51 Pegasi b: The planet that started the revolution of exoplanet research

Abstract

To celebrate its 150 anniversary, Nature collected 10 extraordinary papers published on Nature in November, 2019. One of the papers is on the discovery of the 51 Peg b by Swiss astronomers Michel Mayor and Didier Queloz from University of Geneva. 51 Peg b was the first confirmed planet around a solar-type star other than our own Sun. Mayor and Queloz later won the Nobel Physics Prize in 2019 because they have “explored our cosmic neighbourhoods on the hunt for unknown planets. Their discoveries have forever changed our conceptions of the world.” The discovery was made using the radial velocity technique. A star and a planet orbit each other, and although not much, the star still shows a noticeable wobble due to the gravitational tug of the planet. By observing this wobble motion, scientists can infer the existence of a planet around the star and calculate its mass and distance from the host star. Mayor and Queloz started their planet survey using ELODIE spectrograph in the early 1990s at Haute-Provence Observatory. Through their survey, they found a strange object orbiting the star 51 Pegasi, with a similar mass of Jupiter, but about 7 times closer to its host star than Mercury is to our sun. This object was later named as 51 Pegasi b, and confirmed by other research teams around the world, including Marcy’s research team from America. This new discovery was a big challenge to the traditional models of planet formation. How could a Jupiter-like planet form so close to its host star? Revised planet formation theory suggests 51 Peg b initially formed like a Jupiter farther away from 51 Pegasi, and later migrated inwards towards the star due to interaction with other planets or interaction with the material in the gaseous planetary disk. Since the discovery of 51 Peg b, more than 4200 exoplanets have been confirmed so far, and many more yet coming. The main detection methods of exoplanets are radial velocity technique, transiting technique, microlensing, and direct imaging methods. Most of the planets known today are detected using radial velocity technique and transiting technique. A lot of these planet systems are quite different from our Solar system, which makes people wondering, is our Solar system unique in the Universe? As more and more planets are detected, astronomers keep revising the theory of planet formation to account for the new discoveries. Eventually we will figure out the position of our own solar system in the Univerise, and how planets form. In the present paper, we will first briefly review the discovery of the first confirmed exoplanet around a solar-type star, 51 Peg b by Mayor and Queloz in 1995. Then, we introduce the properties of this extraordinary planet system, and show how this planet system single handly changed our view of planet formation. We shall also introduce the impact of this discovery on the exoplanet community and astronomy community as a whole. In the end, we point out the important implications of the discovery of 51 Peg b.