Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1
File(s)
Accepted version
OA Location
Author(s)
Type
Journal Article
Abstract
Ultracool dwarfs (UCD; Teff < ∼3000 K) cool to settle on the main sequence after ∼1 Gyr. For brown dwarfs, this cooling never stops. Their habitable zones (HZ) thus sweeps inward at least during the first Gyr of their lives. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to enter the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry worlds. Here, we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets recently found around the 0.08 M⊙ dwarf TRAPPIST-1. Despite assumptions maximizing the FUV photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amount of water in the HZ of UCDs, with a sweet spot in the 0.04–0.06 M⊙ range. We also studied the TRAPPIST-1 system using observed constraints on the XUV flux. We find that TRAPPIST-1b and c may have lost as much as 15 Earth oceans and planet d – which might be inside the HZ – may have lost less than 1 Earth ocean. Depending on their initial water contents, they could have enough water to remain habitable. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.
Date Issued
2017-01-01
Date Acceptance
2016-10-06
Citation
Monthly Notices of the Royal Astronomical Society, 2017, 464 (3), pp.3728-3741
ISSN
0035-8711
Publisher
Oxford University Press
Start Page
3728
End Page
3741
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
464
Issue
3
Copyright Statement
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. This is a pre-copy-editing, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The definitive publisher-authenticated version E. Bolmont, F. Selsis, J. E. Owen, I. Ribas, S. N. Raymond, J. Leconte, M. Gillon; Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1, Monthly Notices of the Royal Astronomical Society, Volume 464, Issue 3, 21 January 2017, Pages 3728–3741, is available online at: https://doi.org/10.1093/mnras/stw2578
Subjects
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
Publication Status
Published
Date Publish Online
2016-10-08