Detecting Potentially Habitable Exomoons Around Young Gas Giants

Transits of extrasolar moons around luminous giant planets

Authors:

Heller et al

Abstract:

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 H2O-rich moon around β Pic b would have a transit depth of 1.5×10−3, in reach of near-future technology.

Exomoons in the Habitable Zone may Need to be at Least .5 Earth Mass to Retain an Significant Atmosphere

Origin and Stability of Exomoon Atmospheres - Implications for Habitability

Authors:

Lammer et al

Abstract:

We study the origin and escape of catastrophically outgassed volatiles (H2O, CO2) from exomoons with Earth-like densities and masses of 0.1M⊕, 0.5M⊕ and 1M⊕ 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 less than 0.25M⊕ 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 ∼0.25−0.5M⊕ may evolve to Mars-like habitats. More massive bodies with masses greater than 0.5M⊕, 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.

Explaining the Formation of the Galilean Moons and Titan in the Grand Tack Scenario

The formation of the Galilean moons and Titan in the Grand Tack scenario

Authors:

Heller et al

Abstract:

In the "Grand Tack" (GT) scenario for the young solar system, Jupiter formed beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered an orbital resonance with Saturn. Both planets then supposedly migrated outward for several 105 yr, with Jupiter ending up at ~5 AU. The initial conditions of the GT and the timing between Jupiter's migration and the formation of the Galilean satellites remain unexplored. We study the formation of Ganymede and Callisto, both of which consist of ~50% water and rock, respectively, in the GT scenario. We examine why they lack dense atmospheres, while Titan is surrounded by a thick nitrogen envelope. We model an axially symmetric circumplanetary disk (CPD) in hydrostatic equilibrium around Jupiter. The CPD is warmed by viscous heating, Jupiter's luminosity, accretional heating, and the Sun. The position of the water ice line in the CPD, which is crucial for the formation of massive moons, is computed at various solar distances. We assess the loss of Galilean atmospheres due to high-energy radiation from the young Sun. Ganymede and Callisto cannot have accreted their water during Jupiter's supposed GT, because its CPD (if still active) was too warm to host ices and much smaller than Ganymede's contemporary orbit. From a thermal perspective, the Galilean moons might have had significant atmospheres, but these would probably have been eroded during the GT in < 105 yr by solar XUV radiation. Jupiter and the Galilean moons formed beyond 4.5 (+/-0.5) AU and prior to the proposed GT. Thereafter, Jupiter's CPD would have been dry, and delayed accretion of planetesimals should have created water-rich Io and Europa. While Galilean atmospheres would have been lost during the GT, Titan would have formed after Saturn's own tack, because Saturn still accreted substantially for ~106 yr after its closest solar approach, ending up at about 7 AU.