Water Trapping on Tidally Locked Terrestrial Planets Requires Special Conditions

Yang, Jun; Liu, Yonggang; Hu, Yongyun; Abbot, Dorian S.

doi:10.1088/2041-8205/796/2/l22

Cited by 84 publications

(66 citation statements)

References 33 publications

(35 reference statements)

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“…Godolt et al (2015) shows that taking oceanic circulation into account produces surface habitable conditions for planets orbiting F-type stars instead of a snowball state. Our results would likely be different if we had considered other types of planets such as an Earth-like planet, a land planet, a planet with a Pangea-like continent, or a planet with archipelagos (e.g., Yang et al 2014). …”

Section: Discussionmentioning

confidence: 70%

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

Bolmont

Libert

Leconte

et al. 2016

A&A

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Unlike the Earth, which has a small orbital eccentricity, some exoplanets discovered in the insolation habitable zone (HZ) have high orbital eccentricities (e.g., up to an eccentricity of ∼0.97 for HD 20782 b). This raises the question of whether these planets have surface conditions favorable to liquid water. In order to assess the habitability of an eccentric planet, the mean flux approximation is often used. It states that a planet on an eccentric orbit is called habitable if it receives on average a flux compatible with the presence of surface liquid water. However, because the planets experience important insolation variations over one orbit and even spend some time outside the HZ for high eccentricities, the question of their habitability might not be as straightforward. We performed a set of simulations using the global climate model LMDZ to explore the limits of the mean flux approximation when varying the luminosity of the host star and the eccentricity of the planet. We computed the climate of tidally locked ocean covered planets with orbital eccentricity from 0 to 0.9 receiving a mean flux equal to Earth's. These planets are found around stars of luminosity ranging from 1 L to 10 −4 L . We use a definition of habitability based on the presence of surface liquid water, and find that most of the planets considered can sustain surface liquid water on the dayside with an ice cap on the nightside. However, for high eccentricity and high luminosity, planets cannot sustain surface liquid water during the whole orbital period. They completely freeze at apoastron and when approaching periastron an ocean appears around the substellar point. We conclude that the higher the eccentricity and the higher the luminosity of the star, the less reliable the mean flux approximation.

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Section: Discussionmentioning

confidence: 70%

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

Bolmont

Libert

Leconte

et al. 2016

A&A

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“…This is despite the much lower relative number density of the suprathermal populations (∼5% at 1 AU) as compared to the thermal core [Feldman et al, 1975]. Recent investigations have shown that the heat flux is closely related to the relative drift between the core and the halo populations [Bale et al, 2013], which may be the relic of an electron two stream instability that develops in the low corona [Che and Goldstein, 2014]. It has also been shown that when a clear strahl beam is observed, the majority of solar wind heat flux is carried by the strahl electrons [Pilipp et al, 1987a].…”

Section: Introductionmentioning

confidence: 95%

The evolution of solar wind strahl with heliospheric distance

et al. 2017

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Field‐aligned beams of suprathermal electrons, known as “strahl,” are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1 AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between ∼1 and 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.

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“…Let us assume that the planet starts with a global ocean, possibly covered by sea-ice wherever cold enough. This experiment has been done by Yang et al (2014) 5 with a 325 m-deep ocean. They showed that in such a configuration, winds carry sea-ice toward hot regions and the ocean carries heat toward cold ones, so that an equilibrium can be found with less than 10 m of sea-ice in the coldest regions 6 .…”

Section: Transition From Small To Large Water Inventorymentioning

confidence: 99%

The habitability of Proxima Centauri b

Turbet

Leconte

Selsis

et al. 2016

A&A

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Radial velocity monitoring has found the signature of a M sin i = 1.3 M ⊕ planet located within the Habitable Zone (HZ) of Proxima Centauri (Anglada-Escudé et al. 2016). Despite a hotter past and an active host star the planet Proxima b could have retained enough volatiles to sustain surface habitability (Ribas et al. 2016). Here we use a 3D Global Climate Model (GCM) to simulate Proxima b's atmosphere and water cycle for its two likely rotation modes (1:1 and 3:2 spin-orbit resonances) while varying the unconstrained surface water inventory and atmospheric greenhouse effect. Any low-obliquity low-eccentricity planet within the HZ of its star should be in one of the climate regimes discussed here. We find that a broad range of atmospheric compositions allow surface liquid water. On a tidally-locked planet with sufficient surface water inventory, liquid water is always present, at least in the substellar region. With a non-synchronous rotation, this requires a minimum greenhouse warming (∼10 mbar of CO 2 and 1 bar of N 2 ). If the planet is dryer, ∼0.5 bar/1.5 bars of CO 2 (respectively for asynchronous/synchronous rotation) suffice to prevent the trapping of any arbitrary small water inventory into polar/nightside ice caps. We produce reflection/emission spectra and phase curves for the simulated climates. We find that atmospheric characterization will be possible by direct imaging with forthcoming large telescopes. The angular separation of 7λ/D at 1 µm (with the E-ELT) and a contrast of ∼10 −7 will enable high-resolution spectroscopy and the search for molecular signatures, including H 2 O, O 2 , and CO 2 . The observation of thermal phase curves can be attempted with JWST, thanks to a contrast of 2 × 10 −5 at 10 µm. Proxima b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima b and possibly determine whether this exoplanet's surface is habitable.

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Water Trapping on Tidally Locked Terrestrial Planets Requires Special Conditions

Cited by 84 publications

References 33 publications

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

Habitability of planets on eccentric orbits: Limits of the mean flux approximation

The evolution of solar wind strahl with heliospheric distance

The habitability of Proxima Centauri b

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