It has been conventional wisdom that the best candidates for a habitable planet in another star system would be one that is approximately Earth-sized orbiting around a star's "habitable zone" where liquid water is possible.
However Forbes has a report on a new paper being published in the Monthly Notices of the Royal Astronomical Society that suggests that another candidate would be an Earth-sized moon orbiting a Jupiter-sized planet outside the habitable zone.
An astronomy website defines a habitable zone as that area near a sun where a planet of the right size and right mass would be able to contain liquid water, a prerequisite to life as we know it. Recently NASA's Kepler Space Telescope discovered three planets slightly larger than Earth orbiting their respective stars' habitable zones, holding out the possibility that they might harbor life.
The paper that Forbes reports on postulates another scenario in which a celestial body would contain liquid water and perhaps life. This would involve an Earth-sized moon orbiting a Jupiter-sized planet outside the normal habitable zone of a particular star system. The authors of the paper, astronomers Duncan Forgan of the Royal Observatory in Edinburgh and David Kipping of the Harvard Smithsonian Center for Astrophysics, suggest that a moon of the right size and mass in an eccentric orbit around a Jupiter-sized world would be warm enough to support life because of heating caused by tidal forces. There are four factors that affect the potential habitability of such a moon, besides size and mass, Kipping said: "How far away the moon's planet is from its star; how eccentric the planet's orbit is around the star; how far away the moon is from the planet; how eccentric the moon's orbit is around the planet."
Examples: Europa and Enceladus
Because of observations of planetary probes such as Galileo, which went to Jupiter, and Cassini, currently orbiting Saturn, examples of tidal forces heating moons have already been observed. Europa, orbiting Jupiter, and Enceladus, orbiting Saturn, are both thought to be warmed by tidal heating processes, resulting in liquid oceans and perhaps life of some sort underneath icy crusts.
An objection to the theory
Forbes notes that Jack Lissauer, a planetary scientist at NASA Ames Research Center believes that unless there is some kind of orbital resonance between two massive moons, the tidal heating would not last over geological periods. Kipping responds that that Jupiter's Galilean moons have already been shown to have such resonance, leading to the sort of tidal heating necessary for life to develop over a geologic time scale.
Mark R. Whittington is the author of Children of Apollo, The Last Moonwalker, and Dreams of Barry's Stepfather. He has written on space subjects for a variety of periodicals, including The Houston Chronicle, The Washington Post, USA Today, the L.A. Times, and The Weekly Standard.
- Space & Astronomy
- habitable planet