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ABOUT

Pathways Towards Exomoons is a one-day satellite meeting at the Pathways 2015: Pathways Towards Habitable Planets conference, hosted at the University of Bern, Switzerland on July 15th 2015. The objective of the meeting is to bring the exomoon community together to discuss the diverse pathways towards discovering exomoons, considerations from theory as to where we should look and what the possibilities for habitability may be on such worlds. We hope that this meeting will start a collaborative community and lead to further future meetings to keep the community regularly in contact.

We will host a one-day meeting from 2:00pm to 4:30pm on the third day of the main conference in Lecture Room 106. There will be a number of 12+3 minute talks from invited speakers, selected to try and provide the audience with a broad range of different approaches to solving the exomoon problem. The satellite meeting is being organized by
HEK Principal Investigator David Kipping, who will serve as chair. Students can find information about financial support on the main conference website.

A poster is available
here, which we encourage you to download and put on your institute’s notice board.

SCHEDULE (15 JULY 2015)

1400 - 1410:
Welcome

I. FORMATION & EVOLUTION

1410 - 1425: Stephen Kane: Solar Systems Moons as Exoplanet Analogs
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The field of exoplanetary science has experienced a recent surge of new systems that is largely due to the precision photometryprovided by the Kepler mission. Discoveries have included compact planetary systems in which the orbits of the planets all lie relatively close to the host star, which presents interesting challenges in terms of formation and dynamical evolution. The compact exoplanetary systems are analogous to the moons orbiting the giant planets in our Solar System, in terms of their relative sizes and semi-major axes. In this talk I will present a study that quantifies the scaled sizes and separations of the Solar System moons with respect to their hosts, along with a similar study for a large sample of confirmed Kepler planets in multi-planet systems. The results of this study show that a comparison between the two samples leads to a similar correlation between their scaled sizes and separation distributions. The different gradients of the correlations may be indicative of differences in the formation and/or long-term dynamics of moon and planetary systems.

1425 - 1440: Yu-Cian Hong: Orbital Stability of Exomoons
Planet-planet scattering is currently the best candidate mechanism for explaining the eccentric orbits of observed exoplanets. The dynamical evolution of the primordial satellites of giant planets are simulated in systems that experience strong planetary close encounters. Planet oblateness is incorporated in order to avoid unrealistic instability effect associated with secular resonance between the satellites’ and the planet’s nodal precession. During a close encounter, orbital energy and angular momentum are exchanged between planets and satellites. Common outcomes include the destruction of satellites by ejection out of the system and collision with planets and the star, and satellites on wilder planet-bound orbits. A small fraction of interesting outcomes include satellite exchange between the planets, and satellites bound to ejected free-floating planets. Differences in the satellites’ distribution in the orbital parameter space are imprints of their dynamical histories. Correlation between the survival rate of satellites with various system parameters and properties of close encounters will be discussed.

1440 - 1455: Hagai Perets: Formation and Evolution of Moons & Exomoons
Moons orbiting gas and ice giants are likely formed through collisional growth of planetesimals in circumplanetary disks. Previous studies have shown that the observed (regular) co-planar satellite systems orbiting the Solar system gas and ice giants can be formed in circumplanetary disks, where both accretion and collisions as well as gas drag/migration play important role in the evolution. These studies did not produce irregular, eccentric and/or highly inclined moons. Our studies show that similar in-situ formation of moons can produce both the regular satellites as well as retrograde and high eccentricity satellites such as Triton, when allowing for satellites growth in the external regions of the circumplanetary disks (neglected before) in some cases. Using the same models, we also explore what are the largest moons that can be formed around gas giants, when allowing for more massive circumplanetary disks and/or more massive planets than the Solar system planets; which could be relevant for exomoon formation conditions. We find that relatively large moons can be formed, up to Mars size moons. To date none of our simulations produce Earth-size or larger moons, but further studies of the phase space involved are yet to be done.

II. HABITABILITY

1455 - 1510: Vera Dobos: Tidally Heated Exomoons
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Significant tidal heating on exomoons cause elevated temperature that can melt the ice on the surface. Hence tidal heating may play a key role in their habitability even without significant solar radiation. The Solar System provides examples for tidally heated moons, such as Europa or Enceladus, where the surface ice sheet covers water ocean that is tidally heated. The possibility of life is intensely studied on these satellites. Tidal forces may be even stronger in extrasolar systems, depending on the properties of the moon and its orbit. For studying the surface temperature of tidally heated exomoons, I applied a viscoelastic model for the first time. This model includes the temperature dependency of the tidal heat flux, and the melting of the inner material, unlike the widely used, so-called fixed Q models. Using the more realistic viscoelastic model I introduced the Tidal Temperate Zone (TTZ), which is the region around a planet where the surface temperature of the satellite is between 0 and 100°C, not considering other energy sources than tidal heating. The location and width of the TTZ depends on the orbital period, eccentricity, density and radius of the moon. I made similar calculations with the fixed Q model and investigated the statistical volume of the TTZ using both models. I have found that the viscoelastic model predicts 2.8 times more exomoons in the TTZ than the fixed Q model with orbital periods between 0.1 and 3.5 days for plausible distributions of physical and orbital parameters. The viscoelastic model gives more promising results in terms of habitability, because the inner melting of the body moderates the surface temperature, acting like a thermostat.

