Searching for exoplanets and exo-life: On the 2019 Nobel Prize in Physics

Abstract

The Nobel Prize of 2019 in Physics was awarded to three astronomers for their contributions to our understanding of the evolution of the universe and Earth's place in the cosmos. They are professor James Peebles at Princeton University "for theoretical discoveries in physical cosmology" as well as professors Michel Mayor and Didier Queloz at the University of Geneva "for the discovery of an exoplanet orbiting a solar-type star." The three astronomers have different research directions. Peebles' research is on cosmology, while Mayor and Queloz's research is on searching for extra-solar planets (exoplanets). Mayor and Queloz's discovery of an exoplanet is a milestone for planetary science and astronomy. Since after, searching for exoplanets has become a hot research area for both planetary scientists and astronomers. So far, there have been more than 4000 exoplanets confirmed. These exoplanets demonstrate large diversities in physical and chemical properties as well as orbital eccentricity. The rich-full diversities greatly challenge the classical model of the formation of our own solar system. One example is that so many hot Jupiter-like exoplanets are so close to their parent stars, just like the one discovered by Mayor and Queloz. There are also many super-Earth exoplanets. These require us to re-examine our traditional understanding of the Solar system. Among the more than 4000 exoplanets, 10 to 20 are considered potentially habitable. These exoplanets are located in the habitable zone of their parent stars and have the mass that is comparable to Earth's. They could maintain liquid water on their surface, which is the crucially necessary condition for life existence. It is important to note that most of these potentially habitable exoplanets are around M dwarfs that are cool stars. Due to such close distances, terrestrial exoplanets in the habitable zone of M dwarfs are tidally locked, causing very uneven heating between the day and night sides. Is there atmosphere collapse or water trapping on the extremely cold nightside of these tidally locked exoplanets? It is of great concerns by planetary scientists. Numerical simulations showed that both atmosphere and ocean would transport heats from the day side to the night side of tidal locking exoplanets. It would warm up the nightside although the nightside does not receive any stellar radiation, so that atmosphere collapse and water trapping do not happen. At present, it is unlikely to directly detect exo-life over exoplanets. However, indirect methods can be applied to detecting biosignatures over these potentially habitable exoplanets. One way is to observe gases that are related to biological processes, such as oxygen, ozone, methane, and so on. These gases have significant absorptions at infrared bands. The next generation of infrared space telescopes shall provide us with evidence of biosignatures. It shall win another Nobel Prize if any exoplanet is confirmed with biosiganture. In the present paper, we shall first introduce the scientific accomplishment by Mayor and Queloz and their important impacts on planetary sciences and astronomy. Then, we will present a brief review of the diversity of exoplanets and its great challenges to our traditional understanding of the formation of the Solar system. We shall also introduce the progresses on potentially habitable exoplanets and the search for exo-life. At the end of the paper, we summarize the ongoing extensive studies of exoplanets and look forward to the discovery of habitable exoplanets and exo-life.