Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets

Rice, Ken; Malavolta, L.; Mayo, Andrew W.; Mortier, A.; Buchhave, Lars A.; Affer, L.; Vanderburg, Andrew; López‐Morales, Mercedes; Poretti, E.; Zeng, Li; Cameron, A. Collier; Damasso, M.; Coffinet, A.; Latham, David W.; Bonomo, A. S.; Bouchy, F.; Charbonneau, David; Dumusque, X.; Figueira, P.; Fiorenzano, A. F. Martínez; Haywood, R. D.; Johnson, John Asher; Lopez, Eric; Lovis, C.; Mayor, M.; Micela, G.; Molinari, E.; Nascimbeni, V.; Nava, Chantanelle; Pepe, F.; Phillips, David F.; Piotto, G.; Sasselov, Dimitar; Ségransan, D.; Sozzetti, A.; Udry, S.; Watson, C. A.

doi:10.1093/mnras/stz130

Cited by 44 publications

(30 citation statements)

References 86 publications

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“…However, GJ 9827d is part of a system with two other transiting planets on interior orbits. The density of the closest-in planet (planet b) is consistent with being rocky (Prieto-Arranz et al 2018;Teske et al 2018;Rice et al 2019), which is highly suggestive of photoevaporative mass loss from initially volatile-rich worlds. Planet GJ 9827c is between planets b and d, but its mass is poorly constrained.…”

Section: Introductionmentioning

confidence: 75%

Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

Kasper

Bean

Oklopčić

et al. 2020

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We present a search for helium in the upper atmospheres of three sub-Neptune-sized planets to investigate the origins of these ubiquitous objects. The detection of helium for a low-density planet would be strong evidence for the presence of a primary atmosphere accreted from the protoplanetary nebula because large amounts of helium are not expected in the secondary atmospheres of rocky planets. We used Keck+NIRSPEC to obtain high-resolution transit spectroscopy of the planets GJ 1214b, GJ 9827d, and HD 97658b around the 10833 Å He triplet feature. We did not detect helium absorption for any of the planets despite achieving a high level of sensitivity. We used the nondetections to set limits on the planets' thermosphere temperatures and atmospheric loss rates by comparing grids of 1D models to the data. We also performed coupled interior structure and atmospheric loss calculations, which suggest that the bulk atmospheres (winds) of the planets would be at most modestly enhanced (depleted) in helium relative to their primordial composition. Our lack of detections of the helium triplet for GJ 1214b and GJ 9827d is highly inconsistent with the predictions of models for the present-day mass loss on these planets. Higher signal-to-noise data would be needed to detect the helium feature predicted for HD 97658b. We identify uncertainties in the extreme-ultraviolet fluxes of the host stars and the lack of detailed mass-loss models specifically for cool and metal-enhanced atmospheres as the main limitations to the interpretation of our results. Ultimately, our results suggest that the upper atmospheres of sub-Neptune planets are fundamentally different from those of gas giant planets.

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Section: Introductionmentioning

confidence: 75%

Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

Kasper

Bean

Oklopčić

et al. 2020

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“…With redox diversity, no fine-tuning of H inventory is needed to give a H 2 rich atmosphere for planets with radii in the super-Earth range. This prediction can be tested with ARIEL (Tinetti et al 2016) and perhaps JWST (at HR 858; Vanderburg et al 2019; or at GJ 9827c; Rice et al 2019). Figure 8.…”

Section: 2mentioning

confidence: 99%

Atmosphere Origins for Exoplanet Sub-Neptunes

Kite

Fegley

Schaefer

et al. 2020

ApJ

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Planets with 2 R ⊕ < R < 3 R ⊕ and orbital period <100 d are abundant; these sub-Neptune exoplanets are not well understood. For example, Kepler sub-Neptunes are likely to have deep magma oceans in contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model, assuming that volatiles equilibrate with magma at T ∼ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. For H, most moles go into the magma, so the mass target for both H 2 accretion and H 2 loss models is weightier than is usually assumed. The known span of magma oxidation states can produce sub-Neptunes that have identical radius but with total volatile masses varying by 20-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. This redox diversity degeneracy can be broken by measurements of atmosphere mean molecular weight. We emphasise H 2 supply by nebula gas, but also consider solid-derived H 2 O. We find that adding H 2 O to Fe probably cannot make enough H 2 to explain sub-Neptune radii because >10 3 -km thick outgassed atmospheres have high mean molecular weight. The hypothesis of magma-atmosphere equilibration links observables such as atmosphere H 2 O/H 2 ratio to magma FeO content and planet formation processes. Our model's accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we advocate for such experiments.

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“…These quantities can be measured by a combination of radial velocity and transit observations (e.g. Weiss & Marcy 2014;Rice et al 2019). A new, powerful and potentially more direct approach to measure the bulk composition of exoplanets is to determine the element composition in the atmospheres of polluted white dwarfs.…”

Section: Polluted White Dwarf Abundancesmentioning

confidence: 99%

The atmospheres of rocky exoplanets

Herbort

Woitke

Helling

et al. 2020

A&A

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Context. Little is known about the interaction between atmospheres and crusts of exoplanets so far, but future space missions and ground-based instruments are expected to detect molecular features in the spectra of hot rocky exoplanets. Aims. We aim to understand the composition of the gas in an exoplanet atmosphere which is in equilibrium with a planetary crust. Methods. The molecular composition of the gas above a surface made of a mixture of solid and liquid materials was determined by assuming phase equilibrium for given pressure, temperature, and element abundances. We study total element abundances that represent different parts of the Earth’s crust (continental crust, bulk silicate Earth, mid oceanic ridge basalt), CI chondrites and abundances measured in polluted white dwarfs. Results. For temperatures between ~600 and ~3500 K, the near-crust atmospheres of all considered total element abundances are mainly composed of H2O, CO2, and SO2 and in some cases of O2 and H2. For temperatures ≲500 K, only N2-rich or CH4-rich atmospheres remain. For ≳3500 K, the atmospheric gas is mainly composed of atoms (O, Na, Mg, and Fe), metal oxides (SiO, NaO, MgO, CaO, AlO, and FeO), and some metal hydroxides (KOH and NaOH). The inclusion of phyllosilicates as potential condensed species is crucial for lower temperatures, as they can remove water from the gas phase below about 700 K and inhibit the presence of liquid water. Conclusions. Measurements of the atmospheric composition could, in principle, characterise the rock composition of exoplanet crusts. H2O, O2 and CH4 are natural products from the outgassing of different kinds of rocks that had time to equilibrate. These are discussed as biomarkers, but they do emerge naturally as a result of the thermodynamic interaction between the crust and atmosphere. Only the simultaneous detection of all three molecules might be a sufficient biosignature, as it is inconsistent with chemical equilibrium.

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Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets

Cited by 44 publications

References 86 publications

Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

Atmosphere Origins for Exoplanet Sub-Neptunes

The atmospheres of rocky exoplanets

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