astronomers at the University of Texas at Arlington published results
showing that the habitable zone around 51
Pegasi, where an inner rocky planet (with suitable mass and atmospheric
composition and density) can have liquid water on its surface, lies between 1.20 and 2.0 AU of the star. Unfortunately,
the development of an Earth-like planet in this zone could have been disrupted
since it would likely be expelled by the inward migration of planet b, as
hypothesized by some planetary astronomers. If a small, rocky planet could
have developed without the interference of planet b, however, then stable
orbits appear to be possible over the entire habitable zone. A terrestrial
planet orbiting 51 Pegasi at around the center of the calculated habitable
zone would have an average orbital distance of about 1.6 AU, just beyond
the orbit of Mars in the Solar System. Its
orbital period would be about 2.0 years.
| Host star
||50.21 light-years (15.4 pc)
|luminosity (Sun = 1)
|mass (Sun = 1)
|radius (Sun = 1)
|metallicity (Sun = 1)
||R.A. 22h 57m 28.0s,
Dec. +20° 46' 7"
|other catalog designations
||HR 8729, Gl 882, Hip 113357,
HD 217014, SAO 90896
|mass (Jupiter = 1)
||0.052 AU (7.8 million km, 4.8 million mi.)
|year of discovery
||Mayor and Queloz, Geneva Observatory
|method of discovery
- Boss, A. "Forming a Jupiter-like Companion for 51 Peg," Lunar and
Planetary Science, 27 139 (1996).
- Hatzes, A., Cochran, W., and Bakker, E. "Further Evidence for the
Planet Around 51 Pegasi," Nature, 391, 154 (1998).
- Mayor, D., and Queloz, D. "A Jupiter-mass Companion to a Solar-type
Star," Nature, 378, 355 (1995).
- Noble, M., Musielak, Z. E., and Cuntz, M. "Orbital Stability of Earth-like
Planets in Stellar Habitable Zones." Bulletin of the American Astronomical
Society, Vol. 33, p. 1304 (2001).
PLANETS AND SUBSTELLAR OBJECTS