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51 Pegasi

Coordinates: Sky map 22h 57m 28.0s, +20° 46′ 08″
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51 Pegasi / Helvetios
51 Pegasi is located in 100x100
51 Pegasi

51 Peg (circled) in the constellation Pegasus.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pegasus
Right ascension 22h 57m 27.9805s[1]
Declination +20° 46′ 07.797″[1]
Apparent magnitude (V) 5.49[2]
Spectral type G2IV[3]
Apparent magnitude (B) 6.16[4]
Apparent magnitude (R) 5.0[4]
Apparent magnitude (I) 4.7[4]
Apparent magnitude (J) 4.66[4]
Apparent magnitude (H) 4.23[4]
Apparent magnitude (K) 3.91[2]
U−B color index +0.20[5]
B−V color index +0.67[5]
Radial velocity (Rv)−33.33[1] km/s
Proper motion (μ) RA: +207.328[1] mas/yr
Dec.: +61.164[1] mas/yr
Parallax (π)64.4048 ± 0.0543 mas[1]
Distance50.64 ± 0.04 ly
(15.53 ± 0.01 pc)
Absolute magnitude (MV)4.48[6]
Mass1.12 ± 0.06 [7] M
Radius1.237 ± 0.047[2] R
Luminosity1.36±0.02[6] L
Surface gravity (log g)4.33[8] cgs
Temperature5,768±8[6] K
Metallicity [Fe/H]+0.20[8] dex
Rotation21.9±0.4 days[9]
Rotational velocity (v sin i)5.6[10] km/s
Age6.1±0.6[6] Gyr
Other designations
Helvetios, 51 Peg, GJ 882, HR 8729, BD+19°5036, HD 217014, LTT 16750, GCTP 5568.00, SAO 90896, HIP 113357[11]
Database references
Exoplanet Archivedata

51 Pegasi (abbreviated 51 Peg), formally named Helvetios /hɛlˈvʃiəs/,[12] is a Sun-like star located 50.6 light-years (15.5 parsecs) from Earth in the constellation of Pegasus. It was the first main-sequence star found to have an exoplanet (designated 51 Pegasi b, officially named Dimidium) orbiting it.[13]


51 Pegasi

The star's apparent magnitude is 5.49, making it visible with the naked eye under suitable viewing conditions.

51 Pegasi was listed as a standard star for the spectral type G2IV in the 1989 The Perkins catalog of revised MK types for the cooler stars. Historically, it was generally given a stellar classification of G5V,[14] and even in more modern catalogues it is usually listed as a main-sequence star.[15] It is generally considered to still be generating energy through the thermonuclear fusion of hydrogen at its core, but to be in a more evolved state than the Sun.[6] The effective temperature of the chromosphere is about 5,571 K (5,298 °C; 9,568 °F), giving 51 Pegasi the characteristic yellow hue of a G-type star.[16] It is estimated to be about 6.1 billion years old, somewhat older than the Sun, with a radius 24% larger and 11% more massive. The star has a higher proportion of elements other than hydrogen/helium compared to the Sun; a quantity astronomers term a star's metallicity. Stars with higher metallicity such as this are more likely to host giant planets.[17] In 1996, astronomers Baliunas, Sokoloff, and Soon measured a rotational period of 37 days for 51 Pegasi.[18]

Although the star was suspected of being variable during a 1981 study,[19] subsequent observation showed there was almost no chromospheric activity between 1977 and 1989. Further examination between 1994 and 2007 showed a similar low or flat level of activity. This, along with its relatively low X-ray emission, suggests that the star may be in a Maunder minimum period[14] during which a star produces a reduced number of star spots.

