Massive Satellites of Close-in Gas Giant Exoplanets

Cassidy, Timothy A.; Mendez, Rolando E.; Arras, Phil; Johnson, R. E.; Skrutskie, Michael F.

doi:10.1088/0004-637x/704/2/1341

Cited by 70 publications

(62 citation statements)

References 44 publications

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“…It has long been recognized that moons of giant planets may be warmed by tidal heating from the primary planet and receive sufficient light from a central star to power photosynthesis (70). This provides a model for possible habitable moons orbiting giant exoplanets (71).…”

Section: Titan Lifementioning

confidence: 99%

Requirements and limits for life in the context of exoplanets

McKay

2014

Proc. Natl. Acad. Sci. U.S.A.

124

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The requirements for life on Earth, its elemental composition, and its environmental limits provide a way to assess the habitability of exoplanets. Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life can grow and reproduce at temperatures as low as −15°C, and as high as 122°C. Studies of life in extreme deserts show that on a dry world, even a small amount of rain, fog, snow, and even atmospheric humidity can be adequate for photosynthetic production producing a small but detectable microbial community. Life is able to use light at levels less than 10 −5 of the solar flux at Earth. UV or ionizing radiation can be tolerated by many microorganisms at very high levels and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen may limit habitability. Levels of O 2 over a few percent on an exoplanet would be consistent with the presence of multicellular organisms and high levels of O 2 on Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and salinity are likely to vary and not limit life over an entire planet or moon.extremophiles | Mars | astrobiology T he list of exoplanets is increasing rapidly with a diversity of masses, orbital distances, and star types. The long list motivates us to consider which of these worlds could support life and what type of life could live there. The only approach to answering these questions is based on observations of life on Earth. Compared with astronomical targets, life on Earth is easily studied and our knowledge of it is extensive--but it is not complete. The most important area in which we lack knowledge about life on Earth is its origin. We have no consensus theory for the origin of life nor do we know the timing or location (1). What we do know about life on Earth is what it is made of, and we know its ecological requirements and limits. Thus, it is not surprising that most of the discussions related to life on exoplanets focus on the requirements for life rather than its origin. In this paper we follow this same approach but later return briefly to the question of the origin of life. Limits to LifeThere are two somewhat different approaches to the question of the limits of life. The first approach is to determine the requirements for life. The second approach is to determine the extreme environments in which adapted organisms-often referred to as extremophiles-can survive. Both perspectives are relevant to the question of life on exoplanets.It is useful to categorize the requirements for life on Earth as four items: energy, carbon, liquid water, and various other elements. These are listed in Table 1 along with the occurrence of these factors in the Solar System (2). In our Solar System it is the occurrence of liquid water that appears to limit the occurrence of habitable environments and this appears to be the case for exoplanetary systems as well.From basic thermodynamic considerations it is clear that life ...

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Section: Titan Lifementioning

confidence: 99%

Requirements and limits for life in the context of exoplanets

McKay

2014

Proc. Natl. Acad. Sci. U.S.A.

124

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“…Below, I determine this minimum mass for stars to host potentially habitable exomoons. The P ps /P * p 1/9 constraint is used as a working hypothesis, backed up by numerical studies (Barnes & O'Brien 2002;Domingos et al 2006;Cassidy et al 2009) and observations of solar system satellites. But more extreme scenarios such as retrograde, Super-Ganymede exomoons in a wide orbit about planets in the IHZ of low-mass stars are theoretically possible (Namouni 2010).…”

Section: Orbital Stabilitymentioning

confidence: 99%

“…So far, no extrasolar moon has been confirmed, but dedicated surveys are underway (Kipping et al 2012). Several studies have addressed orbital stability of extrasolar satellite systems (Donnison 2010;Weidner & Horne 2010) and tidal heating has been shown to be an important energy source in satellites (Reynolds et al 1987;Scharf 2006;Cassidy et al 2009). In a recent study (Heller & Barnes 2012, HB12 in the following), we have extended these concepts to the illumination from the planet, i.e.…”

