On this day in 1995, two Swiss astronomers discovered 51 Pegasi, the first major star apart from the Sun to have a planet orbiting it, a breakthrough in astronomical research.
For many years, astronomers thought that the planets that orbited our sun were the only planets in the universe. Swiss astronomers Michel Mayor and Didier Queloz were under that impression in the early 1990s when they were conducting research at the Haute-Provence Observatory in southeastern France. Using an ELODIE spectrograph to detect exoplanets by the radial velocity method, they noticed tiny wobbles in a certain celestial body’s movement. Those wobbles were suggestive of some gravitational interaction with another body. Upon further examination they discovered that the wobbling heavenly body was actually a planet orbiting a star similar to our Sun.
On 6 October 1995, Mayor and Queloz announced that they had discovered the first star other than the Sun to have a planet orbiting it. The discovery of the exoplanet 51 Pegasi b upended astronomers’ ideas of what constituted a planet and marked a breakthrough in astronomical research.
Both the sun-like star, 51 Pegasi, and its exoplanet, 51 Pegasi b, are in the constellation Pegasus, about 50.9 light-years from Earth. Star 51 Pegasi is a yellow dwarf star estimated to be 6.1 billion to 8.1 billion years old.
51 Pegasi’s planet, 51 Pegasi b, is believed to be a superheated gas planet with a radius greater than and a mass about half that of Jupiter. The planet orbits 51 Pegasi in only about four Earth days, leading astronomers to conjecture that it is much closer to its star than Mercury is to our Sun. It’s surface temperatures are also estimated to be a blazing 982°C (1800°F).
The “b” in 51 Pegasi b indicates that this planet was the first discovered orbiting its parent star, 51 Pegasi. Subsequent exoplanet discoveries would be named “c,” “d,” and so on. The planet is also known as Bellerophon, after the Greek hero Bellerophon who tamed Pegasus, the winged horse, an allusion to the constellation in which it was found.
Since the discovery of 51 Pegasi b, a number of other exoplanets orbiting Sun-like stars, including 55 Cancri and tau Bootes, have been detected. This class of exoplanets is called “hot Jupiters,” planets whose mass is close to or exceeds that of Jupiter.
If it weren’t for the work of Mayor and Queloz, this entire class of planets, the most well known of which is 51 Pegasi b, would never have been discovered.
Credit: Image Courtesy NASA
Caption: 51 Pegasi was the first major star apart from our sun to have a planet orbiting it. Since its discovery, many other exoplanets have been found.