PlanetStar Wiki
This is what Bellerophon may look like
Date October 6, 1995
Discoverers Mayor and Queloz et al.
Detection method Radial velocity
Site Observatoire de Haute-Provence in France
Name & designations
Pronunciation /'bel•ər•ōf•on
Adjective Bellerophian
Planet numbers P13, 51 Pegasi P1,
Pegasus P1, Testudo P1,
1995 P1, 1995 Peg-1,
1995 Tes-1
Star designations g Pegasi b, 51 Pegasi b,
BF 3182 b, PH 4 b,
336 Testudi b, P1 Pegasi b,
P1 Testudi b, HD 217014 b,
HIP 113357 b, HR 8729 b,
Gliese 882 b, SAO 90896 b
System 51 Pegasi
Constellation Pegasus
Caelregio Testudo
Right ascension 22h 57m 27.98s (344.366 59°)
Declination +20° 46' 07.8" (+20.768 83°)
Distance 15.608 pc (50.907 ly)
Orbital characteristics
Semimajor axis 0.053 016 AU (7.931 0 Gm)
Periastron 0.052 316 AU (7.826 4 Gm)
Apastron 0.053 715 AU (8.035 6 Gm)
Eccentricity 0.013 191 8
Orbital circumference 0.333 10 AU (49.830 Gm)
Orbital area 0.008 829 AU² (197.59 Gm²)
Orbital period 4.230 768 d (0.011 583 21 yr)
Avg. velocity 136.783 km/s (28.757 AU/yr)
Max. velocity 137.682 km/s (29.215 AU/yr)
Min. velocity 135.878 km/s (28.291 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 18.081° to ecliptic
26.538° to star's equator,
11.847° to invariable plane
Argument of periastron 352.420°
Longitude of ascending node 132.659°
Longitude of periastron 125.079°
Angular separation 3.397 mas
Observing the parent star
Mean angular star size 11.808 04° (708.483')
Max. angular star size 11.965 90° (717.954')
Min. angular star size 11.654 30° (699.258')
Mean star magnitude −33.431
Max. star magnitude −33.460
Min. star magnitude −33.403
Bulk characteristics
Mean radius 1.382 23 RJ (96.677 Mm)
Equatorial radius 1.406 92 EJ (100.524 Mm)
Polar radius 1.329 71 PJ (88.982 Mm)
Mean circumference 607.437 Mm
Equatorial circumference 631.612 Mm
Polar circumference 559.087 Mm
Surface area 1.906 15 SJ (117 070 Mm²)
Volume 2.631 69 VJ (3.766 4 × 106 Mm³)
Flattening 0.119 39 (1:8.375)
Angular diameter 82.809 μas
Mass 1.521 716 MJ
Reciprocal mass
relative to star
Density 0.767 g/cm³
Gravitational influence
Surface gravity 2.107 g (20.67 m/s²)
Weight on Bellerophon
(150 lb on Earth)
316 lb
Standard gravitational parameter 1.928 × 106 km³/s²
Escape velocity 63.15 km/s
Hill radius 1.606 LD (0.617 2 Gm)
Roche limit
(3 g/cm3 satellite)
0.201 13 LD (77.313 Mm)
Stationary orbit 1.997 28 LD (767.753 Mm)
Stationary velocity 14.925 km/s (3.355 LD/d)
Rotation characteristics
Rotation period 101.538 4 h (4.230 768 d)
Rotation velocity 1.728 km/s (3.55°/h)
Direction of rotation
relative to orbit
Axial tilt 42.100°
Longitude of vernal equinox 295.645°
North pole right ascension 15h 18m 54s (226.260°)
North pole declination +26° 03' 22" (+26.056°)
North polar constellation Corona Borealis
North polar caelregio Noctua
South pole right ascension 03h 18m 54s (46.260°)
South pole declination −26° 03' 22" (−26.056°)
South polar constellation Fornax
South polar caelregio Solarium
Thermal characteristics
Surface temperature 1240 K (967°C, 1773°F,
Mean irradiance 672 192 W/m² (491.512 I)
Irradiance at periastron 690 284 W/m² (504.741 I)
Irradiance at apastron 654 802 W/m² (478.797 I)
Albedo 0.064 (bond), 0.037 (geom.)
Scale height 283.8 km
Surface density 0.043 g/m³
Molar mass 2.73 g/mol
Composition 67.148% Hydrogen (H2)
32.598% Helium (He)
2.33‰ Carbon monoxide (CO)
206 ppm Water (H2O)
74.1 ppm Sodium (Na)
33.7 ppm Potassium (K)
1.03 ppm Hydrogen sulfide (H2S)
310 ppb Xenon (Xe)
277 ppb Neon (Ne)
Sodium (Na)
Potassium (K)
Silicates (SiO44−)
Dipole strength 0.379 μT (3.79 mG)
Magnetic moment 3.87 × 1017 T•m³
Dipole tilt 55.03°
Satellite system
Number of moons 0
Number of rings 0

