 An artist's impression of 51 Pegasi b (left) and its star (right). | |
| Discovery | |
|---|---|
| Discovered by | Michel Mayor and Didier Queloz |
| Discovery site | OHP, France |
| Discovery date | 6 October 1995; 30 years ago (6 October 1995) |
| Radial velocity (ELODIE) | |
| Designations | |
| Dimidium | |
| Orbital characteristics | |
| Aphelion | 0.0534 AU (7,990,000 km) |
| Perihelion | 0.0520 AU (7,780,000 km) |
| 0.0527 ± 0.0030 AU (7,880,000 ± 450,000 km) | |
| Eccentricity | 0.013 ± 0.012 |
| 4.230785 ± 0.000036d 101.5388h | |
Averageorbital speed | 136km/s |
| Star | 51 Pegasi |
| Physical characteristics | |
| 1.2±0.1 RJ[1] | |
| Mass | 0.46±0.02 MJ[2] |
Meandensity | 0.35+0.13 −0.08 g/cm3[3] |
| 7.9+1.9 −1.4 m/s2[4] | |
| 6.9+0.5 −0.4 km/s[5] | |
| Temperature | 1,250K[2] |
51 Pegasi b, officially namedDimidium (/dɪˈmɪdiəm/), is anextrasolar planet approximately 50light-years (15parsecs) away in theconstellation ofPegasus. It was the firstexoplanet to be discoveredorbiting amain-sequence star,[6] theSun-like51 Pegasi, and marked a breakthrough in astronomical research. It is theprototype for a class of planets calledhot Jupiters.[7]
In 2017, traces of water were discovered in the planet'satmosphere.[8] In 2019, theNobel Prize in Physics was awarded in part for the discovery of 51 Pegasi b.[9]
51 Pegasi is theFlamsteed designation of the host star. The planet was originally designated 51 Pegasi b byMichel Mayor andDidier Queloz, who discovered the planet in 1995. The following year it was unofficially dubbed "Bellerophon"/bɛˈlɛrəfɒn/ by astronomerGeoffrey Marcy, who followed theconvention of naming planets afterGreek andRoman mythological figures (Bellerophon is a figure from Greek mythology who rode the winged horsePegasus).[10]
In July 2014, theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certainexoplanets and their host stars.[11] The process involved public nomination and voting for the new names.[12] In December 2015, theIAU announced the winning name for thisplanet was Dimidium.[13] The name was submitted by theAstronomische Gesellschaft Luzern (German for 'Astronomical Society of Lucerne'),Switzerland. 'Dimidium' isLatin for 'half', referring to the planet'smass of approximately half the mass ofJupiter.[14]
Theexoplanet's discovery was announced on October 6, 1995, byMichel Mayor andDidier Queloz of theUniversity of Geneva in the journalNature.[15] They used theradial velocity method with theELODIE spectrograph on theObservatoire de Haute-Provence telescope in France and made world headlines with their announcement. For this discovery, they were awarded the 2019Nobel Prize in Physics.[9]
The planet was discovered using a sensitivespectroscope that could detect the slight and regularvelocity changes in the star'sspectral lines of around 70 metres per second. These changes are caused by the planet'sgravitational effects from just 7 million kilometres' distance from the star.
Within a week of the announcement, the planet was confirmed by another team using theLick Observatory inCalifornia.[16]
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After its discovery, many teams confirmed the planet's existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around four days. It is much closer to it thanMercury is to the Sun,[6] moves at anorbital speed of 136 km/s (300,000 mph), yet has a minimum mass about half that of Jupiter (about 150 times that of theEarth). At the time, the presence of a huge world so close to its star was not compatible with theories ofplanet formation and was considered an anomaly. However, since then, numerous other "hot Jupiters" have been discovered[6] (such as those orbiting55 Cancri andτ Boötis), and astronomers are revising their theories of planet formation to account for them by studyingorbital migration.[7]
Assuming the planet is perfectly grey with no greenhouse or tidal effects, and aBond albedo of 0.1, the temperature would be 1,265 K (992 °C; 1,817 °F). This is between the predicted temperatures ofHD 189733 b andHD 209458 b (1,180 K (910 °C; 1,660 °F)–1,392 K (1,119 °C; 2,046 °F)), before they were measured.[17]
In the report of the discovery, it was initially speculated that 51 Pegasi b was the stripped core of a brown dwarf of a decomposed star and was therefore composed of heavy elements, but it is now believed to be agas giant. It is sufficiently massive that its thick atmosphere is not blown away by the star'ssolar wind.
51 Pegasi b probably has a greaterradius than that of Jupiter despite its lower mass. This is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it. Beneath this, the gases that make up the planet would be so hot that the planet would glow red. Clouds ofsilicates may exist in the atmosphere.
The planet istidally locked to its star, always presenting the same face to it.
The planet (withUpsilon Andromedae b) was deemed a candidate for aperturepolarimetry byPlanetpol.[18] It is also a candidate for "near-infrared characterisation.... with theVLTI Spectro-Imager".[17]
A 2015 study alleged the detection of 51 Pegasi b in thevisible light spectrum using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory'sLa Silla Observatory in Chile.[19] This detection, if confirmed, would allow the inference of a true mass of 0.46 Jupiter masses. The findings also could suggest a high albedo for the planet, hence a large radius up to1.9±0.3 Jupiter radii, which could suggest 51 Pegasi b is an inflatedhot Jupiter.[20] The optical detection could not be replicated in 2020, implying the planet has analbedo below 0.15.[21] Measurements in 2021 have marginally detected a polarized reflected light signal, which, while they cannot place limits on the albedo without assumptions made about the scattering mechanisms, could suggest a high albedo.[22]
More recent studies found no evidence of reflected light, ruling out the previous radii and albedo estimates from previous studies. Instead, 51 Pegasi b is likely a low-albedo planet with a radius around1.2±0.1 RJ.[1][23]
