51 Peg b

A gas giant

14.10 Earth radii, Jupiter-class (Jupiter is 11.2). At only 193.9 Earth masses for that size, it must be substantially gas or volatile ices: puffy, not rocky. Orbiting this close, it is a classic 'hot Jupiter', a giant that migrated inward from where it formed.

The orbit

Its year, one full orbit, takes just 4.2 Earth days.

Temperature and starlight

Its equilibrium temperature (the airless-world estimate) is about 1,056 °C. Hot enough to melt copper; any surface would glow. It receives 506.2 times the starlight Earth gets.

Its sun

Its star is Sun-like (5,761 K, G5V; the Sun is 5,772 K).

How far is that, really?

At 50 light-years, tonight's light from this system left it around the year 1975. A Voyager-speed probe would need roughly 892,208 years to get there.

How we found it

It was found by the radial-velocity method: the planet's gravity swings its star in a small circle, and that wobble shows up as a rhythmic Doppler shift in the starlight. The wobble's size gives the planet's minimum mass. The discovery was announced in 1995 from Haute-Provence Observatory.

Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.

01 How the radial velocity trick works

Mayor and Queloz didn't photograph 51 Pegasi b — they listened to its star wobble. Every planet tugs gravitationally on its host star, making the star sway ever so slightly toward and away from Earth. That motion compresses and stretches the wavelengths of starlight in a regular rhythm, the same Doppler shift that makes a passing ambulance siren rise then fall in pitch. The ELODIE spectrograph on the Observatoire de Haute-Provence telescope in France was sensitive enough to detect a velocity change of around 70 metres per second — roughly the jogging pace of a fast human. That tiny signal, repeating on a four-day cycle, was the gravitational fingerprint of a planet sitting just 7 million kilometres from its star. The technique gives only a minimum mass, because the true angle of the orbit relative to our line of sight is unknown. Confirmed within a single week by an independent team at Lick Observatory in California, the detection was essentially airtight from the start.

02 The instrument that changed astronomy Deeper

The hero of the discovery story is a piece of hardware: the ELODIE spectrograph, mounted on the telescope at Observatoire de Haute-Provence in southern France. Spectrographs split starlight into its component wavelengths so precisely that even minuscule Doppler shifts become measurable. ELODIE's sensitivity threshold — around 70 metres per second of stellar motion — turned out to be just sufficient to catch 51 Pegasi b, which induces exactly that kind of wobble because of its enormous mass and extreme proximity to its star. The observation chain ran from France across to a confirmation at Lick Observatory in California within days of the October 6, 1995 announcement in the journal Nature. Later, the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the European Southern Observatory's La Silla Observatory in Chile took over as the instrument of choice for follow-up work, including the contested 2015 attempt to detect the planet directly in visible light.

03 An orbital speed hard to picture

Because 51 Pegasi b sits so close to its star, it has to move extraordinarily fast to stay in orbit. Its orbital speed is 136 km/s — equivalent to 300,000 miles per hour. To put that in perspective, Earth orbits the Sun at about 30 km/s, meaning Dimidium is racing around its star more than four times faster. The orbit itself takes roughly four days to complete, compared with Earth's 365-day year or even Mercury's 88-day year. Mercury, the closest planet in our own solar system, is still far more distant from the Sun than 51 Pegasi b is from 51 Pegasi. Despite this blistering orbital pace and the intense radiation environment, the planet retains a minimum mass around half that of Jupiter — approximately 150 times Earth's mass — because it is sufficiently massive that its thick atmosphere is not stripped away by the star's solar wind.

04 A puffed-up giant that glows from within Deeper

The internal structure of 51 Pegasi b is almost certainly nothing like the cool, banded atmosphere of Jupiter. The planet's superheated atmosphere must be puffed outward into a thick but tenuous layer, giving it a radius that is probably greater than Jupiter's despite its lower mass. Beneath that bloated envelope, the gases would be so ferociously hot that the planet itself would glow red — essentially a self-luminous object radiating its own heat. Clouds of silicates may exist within the atmosphere, mineral grains that on Earth form rocks but here drift as vapor or cloud droplets at extreme temperatures. A 2015 HARPS study suggested a possible radius as large as 1.9 ± 0.3 Jupiter radii, which would make it an inflated hot Jupiter, but a 2022 study found no evidence for reflected light and instead placed the radius at around 1.2 ± 0.1 Jupiter radii. The true size of this world therefore remains genuinely uncertain.

05 One face forever toward the fire

51 Pegasi b is tidally locked to its star, meaning one hemisphere permanently faces the blazing heat of 51 Pegasi while the other hemisphere is in eternal darkness. The same gravitational mechanism locks our Moon so that we always see the same face from Earth, but for 51 Pegasi b the stakes are far more extreme. The daylit side bakes under relentless stellar radiation at temperatures the article pegs at 1,265 K — about 992 degrees Celsius — assuming a simple model with no greenhouse effect or tidal heating and a Bond albedo of 0.1. That figure falls between the predicted temperatures of two other famous hot Jupiters, HD 189733 b and HD 209458 b, which range from 1,180 K to 1,392 K. The permanent night side, by contrast, could be dramatically cooler, creating temperature gradients that would drive ferocious winds roaring around the planet's equator.

