Hydrogen Dominated Atmospheres on Terrestrial Mass Planets: Evidence, Origin and Evolution
- PDF / 1,350,258 Bytes
- 24 Pages / 439.37 x 666.142 pts Page_size
- 61 Downloads / 189 Views
Hydrogen Dominated Atmospheres on Terrestrial Mass Planets: Evidence, Origin and Evolution J.E. Owen1 · I.F. Shaikhislamov2 · H. Lammer3 · L. Fossati3 · M.L. Khodachenko3
Received: 29 November 2019 / Accepted: 21 October 2020 © The Author(s) 2020
Abstract The discovery of thousands of highly irradiated, low-mass, exoplanets has led to the idea that atmospheric escape is an important process that can drive their evolution. Of particular interest is the inference from recent exoplanet detections that there is a large population of low mass planets possessing significant, hydrogen dominated atmospheres, even at masses as low as ∼ 2 M⊕ . The size of these hydrogen dominated atmospheres indicates the envelopes must have been accreted from the natal protoplanetary disc. This inference is in contradiction with the Solar System terrestrial planets, that did not reach their final masses before disc dispersal, and only accreted thin hydrogen dominated atmospheres. In this review, we discuss the evidence for hydrogen dominated atmospheres on terrestrial mass ( 2 M⊕ ) planets. We then discuss the possible origins and evolution of these atmospheres with a focus on the role played by hydrodynamic atmospheric escape driven by the stellar high-energy emission (X-ray and EUV; XUV). Keywords Atmospheric escape
1 Introduction The picture in which planets grow from small, ∼ 1 µm sized, dust-particles in the star’s natal protoplanetary accretion disc is the paradigm in which the majority of planets form (e.g. Lissauer 1993). This protoplanetary disc is initially composed of roughly 1% solids Understanding the Diversity of Planetary Atmospheres Edited by François Forget, Oleg Korablev, Julia Venturini, Takeshi Imamura, Helmut Lammer and Michel Blanc
B J.E. Owen
[email protected]
1
Astrophysics Group, Blackett Laboratory, Imperial College London, Prince Consort Road, London, SW7 2AZ, UK
2
Institute of Laser Physics SB RAS, Novosibirsk, Russia
3
Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
129
Page 2 of 24
J.E. Owen et al.
and 99% hydrogen/helium (H/He) gas by mass (e.g. Armitage 2011). In the core-accretion paradigm, terrestrial and gas-giant planets begin forming in a similar fashion. Mutual collisions between solids lead to growth and eventually, via a debated mechanism, the formation of 1–100 km-sized planetesimals (e.g. Chiang and Youdin 2010; Birnstiel et al. 2016). At ∼ AU separations, within about 105 years, mutual collisions and runaway accretion of planetesimals produce larger planetary embryos with sizes of the Moon- to Mars, some of which grow further via collisions to terrestrial planet sizes (e.g. Morbidelli et al. 2012). At large separations in the Solar System, this collisional growth proceeded to the point where the planetary embryos became massive enough to accrete large amounts of H/He from the nebula, and eventually became gas giants (e.g. Pollack et al. 1996; Helled et al. 2014). In the case of the Solar-System terrestrial planets, it is thought
Data Loading...
