There are plenty of other stars. There are plenty of planets around those stars. The challenge—for Earthlings, at least—is finding planets that are just the right size and distance from the stars to have water on their surface, and therefore be capable of supporting life. On Wednesday, Earthling astronomers happily announced the discovery of a planet inhabiting this so-called "Goldilocks zone."  No life has been found yet, of course, but it's a moment for space-watchers to celebrate, as scientists take it as a promising sign in the search for other such planets. Here's what's good, what's bad, what's known, and what's not known regarding this new discovery.

  • 'Nobody from Earth Will Be Visiting Anytime Soon,' points out The New York Times's Dennis Overbye: "The planet, which goes by the bumpy name of Gliese 581g, is orbiting a star about 20 light-years away in the constellation Libra."

For thousands of years, philosophers and scientists have wondered whether other Earths existed out in the cosmos. And since the first, very un-Eearthlike extrasolar planet was discovered in 1995, astronomers have been inching closer to answering that question. Now, they've evidently succeeded (although to be clear, there's no way at this point to determine whether there actually is life on the new planet).
  • Actually, We've Found Habital-Range Planets Before, points out Ivan Semeniuk at Nature, "but not a planet that is so clearly near Earth's mass range." He also explains that, for now, it's hard to know much. "To detect a world that can be confirmed to have water and possibly life as well, astronomers must search for planets that cross in front of the stars they orbit. In addition to yielding precise sizes, such detections can sometimes provide direct information about a planet's atmosphere."
  • Beware the 'Breathless' Media, agrees Discover's Phil Plait. The "Goldilocks zone" is important, but whether a planet has the right temperature also depends on atmosphere--Venus, for example, "Venus could have liquid water, but its super-thick atmosphere produces a runaway greenhouse effect which has heated it to 460° C (900° F)." For right now, we don't know that the planet is habitable, "or even very Earthlike. It may not even have any water on it."
  • Good News, Bad News Regarding Habitability "Apparently," writes Plait, "all the planets [around this star] have neat, circular orbits, so the system seems to be stable." But here's one problem: the planet is quite close to the star, which turns out not to be a problem for overall temperature since the star is cooler than the Sun, but rather causes another problem:
The star exerts strong tides on the planet, which have the effect of slowing the planet's rotation until it equals the orbital period. This has almost certainly happened to this planet, so in other words, one day on this planet = one year, and the planet always shows the same face to its star like the Moon does to the Earth.

That makes things a bit dicier for habitability. The side facing the star may get very hot, while the dark side gets very cold. If the planet has an atmosphere that gets mitigated somewhat (the hot air on the day side will flow over to the night side and vice versa, smoothing out the highs and lows in temperature), and may make the planet more clement. However, we have no clue if this planet has an atmosphere at all.

  • What This Means for Likelihood of More Earths  "The authors[of the findings]," writes The New York Times's Dennis Overbye, "said the relative ease by which planet was found--in only 11 years--led them to believe that such planets must be common." Or, as Ivan Semeniuk paraphrases astronomer Steven Vogt, "the discovery of another potentially habitable world within 100 light years of our own, significantly improves the likelihood that many more Earthlike worlds capable of hosting life existing across the galaxy." Phil Plait points out that this new planet is really only 20 light years away, "practically in our lap compared to the vast size of our galaxy ... I don't want to extrapolate from a data set of two (us and them), but if this is typical, there could be millions of such planets in the galaxy. Millions."
  • Quips  "This means we've got our backup plan, and there's no need any more to deal with this thorny global warming problem," writes environmental site Grist's Jonathan Hiskes, parodying an argument occasionally advanced seriously. "Henceforth, Grist will be focusing on celebrity gossip, which is what we always wanted to do. Also, 'Goldilocks zone' =  decent female death-metal band name, no?"