Habitability of terrestrial-mass planets in the HZ of M Dwarfs - I. H/He-dominated atmospheres
File(s)
Published version
Author(s)
Owen, JE
Mohanty, S
Type
Journal Article
Abstract
The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterizing planets in the ‘habitable zone’ (HZ). However, Kepler finds that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions (Menv/Mcore) ≳1 per cent. If these planets retain such envelopes over Gyr time-scales, they will not be ‘habitable’ even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether sufficient envelope mass can be photoevaporated away for these planets to become habitable. We improve upon previous work by using hydrodynamic models that account for radiative cooling as well as the transition from hydrodynamic to ballistic escape. Adopting a template active M dwarf XUV spectrum, including stellar evolution, and considering both evaporation and thermal evolution, we show that: (1) the mass-loss is (considerably) lower than previous estimates that use an ‘energy-limited’ formalism and ignore the transition to Jeans escape; (2) at the inner edge of the HZ, planets with core mass ≲ 0.9 M⊕ can lose enough H/He to become habitable if their initial envelope mass-fraction is ∼1 per cent; (3) at the outer edge of the HZ, evaporation cannot remove a ∼1 per cent H/He envelope even from cores down to 0.8 M⊕. Thus, if planets form with bulky H/He envelopes, only those with low-mass cores may eventually be habitable. Cores ≳1 M⊕, with ≳1 per cent natal H/He envelopes, will not be habitable in the HZ of M dwarfs.
Date Issued
2016-07-11
Date Acceptance
2016-04-20
Citation
Monthly Notices of the Royal Astronomical Society, 2016, 459 (4), pp.4088-4108
ISSN
1365-2966
Publisher
Oxford University Press
Start Page
4088
End Page
4108
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
459
Issue
4
Copyright Statement
© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
Sponsor
Imperial College Trust
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council [2006-2012]
Identifier
http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000379830700052&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=1ba7043ffcc86c417c072aa74d649202
Grant Number
N/A
ST/K001051/1
ST/K001051/1
Subjects
Science & Technology
Physical Sciences
Astronomy & Astrophysics
astrobiology
planets and satellites: atmospheres
planets and satellites: composition
planets and satellites: physical evolution
X-rays: stars
STELLAR ASTROPHYSICS MESA
MAIN-SEQUENCE STARS
SOLAR-TYPE STARS
X-RAY
SUPER-EARTHS
KEPLER PLANETS
HOT-JUPITERS
PROTOPLANETARY DISCS
EXTREME-ULTRAVIOLET
RADIUS DISTRIBUTION
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
Publication Status
Published
Date Publish Online
2016-04-22