Measuring the D/H Ratios of Exoplanets and Brown Dwarfs

©NASA

Brown Dwarf

The relative abundance of deuterium and hydrogen is a potent tracer of planet formation and evolution. Jupiter and Saturn have protosolar D/H ratios, a relic of substantial gas accretion from the nebula, while Neptune and Uranus are enhanced in D by accretion of ices into their envelopes. For terrestrial planets, D/H ratios are used to determine the mechanisms of volatile delivery and subsequent atmosphere loss over the lifetime of the planet.

Free-floating planets and brown dwarfs more massive than ~13 M_J quickly fuse their initial D reservoir. Here, we simulate spectra from Teff=200--1800 K including both CH3D and HDO to determine the observability of these dominant deuterium isotopologues in mid-infrared thermal emission spectra. Colder objects have stronger molecular features in their spectra, due to the temperature-dependence of molecular cross sections. CH3D is easier to observe than HDO at all temperatures considered, due to the strength of its absorption bands and locations of features at wavelengths with few other strong absorption features. For objects from 250--450 K closer than 10 pc, a protosolar D/H ratio would be readily observable in 5 hours with JWST. Moderately young Jupiter-mass planets (100--300 Myr) and young Neptunes (10 Myr) may be discovered with JWST and provide the best targets for detecting deuterium on an exoplanet in the coming decade. Future telescope designs should consider the importance of isotopes for understanding the formation and evolution of planetary atmospheres.

Caroline V. Morley, Andrew J. Skemer, Brittany E. Miles, Michael R. Line, Eric D. Lopez, Matteo Brogi, Richard S. Freedman, Mark S. Marley
(Submitted on 9 Oct 2018)

Comments: 8 pages, 5 figures, submitted for publication in ApJ Letters. Comments welcome!
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1810.04241 [astro-ph.EP] (or arXiv:1810.04241v1 [astro-ph.EP] for this version)
Submission history
From: Caroline Morley
[v1] Tue, 9 Oct 2018 21:06:10 GMT (603kb,D)
https://arxiv.org/abs/1810.04241
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