51 Pegasi

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Places in Fenspace
51 Pegasi
Stellar characteristics
Right ascension (Epoch J2000)22h 57m 28s
Declination (Epoch J2000)+20° 46' 7.8"
Spectral typeG4-5 Va
Distance from Sol50.1 ly
Other designations51 Peg, NSV 14374, HR 8729, Gl 882, Hip 113357, HD 217014, BD+19 5036, SAO 90896, LTT 16750.
Planets51 Pegasi b
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51 Pegasi is a yellow-orange main sequence dwarf star of spectral and luminosity type G4-5 Va, but it had been previously classified as G2-2.5 and sometimes as a subgiant (IV). The star has about 1.06 times the mass of Sol[1], 1.15 to 1.4 times its diameter[2], and 1.30 times its visual luminosity[3]. Relatively bright and large for its spectral type and mass, it appears to be running low on core hydrogen and may be as old as 8.5 billion years old or more[4], but one analysis of isochrones suggests that the star could be a billion years younger -- at 7.5 billion years old[5]. 51 Pegasi is 1.6 times more enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron[6].

(Boilerplate from SolStation.com)

The star itself is of apparent magnitude 5.49, and so is visible from the Earth with binoculars, or with the naked eye under dark sky conditions. 51 Pegasi is a yellow dwarf star estimated to be 6.1–8.1 billion years old, somewhat older than the Sun, 4–6% more massive, with more metal content and running low in hydrogen. Its spectral type is listed as either G2.5V or G4-5Va.

In 1996 astronomers Baliunas, Sokoloff, and Soon reported measurements of a sample of stars' Calcium II H and K spectral lines and thereby measured a rotational period of 37 days for 51 Pegasi.[7]

(Boilerplate from Wikipedia)

51 Pegasi b

In 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of a Jupiter-class planet around 51 Pegasi using radial-velocity methods[8]. Planet "b" has at least 46 percent of Jupiter's mass. It moves around 51 Pegasi at an average distance of only 0.05 AUs (a semi-major axis well within Mercury's orbital distance) in a highly circular orbit (e=0.03) that takes only 4.2 days to complete. Its orbit may be inclined by less than 85° from the perspective of an observer on Earth[4]. Assuming a Jupiter-like composition, its radius may be about 1.2 times that of Jupiter, enlarged relative to Jupiter because of greater absorbed stellar radiation in its inner ("torch") orbit. However, the planet may be too hot to hold onto a thick hydrogen atmosphere.

(Boilerplate from SolStation.com)

Places in Fenspace
51 Pegasi b - Bellerophon
Planetary characteristics
Orbit7.89 Gm from primary (Semimajor axis)
Year4.23 days
Daytidally locked to primary
Mean Temperatureestimated 1265K (around 1000 degrees Celsius / 1800 degrees Fahrenheit)
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After the announcement, on October 12, 1995, confirmation came from Dr. Geoffrey Marcy from San Francisco State University and Dr. Paul Butler from the University of California, Berkeley using the Hamilton Spectrograph at the Lick Observatory near San Jose in California.

51 Pegasi b (51 Peg b for short) is the first discovered planetary-mass companion of its parent star. Further such companions would be designated c, d, and so on. The planet has been informally named Bellerophon. After its discovery, many teams confirmed its existence and obtained more observations of its properties, including the fact that it orbits very close to the star, suffers estimated temperatures around 1200°C, and has a minimum mass about half that of Jupiter. At the time, this close distance was not compatible with theories of planet formation and resulted in discussions of planetary migration.

(Boilerplate from Wikipedia)


  1. 51 Peg at exoplanets.org
  2. Henry et al, 2000; and Guillot et al, 1996
  3. 51 Pegasi at the Observatoire de Genève
  4. 4.0 4.1 Henry et al, 1997
  5. Guinan et al, 1999
  6. 51 Peg at exoplanets.org
  7. Baliunas et al. (1996). "Magnetic Field and Rotation in Lower Main-Sequence Stars: An Empirical Time-Dependent Magnetic Bode's Relation?". The Astrophysical Journal Letters 457 (2): L99–L102.
  8. Mayor and Queloz, 1995; and 51 Pegasi at the Observatoire de Genève