The star rotates at an inclination of 79+11
degrees relative to Earth.[9]


51 Pegasi is the Flamsteed designation. On its discovery, the star's planet — and actually the first exoplanet discovered around a main-sequence star — was designated 51 Pegasi b by its discoverers and unofficially dubbed Bellerophon, in keeping with the convention of naming planets after Greek and Roman mythological figures (Bellerophon was a figure from Greek mythology who rode the winged horse Pegasus).[20]

In July 2014, the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[21] The process involved public nomination and voting for the new names.[22] In December 2015, the IAU announced the names of Helvetios for this star and Dimidium for its planet.[23]

The names were those submitted by the Astronomische Gesellschaft Luzern, Switzerland. "Helvetios" is Latin for "the Helvetian" and refers to the Celtic tribe that lived in Switzerland during antiquity; 'Dimidium' is Latin for 'half', referring to the planet's mass of at least half the mass of Jupiter.[24]

In 2016, the IAU organized a Working Group on Star Names (WGSN)[25] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[26] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[12]

Planetary system[edit]

The 51 Pegasi planetary system (shown to scale)

On October 6, 1995, Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of an exoplanet orbiting 51 Pegasi.[13] The discovery was made at Observatoire de Haute-Provence in France. On 8 October 2019, Mayor and Queloz shared the Nobel Prize in Physics for their discovery.[27]

51 Pegasi b (51 Peg b) was the first discovered planetary-mass companion of a main-sequence parent star. It orbits very close to the star, experiences estimated temperatures around 1,200 °C (1,500 K; 2,200 °F) and has a mass at least half that of Jupiter. At the time of its discovery, this close distance was not compatible with theories of planet formation and resulted in discussions of planetary migration.[28] However, several hot Jupiters are now known to be oblique relative to the stellar axis.[29]

The 51 Pegasi planetary system[30]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (Dimidium) ≥ 0.472 ± 0.039 MJ 0.0527 ± 0.0030 4.230785 ± 0.000036 0.013 ± 0.012

See also[edit]


