Wednesday, May 20, 2015

Large Water-Rich Moons around Super-Jovian Planets

Large moons similar in size to the planet Mars (0.53 Earth radii) or Jupiter’s moon Ganymede (0.41 Earth radii) should be detectable in the available dataset of photometric measurements by NASA’s Kepler space telescope. Super-Jovian planets (i.e. planets more massive than Jupiter) appear to be abundant at ~1 AU around Sun-like stars (i.e. 1 AU is the distance of Earth from the Sun). At these distances, temperatures are just right for large moons around these planets to potentially support life. As a result, it is worth considering whether super-Jovian planets at ~1 AU around Sun-like stars can host large moons that are habitable.

Figure 1: Artist’s impression of a gas giant planet like Jupiter with a system of moons orbiting it.

A study by Heller & Pudritz (2015) predicts that water-rich Mars-mass moons can form around super-Jovian planets that lie beyond ~5 AU around Sun-like stars. Jupiter-mass planets that form closer than ~4.5 AU to Sun-like stars are unlikely to form water-rich moons because their accretion disks are less massive and depleted of water-ice. However, a super-Jovian planet with 12 times the mass of Jupiter can potentially form water-rich moons as close as ~3 AU to a Sun-like star as its accretion disk is significantly more massive.

A super-Jovian planet with 10 times the mass of Jupiter forming at 5.2 AU around a Sun-like star can form a moon system with a total mass of roughly 10 times the mass of Ganymede or 2 times the mass of Mars. Simulations indicate that this mass is distributed over 3 to 6 moons in ~90 percent of the cases. If the same planet formed at 9.6 AU, the distance of Saturn from the Sun, the total mass of its moon system can be twice as large.

Figure 2: Artist’s impression of a gas giant planet seen partly through the atmosphere of a large moon in orbit around it.

Figure 3: Artist’s impression of a large, potentially habitable moon in orbit around a gas giant planet.

Due to various interactions, planets like Jupiter tend to migrate away from where they initially form. It is more plausible that observed population of super-Jovian planets at ~1 AU around Sun-like stars formed further out and migrated inwards to their current positions. If these super-Jovian planets formed beyond about 3 to 4.5 AU, they should host water-rich Mars-sized moons. At ~1 AU from a Sun-like star, such a moon can be a potentially habitable ocean world. The abundance of super-Jovian planets at ~1 AU around Sun-like stars could mean that Mars-mass ocean worlds are a common type of habitable environment in the universe.

A super-Jovian planet hosting two or more habitable moons can have interesting astrobiological implications because life on these moons can share a common evolutionary tree of life. This is because a system of moons around a super-Jovian planet is much more compact than a system of planets around a star. As a consequence, it is much easier for the process of panspermia to transfer biological material from one habitable moon to another.

In the less plausible scenario that super-Jovian planets at ~1 AU around Sun-like stars formed in-situ, then they are less likely to host water-rich moons. The absence or presence of water-rich Mars-sized moons orbiting super-Jovian planets at ~1 AU around Sun-like stars can indicate whether these planets tend to form in-situ or tend to migrated to their current positions from further out.

Heller & Pudritz (2015), “Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1 - 20 AU from Sun-like stars”, arXiv:1504.01668 [astro-ph.EP]