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SUMMARY:Habitability of Terrestrial-Mass Planets in the Habitable Zone of 
 Red Dwarfs - Subhanjoy Mohanty (Imperial)
DTSTART:20190509T150000Z
DTEND:20190509T160000Z
UID:TALK124981@talks.cam.ac.uk
CONTACT:Richard Booth
DESCRIPTION:The ubiquity of M dwarfs\, combined with the relative ease of 
 detecting terrestrial-mass planets around them\, has made them prime targe
 ts for finding and characterizing planets in the "Habitable Zone" (HZ). Ho
 wever\, the Kepler mission finds that terrestrial-mass exoplanets are ofte
 n born with voluminous H/He envelopes\, comprising mass-fractions (M_env/M
 _core) >~ 1%. 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 p
 hotoevaporated away for these planets to become habitable. We improve upon
  previous work by using hydrodynamic models that account for radiative coo
 ling as well as the transition from hydrodynamic to ballistic escape. Adop
 ting a template XUV spectrum for an active M dwarf\, including stellar evo
 lution\, 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 esca
 pe\; and (2) if planets form with such bulky H/He envelopes\, only those w
 ith very low-mass cores may eventually be habitable. In particular\,  core
 s with mass >~ 1 M_earth\, and with >~ 1% natal H/He envelopes\, will NOT 
 be habitable in the HZ of M dwarfs in spite of photoevaporation. I will en
 d with a discussion of the general implications for habitability around M 
 dwarfs\, and for the search for habitable M dwarf planets to be undertaken
  by upcoming missions such as TESS\, CHEOPS and JWST. 
LOCATION:Martin Ryle Seminar Room\, Kavli Institute
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