Posted: October 14, 2008
When looking for rocky planets that could support life in other solar systems, astronomers should consider looking outside the ‘Goldilock’s Zone’ of habitability, say scientists presenting their research at the Division of Planetary Sciences meeting.
The so-called habitable zone of a solar system defines the distance from the parent star where conditions are just right for life to prevail. For our own Solar System, planets like Venus and Mercury are too hot for life, and those in the outer Solar System are far too cold. Earth is the only planet awash with liquid water, one of the requisites for life as we know it.
MOA-2007-BLG-192Lb was the first 'small’ exo-Earth to be discovered at 3.3 Earth masses (see our report on this discovery here). It orbits a brown dwarf star with a mass just six percent of our Sun. Scientists speculate that it is a rock-ice planet, possibly with a liquid ocean covering its surface. Image: NASA.
But there is a significant difference between our Solar System and extrasolar systems. While our planets move around the Sun with relatively circular orbits, the majority of extrasolar planets found to date have highly elongated orbits. This causes the planet to be stretched by tides when it is near the star, and less when the planet is farther from its star. The resulting friction generates internal heat, which drives the planet's geophysical processes such as volcanic activity or plate tectonics.
A similar process happens on Io, one of Jupiter’s moons, which is locked into an orbit influenced by the gravity of the other Galilean satellites - Europa, Ganymede and Callisto - and of course Jupiter itself. Jupiter pulls Io inward toward itself, while the gravity of the outer moons pull it in the opposite direction. These opposing forces cause the distance between Io and Jupiter to vary, making Io's orbit slightly elliptical. As a result, Io is subjected to tremendous tidal forces that alternately squeeze and stretch its interior, causing the surface to rise and fall by up to 100 metres a day and forcing molten material and gases out through fractures in the crust and onto the surface as lava. Similarly, this tidal heating is thought to maintain a liquid ocean underneath Europa’s icy shell.
This heating process could create favourable conditions on exoplanets that might otherwise be too cold to sustain life, say Brian Jackson, Rory Barnes and Richard Greenberg of the University of Arizona's Lunar and Planetary Laboratory. As predicted for Europa, tidal heating on an exoplanet could warm that planet’s interior enough to create a liquid ocean. Tidal heating can also enhance outgassing of volatiles that contribute or replenish a planet's atmosphere through volcanism. Tidal heating also drives plate tectonics, a mechanism that checks excessive carbon dioxide from accumulating in a planetary atmosphere, producing the kind of deadly greenhouse atmosphere found on Venus.
Io is the most volcanically active planet in the Solar System, thanks to extensive tidal heating brought about by the push-pull of Jupiter and the Galilean Satellite's gravitational attraction. Image: The Galileo Project, JPL, NASA..
"Our study shows that tidal heating could produce enough heat to drive plate tectonics for billions of years, long enough for life to appear and flourish," Jackson said.
But while this heating process could add the missing ingredient for life to a planet, it could just as easily work against a potentially habitable planet. If the recently discovered ‘super-Earths’, which are about 2 to 10 times as massive as Earth are indeed rocky, tidal heating resulting from their irregular orbits may be great enough to melt them to produce volcanism on par with Io.
"Tidal heating scales with planet mass, so we expect that most easily detectable super-Earths will be dominated by volcanic activity," says Jackson. "That's one of our first conclusions from this work, that the first Earth-like planets found are probably going to be strongly heated and have big volcanoes. Even if Earth-like planets are found within the habitable zone, they may not be habitable because they will be overwhelmed by this tidal heating."