The nice thing about our conventional idea of a habitable zone is that liquid water can exist on the surface. The less helpful part of that definition is that water is more readily available much further out in a planetary system, where it usually shows up as ice. Think in terms of the ‘ice line,’ or the ‘snow line.’ Beyond it is the area around the still-forming star where temperatures are low enough to allow hydrogen compounds to condense into ice grains.
Of course, we’re living proof of the fact that planets in the inner system can be covered with oceans. It’s therefore plausible to think in terms of delivery mechanisms, with icy comets bombarding planets in the inner system to produce oceans like those on Earth. But we’re learning to extend our reach beyond conventional habitable zone notions to places much further out, an idea recently given credence by divers hands.
Consider the work of Scott Gaudi (Ohio State), Eric Gaidos (University of Hawaii) and Sara Seager (MIT), familiar names to long-time Centauri Dreams readers. Recognizing the wealth of water resources in outer solar systems, the trio look to cold super-Earths, planets whose water did not have to be delivered by external means. An internal heat source might keep a liquid water ocean viable under the ice, assuming a massive world in the right place, even if that planet were five times farther out than the Earth.
“It turns out that if super-Earths are young enough, massive enough, or have a thick atmosphere, they could have liquid water under the ice or even on the surface,” Gaudi said. “And we will almost certainly be able to detect these habitable planets if they exist.”
By ‘massive enough,’ Gaidos is talking about a super-Earth ten times as massive as the Earth. The scientist reported these results at the American Geophysical Union meeting in San Francisco on Monday. The issue of detection seems clear enough — we’re making such strides in finding exoplanets that tracking down new super-Earths is more or less a given, especially since some are saying that a third of all solar systems probably contain them.
Right now, gravitational microlensing seems to be the best method for detecting planets at 5 AU or more. The planetary signature is found in changes to the magnification caused when a star passes in front of a more distant one as seen from Earth. A planet around the nearer star creates a secondary boost in the lens-like magnification, allowing not just detections at some distance from the star, but also detections around stars much farther away than would be feasible using radial velocity or transit methods. Even so, recent direct imaging successes remind us that the next generation of telescopes may also deliver many a super-Earth.
Proving the astrobiological case for these super-Earths is a tricky matter indeed. It may well take a dedicated lander on the surface of Europa, for example, to tell us about possible life there by drilling into the ice. How do we resolve the question of life on a distant super-Earth? The issue will remain open for years to come, but in short order we’re going to be finding so many of these interesting worlds that we’ll have plenty to speculate about when it comes to life’s formation around other stars.