1510 - 1525: Duncan Forgan: Exomoon Climates
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Simple calculations of the orbit averaged flux received by Earth-like exomoons orbiting giant planets suggest that the circumplanetary region has an inner habitable edge defined by tidal heating, and no discernible outer edge (at least, until the orbital stability limit is reached at around a third of the planet's Hill Radius). In previous work using 1D climate models that account for atmospheric circulation and ice-albedo feedback processes, we identified an outer circumplanetary habitable edge that is present if the moon's orbit is coplanar to the planet's. Eclipses appear to produce a sufficiently long drop in stellar radiative flux that the moon transitions to a snowball state from which it cannot escape. I will present our recent attempts to improve the climate model, by adding the carbonate-silicate cycle and viscoelastic tidal heating. Will the snowball state be avoided by allowing the moon to regulate its CO2 partial pressure? Will temperature-dependent rigidity/tidal dissipation extend the circumplanetary habitable region to the orbital stability limit? I will address these questions, and consider some routes for future work in exomoon climate modelling.

1525 - 1540: Kristina Kisiyakova: Origin & Stability of Exomoon Atmospheres
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We study the origin and escape of catastrophically outgassed volatiles (H20, CO2) from exomoons with Earth-like densities and masses of 0.1, 0.5 and 1 Earth masses orbiting an extra-solar gas giant inside the habitable zone of a young active solar-like star. We apply a radiation absorption and hydrodynamic upper atmosphere model to the three studied exomoon cases. We model the escape of hydrogen and dragged dissociation products O and C during the activity saturation phase of the young host star. Because the soft X-ray and EUV radiation of the young host star may be up to 100 times higher compared to today's solar value during the first 100 Myr after the system's origin, an exomoon with a mass <0.25 Earth masses located in the HZ may not be able to keep an atmosphere because of its low gravity. Depending on the spectral type and XUV activity evolution of the host star, exomoons with masses between ~0.25-0.5 Earth masses may evolve to Mars-like habitats. More massive bodies with masses >0.5 Earth masses, however, may evolve to habitats that are a mixture of Mars-like and Earth-analogue habitats, so that life may originate and evolve at the exomoon's surface.

III. DETECTION

1540 - 1555: Brianna Lacy: Detecting Exomoons through Imaging
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Direct imaging of extrasolar planets with future space-based coronagraphic missions may provide a means of detecting companion moons for exoplanets with bands of strong atmospheric absorption. Although currently proposed telescopes will not have the angular resolution necessary to spatially resolve a planet and its moon, the angular shift of a point spread function centroid can be measured to precisions better than the angular resolution of the telescope yielding information at angular scales nearer to that of exoplanet-exomoon separation. At wavelengths of light where an exomoon outshines its host due to absorption in the planet's atmosphere, the centroid will shift towards the exomoon. We thus propose a detection strategy based on the variation of the center of light with wavelength, “spectroastrometry," which can both yield a detection of an exomoon and allow for characterization of the moon's orbit and mass of the exoplanet. To explore this detection method, we consider two model systems: an earth moon analogue and a warm Jupiter with earth as its moon. We discuss the instrumentation requirements for detection of these model systems around nearby stars, as well as characterization of the moon-planet systems that may be identified with this approach.

1555 - 1610: David Kipping: Detecting Exomoons through Transits
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One of the first ways proposed to discover exomoons is via the powerful transit technique. Armed with a mature theory and volumes of data from transit survey missions such as Kepler, exomoon hunting via transits is becoming an established enterprise. I’ll discuss the various approaches to finding exomoons in transit and the latest results from the Hunt for Exomoons with Kepler project. An outlook of the potential to discover exomoons with future missions will also be presented.

1610 - 1625: Nadar Haghighipour: Hunt for exomoons- formation, evolution & detection
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In the past few years, exomoons have been searched for extensively and are yet to be verified through either transit photometry or gravitational lensing. From the viewpoint of detectability, the best opportunity to observe an exomoon requires a large mass and/or size ratio between planet and moon. In that respect, the newest category of planets, Super-Earths and Mini-Neptunes, are the best candidates to look for exomoons. Massive satellites of these bodies would also provide an alternate environment for habitability within these systems. In this talk, I will preset the results of our recent study and identify regions of the parameter space where exomoons can successfully form around super-Earth and mini-Neptunes, and be detectable using transit photometry and transit timing variations method.

1625 - 1630: Jean-Philippe Beaulieu: Detecting Exomoons through Microlensing
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A brief discussion of the recent claims of examine candidates through microlensing.