51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Sun-like star located from Earth in the constellation of Pegasus. It was the first

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar ...

star found to have an

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

(designated

51 Pegasi b 51 Pegasi b, officially named Dimidium , and formerly unofficially dubbed Bellerophon , is an extrasolar planet approximately away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the ...

, officially named Dimidium, formerly unofficially dubbed ''

Bellerophon Bellerophon (; Ancient Greek: Βελλεροφῶν) or Bellerophontes (), born as Hipponous, was a hero of Greek mythology. He was "the greatest hero and slayer of monsters, alongside Cadmus and Perseus, before the days of Heracles", and h ...

'') orbiting it.

Properties

The star's

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

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, and even in more modern catalogues it is usually listed as a

star. 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. The

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of the chromosphere is about , giving 51 Pegasi the characteristic yellow hue of a G-type star. It is estimated to be 6.1 – 8.1 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 planets. In 1996, astronomers Baliunas, Sokoloff, and Soon measured a rotational period of 37 days for 51 Pegasi. Although the star was suspected of being variable during a 1981 study, 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 during which a star produces a reduced number of star spots. The star rotates at an inclination of 79 degrees relative to Earth.

Nomenclature

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 (

was a figure from

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities o ...

who rode the winged horse Pegasus). In July 2014, the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the names of Helvetios for this star and Dimidium for its planet. The names were those submitted by the Astronomische Gesellschaft Luzern,

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

. "Helvetios" is

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

for "the

Helveti The Helvetii ( , Gaulish: *''Heluētī''), anglicized as Helvetians, were a Celtic tribe or tribal confederation occupying most of the Swiss plateau at the time of their contact with the Roman Republic in the 1st century BC. According to Juliu ...

an" and refers to the

Celtic Celtic, Celtics or Keltic may refer to: Language and ethnicity *pertaining to Celts, a collection of Indo-European peoples in Europe and Anatolia **Celts (modern) *Celtic languages **Proto-Celtic language *Celtic music *Celtic nations Sports Foo ...

tribe that lived in Switzerland during antiquity; 'Dimidium' is

for 'half', referring to the planet's mass of at least half the mass of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

. In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. In its first bulletin of July 2016, 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.

Planetary system

Artist impression of the exoplanet 51 Pegasi b

On October 6, 1995,

Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places * Swiss, Missouri *Swiss, North Carolina * Swiss, West Virginia *Swiss, Wisconsin Other uses * Swiss-system tournament, in various games and sports * Swiss Internation ...

astronomers

Michel Mayor Michel Gustave Édouard Mayor (; born 12 January 1942) is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory ...

and Didier Queloz announced the discovery of an

orbiting 51 Pegasi. The discovery was made at Observatoire de Haute-Provence in

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

. On 8 October 2019, Mayor and Queloz shared the

Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

for their discovery.

(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 and has a mass at least half that of

. At the time of its discovery, this close distance was not compatible with theories of

planet formation The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...

and resulted in discussions of planetary migration. However, several

hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere t ...

s are now known to be oblique relative to the stellar axis.

References

External links

*
51 Pegasi
at SolStation.com.
nStars database entry

{{Sky, 22, 57, 28.0, +, 20, 46, 08, 50.1 G-type main-sequence stars Pegasi, 51 Maunder Minimum Planetary systems with one confirmed planet Pegasus (constellation) BD+19 5036 Pegasi, 51 0882 217014 113357 8729

Properties

Nomenclature

Planetary system

See also

References

External links