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?

Get the answer.

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?

The planet in question is Gliese 581 c, which orbits the red dwarf star Gliese in the constellation Libra, about 20 light years away from Earth. While Gliese 581 c is about five times the mass of Earth — one of our aforementioned Super-Earths — it has an estimated mean surface temperature of about 290 Kelvin, which is just barely hotter than the mean temperature on Earth . The reason for Gliese 581 c’s relatively temperate surface conditions is owed mostly to the planet’s orbit — it sits in (or at least near) the so-called Goldilocks Zone, an orbital distance from its parent star that gives it an hospitable, regular surface temperature very similar to Earth.

The known habitable temperature range of all life on Earth — the only ecosphere of which we have direct knowledge — is between 258 and 394 Kelvin, which is devastatingly narrow by planetary standards. That slim range also includes the most radical thermal extremophiles known to science: Antarctic cryptoendoliths and thermophilic bacteria found in deep sea volcanic vents. So far, Gliese 581 c is the only exoplanet thought to maintain a mean surface temperature within this range.

As to whether Gliese 581 c has the other unconditional requirement for known forms of life — water — the jury is still out. Scientists have an estimated mass for Gliese 581 c, but not an estimated density. So those five Earth masses could be a life silicate or iron rock, a hydrogen/helium “gas dwarf,” a carbon-crystal “diamond planet,” an exotic super-hot world made of Ice VII, or — most hopefully — a water-ice world with a rocky core and a surface gravity about 1.25 times that of Earth.

As of right now, only one extrasolar planet has a confirmed presence of water vapor in its atmosphere — planet HD 189733 b — and it shouldn’t. HD 189773 b has both water vapor and methane in its atmosphere, and the two should react to create carbon monoxide, but for some reason don’t. And even if scientists could figure out what keeps the water vapor around, HD 189773 b’s mean surface temperature of 1138 Kelvin makes it a pretty inhospitable vacation spot, anyway.

Thus, Gliese 581 c remains our most compelling Class M planet candidate, though one still shrouded in uncertainty. That’s not just some extraordinary extrasolar exceptionalism; it’s a statistically significant slice of star-spanning Geek Trivia.

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