Bellerophon (51 Pegasi b, P13) is a planet that orbits the star 51 Pegasi, located in the constellation Pegasus in the caelregio Testudo 51 light-years away from us. The parent star is similar to our Sun but it is a little more massive, bigger, brighter, and cooler. It is notable for being the first planet discovered around an ordinary star, and is the first hot Jupiter found.

Bellerophon was named after the Greek hero Bellerophon who tamed Pegasus (the Winged Horse).


Prior to discovery[]

In 1992, astronomers discovered and confirmed the existence of planets beyond our solar system for the first time, two orbiting around the pulsar PSR B1257+12. It was a surprising discovery because astronomers thought that every planet would be destroyed by the supernova that created the pulsar. After that unexpected discovery, astronomers scrambled to start looking for planets around main sequence stars. In November 1992, telescopes began to monitor the sky for any star wobbling caused by gravitational tug of planets.


On October 6, 1995 Michel Mayor and Didier Queloz found a signal of a planet around 51 Pegasi, using the radial velocity method at the Observatoire de Haute-Provence with the ELODIE spectrograph. The team determined that the planet takes only as little as four days to orbit the star and determined the mass at half that of Jupiter's. Planets this massive are gas giants. It challenges the theory of planet formation that gas giants can only exist beyond the snow line and thus cannot form this close to the star, so the team believed that this newly discovered planet is a huge lava planet as a class of terrestrial planet. Even the team convinced that this such planet exists, the discovery status was listed unconfirmed until another team (led by Geoff Marcy and Paul Butler) found the same signal six days later. This became the first confirmed planet around an ordinary star. It is now believe that this planet is a gas giant as there are not enough rocky material to form a rocky planet with this much mass.

Bellerophon turned out to be the only planet discovered in the year 1995 as the next planet (Clio (47 Ursae Majoris b, P14)) was discovered on January 17, 1996.


Since the discovery of Bellerophon, many similar planets have been found around other stars, many of them by transit, a method uses to look for slight dimming of the stars caused by planets passing infront of it, unlike the method used to discover this planet. In 2001, the transit of Osiris (HD 209458 b, P30) has been found after first discovering this planet using the wobble method. It even saw a comet-like tail of hydrogen-helium gases trailing the planet as they are boiling off the atmosphere, guarenteeing the existence of hot Jupiters. It forces astronomers to revise the model of planetary formation to include planetary migration that gas giants must've form beyond the snow line and then migrated towards the star caused by interactions with the protoplanetary disk.

Since Bellerophon is the first hot Jupiter found, Bellerophian planet (after Bellerophon) or Pegasean planet (after 51 Pegasi) can be used as synonyms of hot Jupiter.

Orbit and rotation[]


Bellerophon takes only 4.23 days to orbit around 51 Pegasi, meaning that the planet orbits in a very close, torch orbit around the parent star. Every time when Earth orbits the Sun once, Bellerophon completes about 86 revolutions around its star. It orbits at an average distance of only 8 gigameters, compared to 150 gigameters for Earth's orbit. Bellerophon orbits so close to the star that light emanating from the parent star takes just 26 seconds to reach the planet, in comparison, light from our Sun takes over 8 minutes to reach Earth. Like most other subsequent hot Jupiters and most of the Solar System planets, Bellerophon orbits in a circular path with an eccentricity of 0.013, slightly less than Earth's. The planet moves about 137 km/s (85 mi/s), 412 times faster than the orbital movement of our home planet.

This planet orbits in the range named after this planet itself, called Bellerophian orbit (B orbit). Planets with semimajor axes less than 0.1 AU are said to be in the B orbit. Hot Jupiters almost exclusively mean Jupiter-like planets in B orbit.