  1. ^ a b c d e f Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c van Belle, Gerard T.; von Braun, Kaspar (2009). "Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars". The Astrophysical Journal (abstract). 694 (2): 1085–1098. arXiv:0901.1206. Bibcode:2009ApJ...694.1085V. doi:10.1088/0004-637X/694/2/1085. S2CID 18370219.
  3. ^ Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". The Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. ^ a b c d e Monet, David G.; et al. (February 2003). "The USNO-B Catalog". The Astronomical Journal. 125 (2): 984–993. arXiv:astro-ph/0210694. Bibcode:2003AJ....125..984M. doi:10.1086/345888. S2CID 55896673.
  5. ^ a b Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99. Bibcode:1966CoLPL...4...99J.
  6. ^ a b c d e Mittag, M.; Schröder, K.-P.; Hempelmann, A.; González-Pérez, J. N.; Schmitt, J. H. M. M. (2016). "Chromospheric activity and evolutionary age of the Sun and four solar twins". Astronomy & Astrophysics. 591: A89. arXiv:1607.01279. Bibcode:2016A&A...591A..89M. doi:10.1051/0004-6361/201527542. S2CID 54765864.
  7. ^ Fuhrmann, K.; et al. (October 1997). "Solar-type stars with planetary companions: 51 Pegasi and 47 Ursae Majoris". Astronomy and Astrophysics. 326: 1081–1089. Bibcode:1997A&A...326.1081F.
  8. ^ a b Frasca, A.; et al. (December 2009). "REM near-IR and optical photometric monitoring of pre-main sequence stars in Orion. Rotation periods and starspot parameters". Astronomy and Astrophysics. 508 (3): 1313–1330. arXiv:0911.0760. Bibcode:2009A&A...508.1313F. doi:10.1051/0004-6361/200913327. S2CID 118361131.
  9. ^ a b Simpson, E. K.; et al. (November 2010). "Rotation periods of exoplanet host stars". Monthly Notices of the Royal Astronomical Society. 408 (3): 1666–1679. arXiv:1006.4121. Bibcode:2010MNRAS.408.1666S. doi:10.1111/j.1365-2966.2010.17230.x. S2CID 6708869. [as "HD 217014"]
  10. ^ Luck, R. Earle (January 2017). "Abundances in the Local Region II: F, G, and K Dwarfs and Subgiants". The Astronomical Journal. 153 (1): 19. arXiv:1611.02897. Bibcode:2017AJ....153...21L. doi:10.3847/1538-3881/153/1/21. S2CID 119511744. 21.
  11. ^ "51 Peg – Star suspected of Variability". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2011-12-17.
  12. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  13. ^ a b Mayor, Michael; Queloz, Didier (1995). "A Jupiter-mass companion to a solar-type star". Nature. 378 (6555): 355–359. Bibcode:1995Natur.378..355M. doi:10.1038/378355a0. S2CID 4339201.
  14. ^ a b Poppenhäger, K.; et al. (December 2009). "51 Pegasi – a planet-bearing Maunder minimum candidate". Astronomy and Astrophysics. 508 (3): 1417–1421. arXiv:0911.4862. Bibcode:2009A&A...508.1417P. doi:10.1051/0004-6361/200912945. S2CID 118626420.
  15. ^ Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009–2016)". VizieR On-line Data Catalog. Bibcode:2014yCat....1.2023S.
  16. ^ "The Colour of Stars". Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Archived from the original on March 18, 2012. Retrieved 2012-01-16.
  17. ^ Buchhave, Lars A.; Latham, David W.; Johansen, Anders; Bizzarro, Martin; Torres, Guillermo; Rowe, Jason F.; Batalha, Natalie M.; Borucki, William J.; Brugamyer, Erik; Caldwell, Caroline; Bryson, Stephen T.; Ciardi, David R.; Cochran, William D.; Endl, Michael; Esquerdo, Gilbert A. (June 2012). "An abundance of small exoplanets around stars with a wide range of metallicities". Nature. 486 (7403): 375–377. Bibcode:2012Natur.486..375B. doi:10.1038/nature11121. ISSN 1476-4687. PMID 22722196. S2CID 4427321.
  18. ^ Baliunas, Sallie; Sokoloff, Dmitry; Soon, Willie (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. Bibcode:1996ApJ...457L..99B. doi:10.1086/309891.
  19. ^ Kukarkin, B. V.; et al. (1981). "Nachrichtenblatt der Vereinigung der Sternfreunde e.V. (Catalogue of suspected variable stars)". Nachrichtenblatt der Vereinigung der Sternfreunde: 0. Bibcode:1981NVS...C......0K.
  20. ^ "University of California at Berkeley News Release". 1996-11-17. Retrieved 2018-09-27.
  21. ^ "NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars]" (Press release). IAU. 9 July 2014. Retrieved 2018-09-27.
  22. ^ "NameExoWorlds The Process". Archived from the original on 2015-08-15. Retrieved 2018-09-27.
  23. ^ "Final Results of NameExoWorlds Public Vote Released]" (Press release). International Astronomical Union. 15 December 2015. Retrieved 2018-09-27.
  24. ^ "NameExoWorlds The Approved Names". Archived from the original on 2018-02-01. Retrieved 2018-09-27.
  25. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  26. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  27. ^ "Nobel prize for physics: exoplanets and cosmology". The Economist. 2019-10-08. ISSN 0013-0613. Retrieved 2019-10-09.
  28. ^ "51_peg_b". Extrasolar Planets Encyclopaedia. 1995. Retrieved November 12, 2012.
  29. ^ Roberto Sanchis-Ojeda; Josh N. Winn; Daniel C. Fabrycky (2012). "Starspots and spin-orbit alignment for Kepler cool host stars". Astronomische Nachrichten. 334 (1–2): 180. arXiv:1211.2002. Bibcode:2013AN....334..180S. CiteSeerX doi:10.1002/asna.201211765. S2CID 38743202.
  30. ^ Butler, R. P.; et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal. 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode:2006ApJ...646..505B. doi:10.1086/504701. S2CID 119067572.

External links[edit]