Public interest in habitable moons around gas giant planets received a powerful boost from the film Avatar, where a huge world in an Earth-like orbit (Polyphemus) is accompanied by the extraordinary moon Pandora. We have no detections of such moons — exomoons — but as we’ve seen in earlier posts here, David Kipping (Harvard-Smithsonian Center for Astrophysics) continues the hunt through the HEK project (Hunt for Exomoons with Kepler). HEK looks for transit timing variations (TTV) and transit duration variations (TDV), the kind of perturbations that a substantial satellite would create in the orbital motion of the larger world around its star.
While we wait for the first exomoon discovery — a moon down to about 0.2 Earth masses should be detectable with these methods — we’ve just gotten a look at exomoon issues from a new study of magnetic fields around giant planets. The work of René Heller (McMaster University) and Jorge Zuluaga (University of Antioquia, Colombia) finds that merely being in the habitable zone is hardly sufficient protection for any lifeforms that might develop on such moons. Not only are there issues relating to tidal heating and the transport of energy in the moon’s atmosphere, but the magnetic environment in which these moons would move could be a show-stopper.
Heller and Zuluaga’s new paper calculates the size of the magnetospheres of giant planets located in the habitable zone of their host stars. Being inside a planet’s magnetosphere can shield a moon’s surface from high-energy cosmic rays and the effects of the stellar wind from its star, but particles trapped in the magnetosphere itself can create their own problems. The paper notes that the net eﬀect on a moon’s habitability depends on its actual orbit, the extent of the planet’s magnetosphere, and other factors like the intensity of the stellar wind from the star.
Image: An artist’s concept of the Saturnian plasma sheet based on data from Cassini’s magnetospheric imaging instrument. Credit: NASA/JPL.
The researchers have pooled information about the formation and development of magnetic fields in both terrestrial and giant planets and use it to predict the intensity of those fields, based on models developed by Jonathan Fortney and his collaborators at the University of California. They chose to exclude planets around M dwarf stars because of flare activity and excluded G-class stars as being too bright and too massive to allow for exomoon detections in the near future. The compromise was to work with K-class dwarf stars of about 0.7 solar masses.
The work then considers Neptune-, Saturn- and Jupiter-class host planets. As to what may be detected in the ongoing exomoon hunt, the paper argues that moons roughly the mass and size of Mars are likely to exist and should prove detectable around K stars in the near future.
Having determined the scope of a magnetosphere, the other factor that comes into play is the distance of the exomoon from its host planet. Working with Rory Barnes (University of Washington), Heller has previously studied the minimum distance a moon could orbit while sustaining habitability despite the effects of tidal heating (see Assessing Exomoon Habitability for more on this recent work). Get too close to the planet — Barnes and Heller called this moving inside the ‘habitable edge’ — and runaway greenhouse effects can emerge. There is, in other words, a minimum distance an exomoon has to maintain from its planet to remain habitable.
But does the minimum distance conflict with the magnetosphere?
From the paper:
For modest eccentricities, we ﬁnd that satellites around Neptune-sized planets in the center of the HZ around K dwarf stars will either be in an RG [runaway greenhouse] state and not be habitable, or they will be in wide orbits where they will not be aﬀected by the planetary magnetosphere. Saturn-like planets have stronger ﬁelds, and Jupiter-like planets could coat close-in habitable moons soon after formation. Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically inﬂuence their habitability.
Perhaps the odds on finding a Pandora out there, around a Jupiter- or Saturn-class world, are not as good as we might hope. If far enough from its host planet to avoid runaway greenhouse issues and the disrupting effects of tidal heating, the exomoon could outrun the magnetic shielding of the parent world, exposing it to stellar and cosmic high-energy radiation. But Heller and Zuluaga acknowledge that the planet’s composition has much to say about conditions on any moon. The paper goes on to point to the direction of their future work on the subject:
Once a potentially habitable exomoon would be discovered, detailed interior models for the satellite’s behavior under tidal stresses would need to be explored. In a forthcoming study, we will examine the evolution of planetary dipole ﬁelds, and we will apply our methods to planets and candidates from the Kepler sample. Obviously, a range of giant planets resides in their stellar HZs, and these planets need to be prioritized for follow-up search on the potential of their moons to be habitable.
The paper is Heller and Zuluaga, “Magnetic shielding of exomoons beyond the circumplanetary habitable edge,” accepted at the Astrophysical Journal (preprint).