Nobel Prize in Physics 2019 | Exoplanet Science

On 8^th October 2019, James Peebles (Princeton University, USA), Michel Mayor and Didier Queloz (both from the University of Geneva, Switzerland) received the Nobel Prize “for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos”. Two out of the three Physics Nobel laureates received the award for their discoveries in exoplanet science. More specifically, Michel Mayor and Didier Queloz were awarded with the prize for the discovery of the extrasolar planet 51 Pegasi b orbiting a Sun-like star. Their measurements, which were made using a fibre-fed echelle spectrograph called ELODIE at the Haute-Provence Observatory in France, were published in 1995 in the article “A Jupiter-mass companion to a solar-type star” in the scientific journal Nature [1].

51 Pegasi b was discovered using the radial velocity (RV) method, also known as the Doppler method. Besides the transit method, the RV method is another successful means to detect extrasolar planets. It is based on measuring Doppler shifts in a stellar spectrum, which are induced by the gravitational influence of a planet on a star. Thus, Doppler shifts can be most easily detected in star-planet systems in which the orbiting planet is massive and close to its host star. 51 Pegasi b is such a massive and close-in object, having a mass of approx. ~0.5 M_Jupiter and orbiting at a distance of ~0.05 AU [1, 2]. A great overview on exoplanet detection methods and “early milestones” are discussed, for example, in an article by Wright and Gaudi (2012) [3].

Since the first discoveries of extrasolar planets, the number of detections increased rapidly. To date, we count over 4000 confirmed exoplanets [4], which comprise a large diversity of planetary types (e.g., ranging from super-Earths to close-in and hot gas giants). These discoveries have impacted the way we see our Solar System and changed our understanding of planets. Therefore, observations like the one of 51 Pegasi b are exciting, because they give us insight into planetary systems, which are very different to our own Solar System (e.g., in comparison to 51 Peg b, our innermost planet Mercury orbits at ~0.39 AU, Jupiter orbits at ~5.2 AU). Furthermore, they help astronomers to better understand if planetary systems like our Solar System are rather rare or common in the universe.