Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

Hori, Yasunori; Ogihara, Masahiro

doi:10.3847/1538-4357/ab6168

Cited by 36 publications

(28 citation statements)

References 81 publications

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“…This argument, plus the HST/WFC3-IR transit measurements, are strong arguments against the presence of H 2 -dominated envelopes around TRAPPIST-1 planets. This is also supported by recent calculations (Hori and Ogihara 2020 ) showing the total mass loss of hydrogen-rich envelopes around TRAPPIST-1 planets is anyway likely higher than the amount of hydrogen-rich gas they can accrete during their formation.…”

Section: Constraints From Numerical Modellingsupporting

confidence: 81%

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A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

et al. 2020

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TRAPPIST-1 is a fantastic nearby (∼39.14 light years) planetary system made of at least seven transiting terrestrial-size, terrestrial-mass planets all receiving a moderate amount of irradiation. To date, this is the most observationally favourable system of potentially habitable planets known to exist. Since the announcement of the discovery of the TRAPPIST-1 planetary system in 2016, a growing number of techniques and approaches have been used and proposed to characterize its true nature. Here we have compiled a state-of-the-art overview of all the observational and theoretical constraints that have been obtained so far using these techniques and approaches. The goal is to get a better understanding of whether or not TRAPPIST-1 planets can have atmospheres, and if so, what they are made of. For this, we surveyed the literature on TRAPPIST-1 about topics as broad as irradiation environment, planet formation and migration, orbital stability, effects of tides and Transit Timing Variations, transit observations, stellar contamination, density measurements, and numerical climate and escape models. Each of these topics adds a brick to our understanding of the likely—or on the contrary unlikely—atmospheres of the seven known planets of the system. We show that (i) Hubble Space Telescope transit observations, (ii) bulk density measurements comparison with H 2 -rich planets mass-radius relationships, (iii) atmospheric escape modelling, and (iv) gas accretion modelling altogether offer solid evidence against the presence of hydrogen-dominated—cloud-free and cloudy—atmospheres around TRAPPIST-1 planets. This means that the planets are likely to have either (i) a high molecular weight atmosphere or (ii) no atmosphere at all. There are several key challenges ahead to characterize the bulk composition(s) of the atmospheres (if present) of TRAPPIST-1 planets. The main one so far is characterizing and correcting for the effects of stellar contamination. Fortunately, a new wave of observations with the James Webb Space Telescope and near-infrared high-resolution ground-based spectrographs on existing very large and forthcoming extremely large telescopes will bring significant advances in the coming decade.

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Section: Constraints From Numerical Modellingsupporting

confidence: 81%

“…This stems from the fact that any significant change of H 2 content—e.g. arising from variations in the hydrogen-rich gas accretion rates during the planet formation phase (Hori and Ogihara 2020 ) or from variations in the H 2 escape rates (Bolmont et al. 2017b ; Bourrier et al.…”

Section: Constraints From Numerical Modellingmentioning

confidence: 99%

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

et al. 2020

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“…Although transmission spectroscopy cannot rule out H2-dominated atmospheres containing highaltitude aerosols (Moran et al 2018), such configuration is in fact unlikely. This stems from the fact that any small variation of hydrogen content between planets, as expected from (1) variations in the hydrogen-rich gas accretion rates during the planet formation phase (Hori & Ogihara 2020) and from (2) variations in H2 escape rates (Owen & Mohanty 2016;Bolmont et al 2017;Bourrier et al 2017), are expected to produce large variations in density between planets (Turbet et al 2020) that are not observed (Grimm et al 2018;Agol et al 2020).…”