Inclination and early history[]

Bellerophon is almost face-on as the planet tilts 18° to our line of sight. However, a recent study suggests that the parent star rotates at an angle of 79° to our line of sight, meaning that Bellerophon should orbit within few degrees relative to star's rotation. Another study based on Kepler data indicate that over 85% of all planets orbit within 3° relative to star's rotation. Bellerophon's orbit would tilt 61° relative to star's rotation assuming a speculated inclination of 18°, meaning that early in its history, the planet witnessed dynamical interactions with protoplanets that increased the inclination dramatically during the migration. After Bellerophon formed between 3–4 AU from the star, the eccentricity cranked up to between 0.7–0.8 due to gravitation influences of massive giant planets close behind. Bellerophon's periastron distance was only 20–30% of the semimajor axis, at around 1 AU. This disrupted the terrestrial planet formation in the inner regions and the dynamical interactions with planetesimals started the migration as it underwent Kozai mechanism, which means trading high eccentricity and low inclination for low eccentricity and high inclination. This mechanism ends on the opposite end to what it began when the migration halted at present orbit. The planet began migrating a hundred million years after it formed. That's why there are no terrestrial planets around 51 Pegasi because of Bellerophon that otherwise there would of been.

Parent star observation and irradiance[]

Viewed from Bellerophon, its sun would appear 476 times brighter than the Sun seen from Earth because the planet orbits much closer to the light source than Earth is, and the planet receives 492 times more energy from 51 Pegasi than Earth receives from the Sun. From Bellerophon, 51 Pegasi would have an apparent magnitude of −33.43, compared to −26.74 for the Sun seen from Earth. Viewed from our homeworld, 51 Peg has a magnitude of just 5.49, while the magnitude of our home star viewed from Bellerophon is 5.80. In additon to appearing much brighter, its sun would also appear much bigger than ours. The parent star viewed from Bellerophon would appear to be 22 times bigger than our parent star seen from Earth. From that exoplanet, its apparent stellar diameter is 11.81 arcminutes compared to just 0.54 arcminutes from our home planet.


Bellerophon is tidally locked, meaning that every time the planet orbits the star, it rotates exactly once. This means that one side always faces the star while the other is in perpetual darkness and never sees starlight. However, Bellerophon's axis is inclined to the plain of its orbit, roughly 42.1°, and the longitude of vernal equinox is 295.6°. It inclines so steeply that not every areas on one face would perpetually be day or night. North of 48°N latitude on the nightside, it is a perpetual day, conversely south of 48°S latitude on the day side, it is a perpetual night.

Bellerophon's north pole points to the constellation Corona Borealis while its south pole points to the constellation Fornax.

Structure and composition[]

Mass and size[]

Bellerophon has a determined mass of 0.472 MJ, but it is the lower limit. The speculated mass, taking speculated inclination into account, is 1.522 MJ, or 2.889 wekagrams. Its radius is 95,677 kilometers (60,072 miles), which is 38% bigger than Jupiter, and seen from Earth, Bellerophon would have an apparent diameter of 82.8 μpc. However, the planet would be smaller if it orbits further away from the star. Calculating between mass and radius, it yields that the density is similar to Saturn's in our solar system. Like Saturn, Bellerophon would float on a tub of water if it is big enough.

Bellerophon has a flattening (ellipticity) of 0.119, higher than any planet of our solar system, meaning that its equatorial diameter is merely 11,542 kilometers (12%) greater than its polar diameter.

Gravitational influence[]

Despite the density is 17 that of Earth's, its surface gravity is more than twice as strong. If you weigh 150 pounds on Earth, you would weigh 316 pounds on Bellerophon.

Its hill sphere is very small because Bellerophon orbits so close to the star that the gravitational influence is so strong. The stationary orbit is outside the hill sphere so that orbit is unstable. The stationary orbit lies 0.391 lunar distances beyond the hill sphere. The tidal forces of the planet would tear up satellites if it orbits too close to the planet (within the roche limit). The roche radius is 18 of the hill radius. Unlike the hill radius, stationary radius and roche radius does not depend on the planet's distance from the parent star.


Bellerophon is a gas giant with no solid surface, meaning that this planet does not have a crust like Earth and other terrestrial planets. Even though this planet is a hot Jupiter, Bellerophon has an interior structure similar to Jupiter since it is within the same class of mass. This planet has a mantle made of liquid hydrogen and helium mixture with metallic hydrogen deeper down. At its center lies a core of rock and metal with a mass of 5.38 Earth masses, roughly 1.2% of the total planet's mass.


