Astronomers from the University of Washington's Virtual Planetary Laboratory say they've developed a formula to figure out which planets outside our solar system are most likely to contain life.

A lesser person might call that formula something catchy like the Alien Index, but these professionals call it the "Habitability Index for Transiting Planets," and it will soon be published in the Astrophysical Journal.

"Basically, we've devised a way to take all the observational data that are available and develop a prioritization scheme," said co-author Rory Barnes, "so that as we move into a time when there are hundreds of [planets to consider] available, we might be able to say, 'OK, that's the one we want to start with.' "

The Alien Index (we're still hanging onto that phrase) takes the most important details of a planet that make it suitable for life, and turns it into quantifiable data. Those figures are then plugged into the formula, to see which planets are the best candidates.

The most important elements of a planet that could have the next K-PAX on it are:

Being In "The Goldilocks Zone"

In order to be a candidate for life, a planet has to be in The Goldilocks Zone, an area that's just the right distance from its sun to have liquid water on its surface. Water is considered one of the most important indicators of whether a planet can have life. That's why NASA's recent news of water on Mars made such a splash.

For a while, being in the Goldilocks Zone was all we had to go on. But as scientists identify more and more planets, the number of planets in the Goldilocks Zone became blissfully unmanageable, and these new factors had to be taken into account to whittle down the number of places to look.

Here's what to look for in a potentially alien-run planet:

Rocky Is Good

A good planet for life has to be rocky. On Earth, our rocky terrain helps the planet to insulate itself, keeping our temperatures relatively stable and life-friendly.

Friendly With The Sun

In order to sustain life, a planet must be able to get a lot of energy from the sun around which it orbits. One of the ways to measure that is to look at what astronomers call "eccentricity-albedo degeneracy" (also an accurate description of Natalie Portman's character in Garden State). That's a figure that represents how much energy is reflected back off the surface of the planet, and how that relates to its orbit. Those two elements together make up its eccentricity-albedo degeneracy, which is basically a fancy way of saying it can get energy reliably from the sun, without losing that energy, or getting too hot or cold.

Just Right Solar Radiation

The researchers found that the best candidates for worlds with life are those planets that get about 60 percent to 90 percent of the solar radiation that the Earth receives from the sun. In other words, it's getting too hot up in here.

The Alien Index will continue to get more specific as astronomers study the planets earmarked as possible life-sustaining worlds.

Now, go find some aliens, y'all.

Photo: Garrette | Flickr

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