Which of the several hundred known and suspected planets outside our local solar system has enough characteristics in common with Earth that scientists believe it has a reasonable chance of supporting a recognizable form of life?
In recent weeks, astronomers have been going gaga over the first direct, visible-light image of a planet outside our solar system. While planet Fomalhaut b has been in the exoplanet record books for a while, this represents the first time that a telescope has been able to spot an extrasolar planet using direct imaging with the same wavelengths of light that us mere humans use to see everyday.
We're becoming pretty competent planet hunters, which means we just might start finding some potentially life-bearing worlds in the near future; or, at least, planets that could hold life in a form we'd recognize. And it has nothing to do with the fact that we actually got a direct-light glamour photo of Fomalhaut b, either.
You see, Fomalhaut b's relative proximity isn't what made it so easy to spot. Fomalhaut b was first discovered by observing the debris disk surrounding its parent star, which made pinpointing its likely location rather straightforward (though it still took several years). After that, it was merely a question of poring over Hubble Space Telescope images of the right area of around Fomalhaut and figuring out which speck of light was a planet.
What makes the Fomalhaut image so spectacular, besides being the first direct visible-light image of an exoplanet, is that Fomalhaut b is actually a pretty small, cool world, by known extrasolar standards. It has a mass between one-half to three times that of Jupiter (we're a little early in Fomalhaut's observational history to be more precise), and an estimated mean surface temperature of around 72 Kelvin. For comparison, Earth's mean surface temperature is about 287 K.
Until recently, the discovery of exoplanets suffered from an extraordinary selection bias, as highly massive, extremely hot, or extremely bizarre planets, most of which were gas giants were simply easier to detect using the indirect discovery methods at astronomers' disposal. Basically, they looked at a star and then looked for something weird, which usually led to finding a planet that was, well, very different from good old Earth.
In fact, we've yet to find anything that would strictly qualify as an Earth-like terrestrial planet outside our solar system. The best we can do right now is a modest collection of so-called Super-Earths, planets that are far less massive than gas giants, but are nonetheless several times the mass of Earth.
That said, there is at least one known exoplanet that has enough in common with humanity's homeworld that scientists believe it has at least a reasonable chance to support a form of life similar to that found on Earth.
WHAT EXTRASOLAR PLANET IS CONSIDERED MOST LIKELY TO SUPPORT A RECOGNIZABLE FORM OF LIFE?