What is the best way to locate the galaxy's habitable planets?

We've officially found over 700 exoplanets, and with so many worlds to explore the big question is which, if any, could support life. But should we be looking for Earth-like planets — or is life hiding on far more alien worlds?

Based on our current rate of discovery, it's entirely probable that we will have found thousands of exoplanets by 2020. Most of these, of course, will be large gas planets and worlds in close proximity to their star, meaning the odds of them harboring life is very low. We're going to have to look a little harder to find planets that could potentially support life, and now an international team of researchers have developed a new pair of models to assess a planet's likelihood of supporting life.

Team member and Washington State astrobiologist Dirk Schulz-Makuch explains the two basic questions that have driven the development of their models:

"The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life. The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not. As a practical matter, interest in exoplanets is going to focus initially on the search for terrestrial, Earth-like planets."

To that end, Schulz-Makuch and his colleagues propose categorizing all new expolanets along two indices. First, there's the Earth Similarity Index, which does pretty much exactly what it says on the tin: assessing a planet in terms of its Earth-like features. Second, there's the Planet Habitability Index, which expands the planetary features under consideration to any that could conceivably support life, even if they're far from what we find on Earth.

Here's a breakdown of various objects' ESI, courtesy of Alan Boyle at Cosmic Log:

The Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as the size and temperature of its parent star, and compares those parameters with Earth's. Earth has the maximum global ESI of 1. Mars has a 0.70 rating, and Mercury is the next on the list with 0.60. For what it's worth, the dwarf planet Pluto and Neptune's moon Triton register a measly 0.075 and 0.074, respectively. And Enceladus, the icy Saturnian moon that is thought to harbor a subsurface ocean and perhaps life, is right down there with them at 0.094. Looking beyond the solar system, the researchers worked up ESI values for a variety of extrasolar planets. The top finishers were Gliese 581g (whose existence is in dispute) with 0.89, and Gliese 581d with 0.74.

As for Earth's PHI, it actually doesn't rate a perfect 1 on the scale - that would mean that life was basically destined to evolve here, and of course nothing is ever that certain. Instead, our planet rates a 0.96, which is about as good a score as possible. After that, the rest of the solar system sees a sharp drop-off:

Based on what's known about the rest of the solar system, the runner-up is not Mars, as you might expect, but the Saturnian moon Titan (0.64 vs. 0.59 for Mars). The Jovian moon Europa is next on the list (0.47), but Enceladus (0.35) ranks lower than Venus, Jupiter and Saturn (0.37).

The PHI could be particularly powerful in finding habitable exoplanets. While an Earth-like planet is relatively obvious to spot, categorizing a planet's habitability means thinking in broader terms about its chemical composition and what combination of natural processes might result in the emergence of life. In their paper, the researchers point to a nearby body that is far from Earth-like and yet most likely does support life: Saturn's moon Titan.

They explain:

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star. For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life. Our proposed PHI is informed by chemical and physical parameters that are conducive to life in general. It relies on factors that, in principle, could be detected at the distance of exoplanetsfrom Earth, given currently planned future (space) instrumentation."

Via Washington State University. Image by NASA.