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Exomoon Habitability and Tidal Evolution in Low-Mass Star Systems

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Exomoon Habitability and Tidal Evolution in Low-Mass Star Systems MNRAS 000,1{20 (2017) Preprint 25 July 2017 Compiled using MNRAS LATEX style file v3.0 Exomoon Habitability and Tidal Evolution in Low-Mass Star Systems Rhett R. Zollinger,1? John C. Armstrong,2y Rene´ Heller3z 1Southern Utah University, Cedar City, Utah 84720, USA 2Weber State University, Ogden, Utah 84408, USA 3Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 G¨ottingen, Germany Accepted 2017 July 20. Received 2017 July 14; in original form 2016 August 9 ABSTRACT Discoveries of extrasolar planets in the habitable zone (HZ) of their parent star lead to questions about the habitability of massive moons orbiting planets in the HZ. Around low-mass stars, the HZ is much closer to the star than for Sun-like stars. For a planet- moon binary in such a HZ, the proximity of the star forces a close orbit for the moon to remain gravitationally bound to the planet. Under these conditions the effects of tidal heating, distortion torques, and stellar perturbations become important considerations for exomoon habitability. Utilizing a model that considers both dynamical and tidal interactions simulta- neously, we performed a computational investigation into exomoon evolution for sys- tems in the HZ of low-mass stars (. 0:6 M ). We show that dwarf stars with masses . 0:2 M cannot host habitable exomoons within the stellar HZ due to extreme tidal heating in the moon. Perturbations from a central star may continue to have delete- rious effects in the HZ up to ≈ 0:5 M , depending on the host planet's mass and its location in the HZ, amongst others. In addition to heating concerns, torques due to tidal and spin distortion can lead to the relatively rapid inward spiraling of a moon. Therefore, moons of giant planets in HZs around the most abundant type of star are unlikely to have habitable surfaces. In cases with lower intensity tidal heating the stel- lar perturbations may have a positive influence on exomoon habitability by promoting long-term heating and possibly extending the HZ for exomoons. Key words: planets and satellites: dynamical evolution and stability { planets and satellites: physical evolution 1 INTRODUCTION on the horizon (Kipping et al. 2009, 2012; Heller et al. 2014) now that modern techniques enable detections of sub-Earth The exploration of the moons of Jupiter and Saturn has pro- sized extrasolar planets (Muirhead et al. 2012; Barclay et al. vided immense understanding of otherworldly environments. 2013). A recent review of current theories on the formation, Some of these moons have reservoirs of liquids, nutrients, detection, and habitability of exomoons suggests that nat- and internal heat (Squyres et al. 1983; Hansen et al. 2006; ural satellites in the range of 0.1-0.5 Earth masses (i) are Brown et al. 2008; Saur et al. 2015) { three basic components arXiv:1707.07040v1 [astro-ph.EP] 21 Jul 2017 potentially habitable, (ii) can form within the circumplane- that are essential for life on Earth. Recent technological and tary debris and gas disk or via capture from a binary, and theoretical advances in astronomy and biology now raise the (iii) are detectable with current technology (Heller et al. question of whether life might exist on any of the moons be- 2014). Considering the potential for current observation, we yond the solar system (\exomoons"). Given the abundant explore the expected properties of such exomoons in this population of moons in our system, exomoons may be even paper. more numerous than exoplanets (Heller & Pudritz 2015). No moon outside the Solar System has been detected, When investigating planet habitability, the primary but the first detection of an extrasolar moon appears to be heat source is typically the radiated energy from a central star. Considerations of stellar radiation and planet surface temperatures have led to the adoption of a concept referred ? E-mail: [email protected] to as the stellar \habitable zone" (HZ) that is the region y E-mail: [email protected] around a star in which an Earth-like planet with an Earth- z E-mail: [email protected] like atmosphere can sustain liquid surface water (Kasting © 2017 The Authors 2 R. R. Zollinger et al. et al. 1993). Tidal heating is typically less important as an moon will slowly spiral outward. This action could eventu- energy source for planets in the HZ, though it might be rel- ally destabilize the moon's orbit, leading to its ejection. On evant for eccentric planets in the HZs around low-mass stars the other hand, if the planet's rotation period is longer, the of spectral type M (Barnes et al. 2008). For the moon of a bulge will lag and the moon will slowly spiral inward. As this large planet, tidal heating can work as an alternative inter- happens, the tidal forces on the moon become increasingly nal