Extrasolar Planet 51 Pegasi b

By now the exoplanet known as 51 Pegasi b (hot Jupiter) is not particularly different to other planets in its class. However, in 1995, it was the first confirmed exoplanet discovery around a sunlike star. See When was the first exoplanet discovered? for historical details about other exoplanets discovered before 1995).

The star 51 Pegasi is about 47.9 light years from Earth, its mass is similar to our Sun, and its size not more than about 30% greater than our Sun. The planet itself has a mass of about half a Jupiter mass ($0.0468\pm0.007 \ M_{j}$), and a maximum star-planet distance (semimajor axis) of just 5% of the Earth-Sun distance (0.052 astronomical units (AU)).

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The radius of the planet is unknown, and therefore so is it's density. The planet was discovered by the radial velocity (Doppler, or RV) method.

Advanced Readers: The original discovery paper is Mayor and Queloz 1995, Nature, 378, 355, A Jupiter-mass companion to a solar-type star. The abstract of the paper is very short and sweet, and non-sensationalist with no frills or fanfare, as is the title of the paper.

Learn more about exoplanets with the book Exoplanets and Alien Solar Systems.

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File under: 51 Pegasi b; First exoplanet discovered; First hot Jupiter discovered.