Abstract
Abstract Terrestrial planets orbiting M dwarfs may soon be observed with the James Webb Space Telescope (JWST) to characterize their atmospheric composition and search for signs of habitability or life. These planets may undergo significant atmospheric and ocean loss due to the superluminous pre-main-sequence phase of their host stars, which may leave behind abiotically generated oxygen, a false positive for the detection of life. Determining if ocean loss has occurred will help assess potential habitability and whether or not any O2 detected is biogenic. In the solar system, differences in isotopic abundances have been used to infer the history of ocean loss and atmospheric escape (e.g., Venus, Mars). We find that isotopologue measurements using transit transmission spectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may be possible with JWST, if the escape mechanisms and resulting isotopic fractionation were similar to Venus. We present analyses of post-ocean-loss O2- and CO2-dominated atmospheres containing a range of trace gas abundances. Isotopologue bands are likely detectable throughout the near-infrared (1–8 μm), especially 3–4 μm, although not in CO2-dominated atmospheres. For Venus-like D/H ratios 100 times that of Earth, TRAPPIST-1b transit signals of up to 79 ppm are possible by observing HDO. Similarly, 18O/16O ratios 100 times that of Earth produce signals at up to 94 ppm. Detection at signal-to-noise ratio = 5 may be attained on these bands with as few as four to 11 transits, with optimal use of JWST’s NIRSpec Prism. Consequently, H2O and CO2 isotopologues could be considered as indicators of past ocean loss and atmospheric escape for JWST observations of terrestrial planets around M dwarfs.
Highlights
In the near future, terrestrial exoplanets around small M dwarf stars will be observed by the James Webb Space Telescope (JWST) and extremely large ground-based telescopes (Cowan et al 2015; Quanz et al 2015; Snellen et al 2015; Greene et al 2016; Lovis et al 2017; Morley et al 2017; Lincowski et al 2018)
We find that isotopologue measurements using transit transmission spectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may be possible with JWST, if the escape mechanisms and resulting isotopic fractionation were similar to Venus
We present noiseless simulated transit transmission spectra at 1 cm−1 resolution demonstrating the signal present due to different levels of extreme isotopic fractionation for δD up to 100 Vienna Standard Mean Ocean Water (VSMOW) and δ18O up to 100 VSMOW
Summary
Terrestrial exoplanets around small M dwarf stars will be observed by the James Webb Space Telescope (JWST) and extremely large ground-based telescopes (Cowan et al 2015; Quanz et al 2015; Snellen et al 2015; Greene et al 2016; Lovis et al 2017; Morley et al 2017; Lincowski et al 2018). The superluminous pre-main-sequence phase of M dwarf stars may drive significant loss of a planet’s surface water and atmosphere, and potentially produce large quantities of atmospheric oxygen. This superluminous phase could last for up to one billion years for the smallest stars (Baraffe et al 2015). During this time, ocean-bearing planets that formed in what is presently the habitable zone would have been subjected to fluxes up to 100 times the stellar irradiation of the main-sequence. These processes include oxidation of the surface, interaction with a magma ocean that reincorporates the O2 into the mantle (Schaefer et al 2016; Wordsworth et al 2018), or loss of O2 to space either via hydrodynamic escape early on or by a number of ongoing escape mechanisms (e.g. Hunten 1982; Lammer et al 2007; Ribas et al 2016; Airapetian et al 2017; Dong et al 2017; Garcia-Sage et al 2017; Egan et al 2019)
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