Atmospheres of Rocky Exoplanets

Wordsworth, R.; Kreidberg, L.

doi:10.48550/arxiv.2112.04663

“…The conditions under which terrestrial planets can retain sizable atmospheres under different irradiation levels, timescales, types of host star, and planet masses, radii, and surface gravity all remain areas of active research. While an exoplanet's atmosphere can be studied via transit and/or eclipse observations, transmission spectroscopy has so far failed to conclusively determine the properties (or absence of) any rocky planet's atmosphere (e.g., see Wordsworth & Kreidberg 2021). To date, emission measurements have offered the best prospects for studying the properties of terrestrial exoplanets.…”

Section: Introductionmentioning

confidence: 99%

GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere

Crossfield

¹

,

Malik

²

,

Hill

³

et al. 2022

ApJL

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In recent years, the discovery of increasing numbers of rocky, terrestrial exoplanets orbiting nearby stars has drawn increased attention to the possibility of studying these planets’ atmospheric and surface properties. This is especially true for planets orbiting M dwarfs, whose properties can best be studied with existing observatories. In particular, the minerological composition of these planets and the extent to which they can retain their atmospheres in the face of intense stellar irradiation both remain unresolved. Here, we report the detection of the secondary eclipse of the terrestrial exoplanet GJ 1252b, obtained via 10 eclipse observations using the Spitzer Space Telescope’s IRAC2 4.5 μm channel. We measure an eclipse depth of 149 − 32 + 25 ppm, corresponding to a dayside brightness temperature of 1410 − 125 + 91 K. This measurement is consistent with the prediction for a bare rock surface. Comparing the eclipse measurement to a large suite of simulated planetary spectra indicates that GJ 1252b has a surface pressure of ≲10 bar, i.e., substantially thinner than the atmosphere of Venus. Assuming energy-limited escape, even a 100 bar atmosphere would be lost in <1 Myr, far shorter than our gyrochronological age estimate of 3.9 ± 0.4 Gyr. The expected mass loss could be overcome by mantle outgassing, but only if the mantle’s carbon content were >7% by mass—over two orders of magnitude greater than that found in Earth. We therefore conclude that GJ 1252b has no significant atmosphere. Model spectra with granitoid or feldspathic surface composition, but with no atmosphere, are disfavored at >2σ. The eclipse occurs just +1.4 − 1.0 + 2.8 minutes after orbital phase 0.5, indicating e cos ω = +0.0025 − 0.0018 + 0.0049 , consistent with a circular orbit. Tidal heating is therefore likely to be negligible with regard to GJ 1252b’s global energy budget. Finally, we also analyze additional, unpublished TESS transit photometry of GJ 1252b, which improves the precision of the transit ephemeris by a factor of 10, provides a more precise planetary radius of 1.180 ± 0.078 R ⊕, and rules out any transit-timing variations with amplitudes ≳1 minute.

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“…The atmosphere-reducing efficacy of late-accreting debris is further dependent on the timing of the impact event. Mdwarf planetary systems undergo an extended luminous pre-main sequence phase, during which initially waterrich or hydrogen-dominated planets undergo prolonged magma ocean phases (Schaefer et al 2016;Barth et al 2021;Wordsworth & Kreidberg 2021;Lichtenberg et al 2021aLichtenberg et al , 2022. Prebiotically relevant impact events onto formed planets must thus happen after the runaway greenhouse transition has receded to shorter orbital distances.…”

Section: Planetesimal Composition and Impact Timingmentioning

confidence: 99%

“…Second, the arrival of the most massive impactors, which are the most effective in triggering extended reduced atmospheres by equilibrating with sufficient surface water, is limited to a few tens of Myr in M star systems. At this time the runaway greenhouse threshold is far outside the orbit of planets that reside in potentially habitable regions during the stellar main-sequence phase (Luger & Barnes 2015;Schaefer et al 2016); meaning they will be in a global magma ocean regime (Wordsworth & Kreidberg 2021;Lichtenberg et al 2022), because a fraction of an Earth's ocean mass is already sufficient to keep the planetary surface temperature above the melting temperature of rocks (Boukrouche et al 2021). If the planetary mantle is a magma ocean, impactor iron merges with the target body core (Kendall & Melosh 2016).…”

Section: Impact-generated Reduced Climate States On Exoplanetsmentioning

confidence: 99%

“…While G-dwarf systems hosting terrestrial planets would provide an ideal comparison to the Solar System (Jontof-Hutter 2019; Wordsworth & Kreidberg 2021), their scarcity in close proximity to the Sun coupled with the limitations of the transit and radial velocity techniques greatly restrict the detailed exploration of these systems until space-based direct imaging surveys become technically viable (Apai et al 2019;Gaudi et al 2020;Quanz et al 2021). However, formation models (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Reduced late bombardment on rocky exoplanets around M-dwarfs

Lichtenberg¹,

Clement²

2022

Preprint

0

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Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyse atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts' extended pre-main sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean analog environments. We contrast the Solar System scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars ≤ 0.4 M experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favoured around FGK-stellar types relative to M-dwarfs.

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“…Exoplanet observations in the 2020s will have the potential to probe the atmospheres and climates of select exoplanets on short-period orbits. To understand the history of these planets' atmospheres will require independent constraints on the importance of short-lived radioisotope heating during planet formation throughout the galaxy (Wordsworth & Kreidberg 2021;Lichtenberg et al 2022).…”

Section: Introductionmentioning

confidence: 99%

Prevalence of short-lived radioactive isotopes across exoplanetary systems inferred from polluted white dwarfs

Curry,

Bonsor,

Lichtenberg

et al. 2022

Preprint

1

0

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In the Solar System short-lived radioisotopes, such as 26 Al, played a crucial role during the formation planetary bodies by providing a significant additional source of heat. Notably, this led to early and large-scale melting and iron core formation in planetesimals and their loss of volatile elements, such as hydrogen and carbon. In the context of exoplanetary systems, therefore, the prevalence of short-lived radioisotopes is key to interpreting the observed bulk volatile budget and atmospheric diversity among low-mass exoplanets. White dwarfs that have accreted planetary material provide a unique means to infer the frequency of iron core formation in extrasolar planetesimals, and hence the ubiquity of planetary systems forming with high short-lived radioisotope abundances. Here, we devise a quantitative method to infer the fraction of planetary systems enriched with shortlived radionuclides upon planetesimal formation from white dwarf data. We argue that the current evidence from white dwarfs point towards a significant fraction of exo-planetesimals having formed an iron core. Although the data may be explained by the accretion of exo-moon or Pluto-sized bodies that were able to differentiate due to gravitational potential energy release, our results suggest that the most likely explanation for the prevalence of differentiated material among polluted white dwarfs is that the Solar System is not unusual in being enriched in 26 Al. The models presented here suggest a ubiquitous pathway for the enrichment of exoplanetary systems by short-lived radioisotopes, disfavouring short-lived radioisotope enrichment scenarios relying on statistically rare chance encounters with single nearby supernovae, Wolf-Rayet or AGB stars.

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Atmospheres of Rocky Exoplanets

Cited by 5 publications

References 263 publications

GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere

GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere

Reduced late bombardment on rocky exoplanets around M-dwarfs

Prevalence of short-lived radioactive isotopes across exoplanetary systems inferred from polluted white dwarfs

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