Bellerophon's atmosphere is comprised mainly of hydrogen and helium, like all other gas giants including all four in our solar system. It contains trace amounts of carbon monoxide, water vapor, sodium and potassium vapors, and silicates. The atmospheric temperature at 1-bar level is 1240 K (967°C, 1773°F), hot enough for clouds made off alkali metals to form with silicate clouds just below it. Because alkali metal clouds are dark, the planet's albedo is really low, reflecting only 6% of the radiation falling to it while reflecting just 4% of its sunlight. It is also hot enough for atmosphere to glow faintly red.

Wind and storms[]

Because the planet is tidally locked causing large temperature differences between dayside and nightside, there would be supersonic winds blowing at around 1 mi/s, (1.5 km/s) (3000–4000 mph), transferring heat from the dayside to the nightside in the process, reducing the temperature contrast between both hemispheres. Bellerophon has numerous storm features with each feature having storms far more violent than any of the solar system planets because of the extremely high temperature. Lightning on Bellerophon are up to about 300,000 times more powerful than Earth's and over 300 times more powerful than lightning on Jupiter and Saturn.

Bellerophon receives almost 500 times more energy from its star than Earth receives from the Sun, because the planet orbits nearly 20 times closer to the star than Earth is to the Sun that is 40% more luminous than our Sun. So unlike Solar System gas giants, Bellerophon and other hot Jupiters receive more energy from their parent star than is emitted from their interior. Storms on hot Jupiters including this planet are fueled almost exclusively by stellar energy unless for planets at least five times more massive than Jupiter.

Magnetic field[]

Bellerophon has a tedious magnetic field because the planet rotates so slowly caused by tidal forces from the nearby parent star. A slow rotation means that the metallic hydrogen in the lower mantle does not churn much and does not conduct electricity as readily as gas giants orbiting further away from their parent stars. The magnetic dipole tilts 55° with respect to rotational poles. Planets with high axial tilts tend to have high dipole tilts of a similar degree because stellar winds align the dipole to face the incoming winds more directly.

Moons and rings[]

Bellerophon could have moons around it, but it orbits too close to the star to have numerous moons like solar system's gas giants. Despite the hill radius being 1.6 lunar distances (0.6 gigameters), Bellerophon has no moons, but the planet once had moons, but they escaped planet's grip caused by the exchange of tidal forces between the planet and the star. With no moons means that planet doesn't have rings like all four gas giants around the Sun.

Future studies[]

Bellerophon would be a very interesting planet to study its nature since it is the planet of firsts. Astronomers were monitoring for transits of this planet by watching if 51 Pegasi dims once every four days, but no evidence for dimming, meaning that the transit of Bellerophon is now ruled out. This makes the study of this hot Jupiter more far-fetch. Another way to study is to use reflecting light using near-infrared or polarimetry like it has been done on two other hot Jupiters, Arcas (Tau Boötis b) and Heleus (Upsilon Andromedae b). This method enable to study the atmosphere by studying the starlight coming off the planet. Direct imaging of Bellerophon is virtually impossible as it orbits much too close to the glaring effects of the star, but ATLAST (due to launch between 2025–35) might be able to image this planet. Astrometry can be used to constrain the actual inclination, and thus its real mass.

Planets beyond Bellerophon[]

After discovering Bellerophon, astronomers wondered if there are other planets around 51 Pegasi. Astronomers are monitoring the parent star for more planets but so far no signal was detected. However, I speculate that there are two undetected planets orbiting far beyond Bellerophon in the 2:1 resonance, designated 51 Pegasi c and 51 Pegasi d. These planets have orbital properties similar to Jupiter and Saturn. Their orbital periods are 13 and 26 years, and orbit at average distances of 5.71 and 9.06 AU, respectively. Both planets are more massive, but both considerably smaller than Bellorophon. The inner masses 4.26 MJ while the outer masses 2.03 MJ. The prospects for detecting these planets are good. The present wobble method should be able to find both planets, but 51 Peg c is lot easier because it is more massive, orbits closer to the star, and require less observation than 51 Peg d. Gaia (currently in testing phase) may possibly discover these two planets using the astrometry.