Section: Introductionmentioning

confidence: 99%

Exomoon habitability constrained by energy flux and orbital stability

Heller

2012

A&A

106

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Context. Detecting massive satellites that orbit extrasolar planets has now become feasible, which led naturally to questions about the habitability of exomoons. In a previous study we presented constraints on the habitability of moons from stellar and planetary illumination as well as from tidal heating. Aims. Here I refine our model by including the effect of eclipses on the orbit-averaged illumination. I then apply an analytic approximation for the Hill stability of a satellite to identify the range of stellar and planetary masses in which moons can be habitable. Moons in low-mass stellar systems must orbit their planet very closely to remain bounded, which puts them at risk of strong tidal heating. Methods. I first describe the effect of eclipses on the stellar illumination of satellites. Then I calculate the orbit-averaged energy flux, which includes illumination from the planet and tidal heating to parametrize exomoon habitability as a function of stellar mass, planetary mass, and planet-moon orbital eccentricity. The habitability limit is defined by a scaling relation at which a moon loses its water by the runaway greenhouse process. As a working hypothesis, orbital stability is assumed if the moon's orbital period is less than 1/9 of the planet's orbital period. Results. Due to eclipses, a satellite in a close orbit can experience a reduction in orbit-averaged stellar flux by up to about 6%. The smaller the semi-major axis and the lower the inclination of the moon's orbit, the stronger the reduction. I find a lower mass limit of ≈0.2 M for exomoon host stars that allows a moon to receive an orbit-averaged stellar flux comparable to the Earth's, with which it can also avoid the runaway greenhouse effect. Precise estimates depend on the satellite's orbital eccentricity. Deleterious effects on exomoon habitability may occur up to ≈0.5 M if the satellite's eccentricity is 0.05. Conclusions. Although the traditional habitable zone lies close to low-mass stars, which allows for many transits of planet-moon binaries within a given observation cycle, resources should not be spent to trace habitable satellites around them. Gravitational perturbations by the close star, another planet, or another satellite induce eccentricities that likely make any moon uninhabitable. Estimates for individual systems require dynamical simulations that include perturbations among all bodies and tidal heating in the satellite.

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“…This planet needs to orbit the star beyond several AU, where alternative energy sources are required on its putative moons to keep their surfaces habitable, i.e., to prevent freezing of H 2 O. This heat could be generated by (1) tides (Reynolds et al 1987;Scharf 2006;Cassidy et al 2009;Heller & Barnes 2013); (2) planetary illumination (Heller & Barnes 2015); (3) release of primordial heat from the moon's accretion (Kirk & Stevenson 1987); or (4) radiogenic decay in its mantle and/or core (Mueller & McKinnon 1988). All of these sources tend to subside on a Myr timescale, but (1) and (2) can compete with stellar illumination over hundreds of Myr in extreme, yet plausible, cases.…”

Section: Evolution Of Exomoon Habitabilitymentioning

confidence: 99%

Transits of extrasolar moons around luminous giant planets

Heller

2016

A&A

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Beyond Earth-like planets, moons can be habitable, too. No exomoons have been securely detected, but they could be extremely abundant. Young Jovian planets can be as hot as late M stars, with effective temperatures of up to 2000 K. Transits of their moons might be detectable in their infrared photometric light curves if the planets are sufficiently separated ( 10 AU) from the stars to be directly imaged. The moons will be heated by radiation from their young planets and potentially by tidal friction. Although stellar illumination will be weak beyond 5 AU, these alternative energy sources could liquify surface water on exomoons for hundreds of Myr. A Mars-mass H 2 O-rich moon around β Pic b would have a transit depth of 1.5 × 10 −3 , in reach of near-future technology.

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Massive Satellites of Close-in Gas Giant Exoplanets

Cited by 70 publications

References 44 publications

Requirements and limits for life in the context of exoplanets

Requirements and limits for life in the context of exoplanets

Exomoon habitability constrained by energy flux and orbital stability

Transits of extrasolar moons around luminous giant planets

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