Section: Transmission Spectra Of the Planetsmentioning

confidence: 99%

TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

Ducrot

Gillon

Delrez

et al. 2020

A&A

112

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Context. With more than 1000 h of observation from Feb. 2016 to Oct. 2019, the Spitzer Exploration Program Red Worlds (ID: 13067, 13175 and 14223) exclusively targeted TRAPPIST-1, a nearby (12 pc) ultracool dwarf star, finding that it is orbited by seven transiting Earth-sized planets. At least three of these planets orbit within the classical habitable zone of the star, and all of them are well-suited for a detailed atmospheric characterization with the upcoming JWST. Aims. The main goals of the Spitzer Red Worlds program were (1) to explore the system for new transiting planets, (2) to intensively monitor the planets’ transits to yield the strongest possible constraints on their masses, sizes, compositions, and dynamics, and (3) to assess the infrared variability of the host star. In this paper, we present the global results of the project. Methods. We analyzed 88 new transits and combined them with 100 previously analyzed transits, for a total of 188 transits observed at 3.6 or 4.5 μm. For a comprehensive study, we analyzed all light curves both individually and globally. We also analyzed 29 occultations (secondary eclipses) of planet b and eight occultations of planet c observed at 4.5 μm to constrain the brightness temperatures of their daysides. Results. We identify several orphan transit-like structures in our Spitzer photometry, but all of them are of low significance. We do not confirm any new transiting planets. We do not detect any significant variation of the transit depths of the planets throughout the different campaigns. Comparing our individual and global analyses of the transits, we estimate for TRAPPIST-1 transit depth measurements mean noise floors of ~35 and 25 ppm in channels 1 and 2 of Spitzer/IRAC, respectively. We estimate that most of this noise floor is of instrumental origins and due to the large inter-pixel inhomogeneity of IRAC InSb arrays, and that the much better interpixel homogeneity of JWST instruments should result in noise floors as low as 10 ppm, which is low enough to enable the atmospheric characterization of the planets by transit transmission spectroscopy. Our analysis reveals a few outlier transits, but we cannot conclude whether or not they correspond to spot or faculae crossing events. We construct updated broadband transmission spectra for all seven planets which show consistent transit depths between the two Spitzer channels. Although we are limited by instrumental precision, the combined transmission spectrum of planet b to g tells us that their atmospheres seem unlikely to be CH4-dominated. We identify and model five distinct high energy flares in the whole dataset, and discuss our results in the context of habitability. Finally, we fail to detect occultation signals of planets b and c at 4.5 μm, and can only set 3-σ upper limits on their dayside brightness temperatures (611 K for b 586 K for c).

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“…Lee et al 2014), the continual recycling of the accreting gas around the planet (e.g. Ormel et al 2015;Cimerman et al 2017;Kurokawa & Tanigawa 2018;Kuwahara et al 2019), the delay of gas accretion by polluted envelopes (Brouwers & Ormel 2020), and disk dissipation (Ikoma & Hori 2012;Hori & Ogihara 2020). We have recently proposed the possibility of suppressing envelope accretion via a limit due to the disk accretion rate (Ogihara & Hori 2018), and similar solutions have been discussed in other studies (e.g.…”

Section: Introductionmentioning

confidence: 69%

Unified Simulations of Planetary Formation and Atmospheric Evolution: Effects of Pebble Accretion, Giant Impacts, and Stellar Irradiation on Super-Earth Formation

Ogihara

Hori

2020

ApJ

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A substantial number of super-Earths have been discovered, and atmospheres of transiting super-Earths have also been observed by transmission spectroscopy. Several lines of observational evidence indicate that most super-Earths do not possess massive H 2 /He atmospheres. However, accretion and retention of less massive atmospheres on super-Earths challenge planet formation theory. We consider the following three mechanisms: (i) envelope heating by pebble accretion, (ii) mass loss during giant impacts, and (iii) atmospheric loss by stellar X-ray and EUV photoevaporation. We investigate whether these mechanisms influence the amount of the atmospheres that form around super-Earths. We develop a code combining an N -body simulation of pebble-driven planetary formation and an atmospheric evolution simulation. We demonstrate that the observed orbital properties of super-Earths are well reproduced by the results of our simulations. However, (i) heating by pebble accretion ceases prior to disk dispersal, (ii) the frequency of giant impact events is too low to sculpt massive atmospheres, and (iii) many super-Earths having H 2 /He atmospheres of 10 wt% survive against stellar irradiations for 1 Gyr. Therefore, it is likely that other mechanisms such as suppression of gas accretion are required to explain less massive atmospheres ( 10 wt%) of super-Earths.

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Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

Cited by 36 publications

References 81 publications

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

Unified Simulations of Planetary Formation and Atmospheric Evolution: Effects of Pebble Accretion, Giant Impacts, and Stellar Irradiation on Super-Earth Formation

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