heat source as well. Several studies have addressed the greater. If the inward migration continues past the Roche importance of tidal heating and its effects on the habitability limit, the satellite can be disintegrated. of exomoons (Reynolds et al. 1987a; Scharf 2006; Henning (ii) Eccentricity: Non-circular orbits will be circularized et al. 2009; Porter & Grundy 2011a; Heller 2012; Forgan & over the longterm. The timescale for which the eccentricity Kipping 2013; Heller & Barnes 2013; Heller & Zuluaga 2013; is damped can be estimated as (Goldreich & Soter 1966) Heller & Barnes 2015; Dobos & Turner 2015; Dobos et al. 5 0 2017). Tidal heat can potentially maintain internal heat- 4 Ms a Qs τe ≈ ; (1) ing over several Gyr (see Jupiter's moon Io; Spencer et al. 63 Mp Rs n 2000a), which contributes to surface heating and potentially where n is the orbital mean motion frequency, Q0 is the tidal drives important internal processes such as plate tectonics. s dissipation function, a is the semi-major axis, Ms and Rs are Tidal interactions between a massive moon and its host the mass and radius of the satellite, and Mp the mass of the planet become particularly interesting for planets in the HZs host planet. of M dwarfs, which are smaller, cooler, fainter, and less mas- (iii) Rotation frequency: The moon will have its ro- sive than the Sun. They are the predominant stellar popula- tation frequency braked and ultimately synchronized with tion of our Galaxy (Chabrier & Baraffe 2000), and as such, its orbital motion around the planet (Dole 1964; Porter & these low-mass stars may be the most abundant planet hosts Grundy 2011b; Sasaki et al. 2012), a state that is commonly in our Galaxy (Petigura et al. 2013; Dressing & Charbon- known as tidal locking. neau 2015). Their lower core temperatures and decreased (iv) Rotation axis: Any initial spin-orbit misalignment energy output result in a HZ that is much closer to the (or obliquity) will be eroded, causing the moon's rotation star in comparison to Sun-like stars. If a planet in the HZ axis to be perpendicular to its orbit about the planet. In ad- around an M dwarf star has a massive moon, the close-in dition, a moon will inevitably orbit in the equatorial plane of orbital distances could potentially influence the orbit of the the planet due to both the Kozai mechanism and tidal evolu- moon around the planet (Heller 2012). The relatively close tion (Porter & Grundy 2011b). The combination of all these central star can serve as a continual source of gravitational effects will result in the satellite having the same obliquity perturbation to the moon's orbit, which has important im- as the planet with respect to the circumstellar orbit. As for plications to tidal heating and orbital evolution of moons in the host planet, massive planets are more likely to maintain these systems. their primordial spin-orbit misalignment than small planets With the possible abundance of planetary systems (Heller et al. 2011b). Therefore, satellites of giant planets around low-mass stars and their unique characteristics for are more likely to maintain an orbital tilt relative to the habitability, we are interested in exploring their potential star than even a single terrestrial planet at the same dis- for exomoon habitability. To accomplish this we employed a tance from a star. distinctive approach to simultaneously consider both orbital and tidal influences. In this paper, we review some theories The issue of tidal equilibrium in three-body hierarchical on tidal interactions between two massive bodies and then star-planet-moon systems in the Keplerian limit has recently conduct a computational investigation into the importance been treated by Adams & Bloch(2016), who demonstrated of these interactions on the long-term evolution of hypo- that a moon in tidal equilibrium will have its circumplane- thetical exomoons. For our study we focus on massive moons tary orbit widened until it ultimately gets ejected by stellar around large planets in the HZ of low-mass stars (. 0:6 M ) perturbations. N-body gravitational perturbations from the which demonstrates the long-term effect of gravitational per- star set an upper limit on the maximum orbital radius of the turbations from a central star. moon around its planet at about half the planetary Hill ra- dius for prograde moons (Domingos et al. 2006a). However, moons can be ejected from their planet even from close or- 2 DYNAMICAL AND TIDAL EVOLUTION bits around their planets, e.g. when the planet-moon system migrates towards the star and stellar perturbations increase Tidal bulges raised in both a planet and satellite will dissi- the moon's eccentricity fatally (Spalding et al. 2016). Per- pate energy and apply torques between the two bodies. The turbations from other planets can also destruct exomoon rate of dissipation strongly depends on the distance between systems during planet-planet encounters (Gong et al.
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