Where should we look for life on other planets? Even as we're contemplating sending telescopes to space to look, the answer is becoming more complicated. The incredible pace of discovery that's sent the number of exoplanets to well over 3,000 has also inspired people to think more carefully about what might be involved in making a planet habitable.

The traditional definition simply involves the possible presence of liquid water on a planet's surface. But even figuring out that turns out to be fiendishly complicated. In the latest attempt to provide some perspective on the challenge, a paper in PNAS tackles the question of what might make a planet habitable by stepping back and looking at the problem afresh.

The paper backs all the way off to considering whether water should be a requirement at all. There have been some suggestions that the atmosphere of Saturn's moon Titan shows signs of a chemical process that could be consistent with life. But the paper's authors argue that, until we actually confirm that there's life on Titan, the whole concept of hydrocarbon-based life should be considered speculative. We shouldn't ignore any data that comes in that might be consistent with that, but it's probably not something we should spend time and money to go out and look for.

The outer limits

With that out of the way, the authors turn to water, which stays liquid within a narrow temperature range. The outside of that range is set empirically by early Mars, which clearly had liquid water flowing on its surface even though the planet is now too cold for that. The explanation for the early warmth? A thick atmosphere rich in greenhouse gasses.

Initially, people considered the outer limit of this greenhouse warming to be the temperature at which carbon dioxide starts to freeze out of the atmosphere anyplace close to a star. But models suggest that the initial CO 2 clouds would have a warming effect on their own, extending the habitable zone even further. Now, the paper argues, we should consider the outer limit to be where the greenhouse impact is maximized, or 1.67 Astronomical Units for a Sun-like star. That's somewhat closer than the 1.77 AU where Mars resides.

What could push things further? Hydrogen. Once H 2 starts bumping into other molecules in the atmosphere, its absorption spectrum spreads out to cover the entire thermal infrared spectrum. Unlike carbon dioxide, it's not going to freeze out of the atmosphere. Mars isn't big enough to hang on to hydrogen in its atmosphere, making any warming driven by it temporary. But calculations suggest that a heavy planet with a hydrogen-rich atmosphere could remain habitable indefinitely out to roughly Saturn's orbit.

Unfortunately, these would be so far out from the host star that they'd receive very little light to reflect, making them extremely difficult to spot unless we built an enormous telescope. So the authors recommend not bothering.

On the inside

Things are quite a bit more complicated on the inner side of the habitable zone. Venus again sets an empirical limit since it wouldn't have had water present any time within the last billion years (Venus is at 0.75 AU). Farther out, water becomes part of the problem, as warm temperatures put it into the atmosphere where it has a potent greenhouse effect. Close enough to the star, and you get a runaway greenhouse, with temperatures sufficient to boil off all the planet's water. This happens at 0.84 AU for a Sun-like star; it will shift depending on the brightness of the host star.

But problems can happen at even lower temperatures. If water vapor enters the stratosphere, then UV radiation can break its bonds, allowing hydrogen to escape off into space. That's calculated to happen at 0.95 AU, which suggests that Earth is dangerously close to losing its water.

The authors term these estimates "pessimistic." They rely on what's called a 1-D model of the planetary atmosphere, in effect a single core through its entire height. In these models the entire lower atmosphere of the planet behaves identically, becoming saturated with water vapor. On Earth, this doesn't actually happen, as there are spatial differences in how the water rises into the atmosphere and falls back to Earth that ensure the lower atmosphere never gets saturated. The authors of the article note that this sort of behavior would show up in a 3D atmospheric model, and we'd do well to consult those before defining the inner edge of the habitable zone.

The other option the authors consider is a Dune-style desert planet (yes, Frank Herbert's books have entered the scientific lexicon). A dry enough planet might keep from overheating, while still having liquid water near the poles. Whether it does or not would depend on its geology, however, as minerals could lock up some of the water or keep it circulating through a sub-surface hydrological cycle. Plate tectonics would also circulate some of the water through subduction and volcanism. All of which could mean that water at the poles could be a temporary thing, much as ice at the poles has been throughout our planet's history.

What to look for

All of this should feed into the design of whatever telescope we send up to search for life on other planets, according to the authors of the paper. We now have a reasonable ability to make statistical inferences about how many planets are likely to be orbiting nearby stars. To figure out the telescope's requirements, we need to combine those with estimates of how many of those planets might be habitable and how close they'd be to the host star.

The news here is good. There are lots of planets, and most of them will be orbiting close enough to stars that we'd have a decent chance of spotting the light reflected off them provided we build the right hardware.

The problem is that we're getting less and less sure of how to set rules for what to look for. Small molecules that are associated with life might make their way into the atmosphere but not at high enough levels to be detectable. This leaves us looking at major gasses like oxygen and methane, both of which are present in our atmosphere. Left on their own, the oxygen would convert the methane to water and CO 2 over time. It's only because life continuously produces each that we've got them both in our atmosphere.

The problem is that, given the right atmospheric components and surface chemistry, a planet could produce almost any combination of gasses we can think of. So, coming up with a definitive idea about what we should be looking for might be the biggest roadblock to finding life on another planet.

But that only affects the data interpretation, not the data collection. An argument could be made—although the authors of this paper don't do so—that it's worth sending something up and seeing what our neighboring planets' atmospheres look like. The scientists can argue about life once the data's in hand.

PNAS, 2013. DOI: 10.1073/pnas.1309107110 (About DOIs).