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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Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can also follow him on his personal blog.