Scientists spotted water in the atmosphere of 51 Pegasi b, one of the first exoplanets ever been discovered. It is around 50 light-years away – so we can call it a “nearby” exoplanet – and it is in the constellation of Pegasus.

51 Pegasi b has not marked a breakthrough in astronomical research for the first time: back in 1995, it was the first exoplanet to be discovered orbiting a main-sequence star (sun-like star). The first confirmed exoplanet discovery came in 1992 when several terrestrial-mass planets orbiting the pulsar PSR B1257+12.

Water on 51 Pegasi b

Artist impression of the exoplanet 51 Pegasi b
An artist’s impression of 51 Pegasi b (center) and its star (right).

A team led by Dr. Jayne Birkby of the Harvard-Smithsonian Center for Astrophysics observed 51 Pegasi b and its host star for 4 hours using the Very Large Telescope (see notes 1) in Chile. As the planet shifted away from and then towards Earth, its light shifted towards redder and then bluer wavelengths, thanks to the Doppler effect (see notes 2). The team analyzed its spectrum and spotted a watery signature.

The discovery is really exciting since it is also an important step towards detecting water molecules in smaller, more habitable worlds, like Proxima Centauri b, discovered in 2016, which is only 4.37 light-years away.

Why water is important

Water’s extensive capability to dissolve a variety of molecules has earned it the designation of “universal solvent,” and it is this ability that makes water such an invaluable life-sustaining force.

On a biological level, water’s role as a solvent helps cells transport and use substances like oxygen or nutrients. Water-based solutions like blood help carry molecules to the necessary locations. Thus, water’s role as a solvent facilitates the transport of molecules like oxygen for respiration.

Despite having water in its atmosphere, unfortunately, 51 Pegasi b cannot host life forms. It is also the first discovered exoplanet for a class of planets called hot Jupiters, a class of exoplanets that are physically similar to Jupiter (gas giants), and orbit their stars from a very short distance.

Some 51 Pegasi b facts

  • 51 Pegasi b is the first exoplanet discovered orbiting a sun-like star.
  • It has a surface temperature of 1000 °C (1832 °F).
  • 51 Pegasi b is 47% less massive but 50% larger than Jupiter, solar system’s largest planet. So, it is considered a “Hot Jupiter”.
  • Its host star, 51 Pegasi is a main-sequence (sun-like) star, but it is 11% more massive and 23% larger than our sun.
  • Hot Jupiters were not thought to be possible until 1995. The discovery of 51 Pegasi b on October 6, 1995 challenged existing theories about planet formation. Astronomers now know large gas giants may form far from their stars and migrate closer to their stars over millions of years.
  • 51 Pegasi b was the first planet to have its reflected light spectrum detected.


  1. The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2 m across, which are generally used separately but can be used together to achieve very high angular resolution.
  2. Doppler effect: an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the redshift seen by astronomers. It is named after the Austrian physicist Christian Doppler (1803-1853), who proposed it in 1842 in Prague.


M. Özgür Nevres

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