51 Pegasi

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51 Pegasi
Pegasus 51 location.png
Observational Data
Designation 51 Pegasi
HD 217014
HIP 113357
Right ascension 22h 57m 27.9804s[1][2]
Declination +20° 46′ 07.7822″[1][2]
Constellation Pegasus
Type of object Star
Magnitude Apparent Mag: 5.49[3]
Absolute Mag: 4.51
Distance from Earth 50.1 ly[4]
Radial velocity -33.24±0.43 km/s[1][2]
Proper motion RA: 207.363 ± 0.31 mas/yr[1][2]
Dec.: 62.094 mas/yr[1][2]
Parallax 64.64 ± 0.12 mas[1][2]

51 Pegasi is a star located 50.1 light years away in the constellation Pegasus, which is known for both being very much like our own sun and for the being the first star system outside our own where an extrasolar planet was discovered, in 1995.[5][6]

From Earth, the star has an apparent magnitude of 5.49 allowing it to be seen by the unaided eye under very dark conditions, or with binoculars.

The Star

51 Pegasi is a yellow-orange main sequence star of spectral class of G4-5 Va, although it has been previously classified as G2IV-V (meaning either a dwarf or a subgiant). The star is remarkably similar to our own sun, having 1.06 times the mass, 1.15 to 1.4 times the diameter, and 1.30 times the visual luminosity. The star is also 60% more abundant in elements heavier then hydrogen and helium (metallicity).[4] 51 Pegasi rotates once every 37 days, somewhat slower then the Sun's 25 day rotation.

The star is believed to be farther along on its path of stellar evolution and appears to be running low on its hydrogen fuel needed for stellar fusion.


51 Pegasi was the star where the first extrasolar world was discovered using radial-velocity methods; astronomers Michel Mayor and Didier Queloz announced the discovery the Jupiter class gas giant on October 12, 1995.[6] Independent confirmation of the world came from San Francisco State University, University of California, and the Lick Observatory. The world is nicknamed "Bellerophon", although its official astronomical designation is 51 Pegasi b.[7]

The planet itself has at least 46 percent of Jupiter’s mass and maintains a very close orbit of 0.052 AUs from its parent star. Due to how close the planet is to 51 Pegasi, its orbit only takes 4.2 days to complete. Because of this distance, the world is tidally locked to its parent star. Temperatures are estimated to be 1,265 kelvin (around 1,000 degrees Celsius / 1,800 degrees Fahrenheit) on average.

The world is also remarkable because a world so close to its parent star was not compatible with planet formation theories at the time, and the planet was considered an anomaly. Since then other such “hot Jupiters” have been discovered challenging astronomers to revise the theories of planetary formation, one way is by studying orbit migration.

A terrestrial planet that could maintain liquid water on its surface could hypothetically exist in a habitable zone that ranges from 1.2 AU to 2.0 AUs away for 51 Pegasi.[8] The star is a target for NASA’s Terrestrial Planet Finder (TPF) and the ESA's Darwin in the future.

No other worlds are known at this time.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 51 Pegasi. Simbad Astronomical Database. simbad.u-strasbg.fr. Retrieved on 2019-07-25.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Gaia Colaboration. VizieR Online Data Catalog: Gaia DR2 (Gaia Collaboration, 2018). (2018). VizieR Online Data Catalog. Bibcode:2018yCat.1345....0G
  3. Jim Kaler. 51 Pegasi. Stars. stars.astro.illinois.edu. Retrieved on 2019-07-25.
  4. 4.0 4.1 51 Pegasi. solstation.com. Retrieved on 2019-07-25.
  5. 51 Pegasi. britannica.com. Retrieved on 2019-07-25.
  6. 6.0 6.1 Mayor, M.; Queloz, D. (1995). "A Jupiter-mass companion to a solar-type star". Nature 378 (6555): 355-359. doi:10.1038/378355a0. Bibcode1995Natur.378..355M.  arXiv:
  7. 51 Pegasi and Bellerophon. jumk.de. Retrieved on 2019-07-25.
  8. Noble, M.; Musielak, Z. E.; Cuntz, M. (2001). "Orbital Stability of Earth-like Planets in Stellar Habitable Zones". American Astronomical Society 331: 1304. Bibcode2001AAS...199.0310N. 

External links