06 The albedo debate: bright world or dark? Deeper

How reflective is 51 Pegasi b? The answer has swung back and forth as successive studies have contradicted one another. The 2015 claim of a direct visible-light detection using HARPS hinted at a high albedo and a radius up to 1.9 ± 0.3 Jupiter radii — consistent with a brilliantly cloud-covered, inflated planet. Then in 2021, researchers failed to replicate the optical detection, implying an albedo below 0.15, meaning the planet absorbs the vast majority of starlight rather than reflecting it. A separate 2021 measurement did marginally detect a polarized reflected light signal, which could hint at high albedo, but the authors noted they could not place firm limits without assumptions about the scattering mechanisms involved. Finally, a 2022 study found no evidence of reflected light at all, ruling out the high-albedo, large-radius picture and pointing instead to a low-albedo planet with a radius around 1.2 ± 0.1 Jupiter radii. The question is still open.

07 Water in an alien sky

In 2017, traces of water were discovered in the atmosphere of 51 Pegasi b — a headline-grabbing finding for a planet whose surface conditions would instantly destroy any liquid water. The detection doesn't mean the planet is habitable in any sense. At temperatures around 1,265 K, water exists only as a dissociated vapor in the upper atmosphere rather than as liquid droplets or ice. Still, finding water molecules in the atmosphere of a gas giant 50 light-years away demonstrated the power of spectroscopic analysis and signaled that atmospheric chemistry of hot Jupiters was becoming accessible to observation. Alongside the possibility of silicate clouds drifting through the atmosphere, the water detection paints a picture of a chemically complex environment very different from anything in our own solar system, and it opens the door to more detailed characterizations of hot Jupiter atmospheres as telescope technology continues to improve.

08 A name chosen by public vote

For its first two decades, the planet went by two names simultaneously. Its formal scientific designation was 51 Pegasi b, assigned by discoverers Michel Mayor and Didier Queloz. Then in 1996, astronomer Geoffrey Marcy informally dubbed it Bellerophon, following the classical tradition of naming planets after Greek and Roman mythological figures. Bellerophon was the Greek hero famous for riding the winged horse Pegasus — a tidy link to the Pegasus constellation where the star sits. The official name Dimidium came through a different route entirely. In July 2014 the International Astronomical Union launched NameExoWorlds, a public nomination and voting process. In December 2015 the IAU announced the winner: Dimidium, submitted by the Astronomische Gesellschaft Luzern — the Astronomical Society of Lucerne, Switzerland. The Latin word dimidium means 'half,' a direct reference to the planet's mass of approximately half the mass of Jupiter.

09 Migration: how it got so close Deeper

When 51 Pegasi b was announced, its existence directly contradicted the prevailing model of planetary formation, which held that gas giants could only coalesce far from their stars where temperatures are cold enough for ices and other materials to accumulate. A giant planet practically kissing its star was considered an anomaly — and the article notes that the initial discovery report even speculated it might be the stripped core of a brown dwarf or a decomposed star made of heavy elements. That idea was abandoned as the scientific community accepted it as a gas giant. The resolution came through orbital migration theory: a gas giant can form far out, then gradually spiral inward as it loses orbital energy through interactions with the disk of gas and dust around the young star. Since 1995, numerous other hot Jupiters have been discovered around stars including 55 Cancri and τ Boötis, turning what seemed like a bizarre exception into an entire recognizable class of planets.

10 Direct detection: harder than it sounds Deeper

Inferring a planet's existence from a star's wobble is one thing; actually catching photons that bounced off the planet itself is far more challenging. The 2015 attempt to directly detect 51 Pegasi b in visible light used HARPS at La Silla Observatory in Chile. Had it been confirmed, it would have allowed scientists to calculate a true mass of 0.46 Jupiter masses — not just the lower-bound minimum that radial velocity provides — because the direct detection geometry fixes the orbital inclination. The result could not be replicated in 2021, and a 2022 study actively ruled out the implied radius and albedo estimates. Beyond the albedo controversy, the planet is also a candidate for aperture polarimetry by the Planetpol instrument and has been flagged as a candidate for near-infrared characterization using the VLTI Spectro-Imager. Each technique probes a different aspect of the planet's light, and together they map the slow, contested process of learning what a world 50 light-years away actually looks like.

11 Nobel recognition, 24 years later

The 2019 Nobel Prize in Physics was awarded in part for the discovery of 51 Pegasi b, honoring Michel Mayor and Didier Queloz of the University of Geneva nearly a quarter-century after their announcement in October 1995. Nobel Prizes are almost never awarded immediately after a discovery — the committee waits to see whether findings hold up and transform a field. By 2019, the transformation was undeniable: the detection of 51 Pegasi b had spawned an entire discipline of exoplanet science, launched the hot Jupiter classification, driven the construction of purpose-built instruments, and prompted the revision of planetary formation theory. The prize formally acknowledged that a detection made with a ground-based spectrograph on a French hillside, identifying a wobble of around 70 metres per second, was one of the most consequential astronomical observations of the twentieth century.

About this record

The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.