Sister Earth?

Last week, a group of European astronomers announced that they had discovered the "most Earth-like planet yet" around a star in the constellation Libra. The planet, known as GL581c, is the first of more than 200 planets discovered outside our solar system that is apparently in the so-called "habitable zone" of its star -- the range of orbital distances where liquid water can theoretically exist. In this sense, the discovery is a landmark in the search for life elsewhere in the Universe.

But if you examine the details more closely, you will discover a planet that is very different from the Earth. First of all, it is at least 5 times the mass of the Earth -- and it orbits the star every 13 days (instead of our leisurely 365 days), at less than 1/5 the distance from the Sun to the sweltering planet Mercury. The only reason the planet isn't burnt to a crisp is that its parent star is much cooler than the Sun -- a tiny red dwarf. As a consequence, it is likely that the surface of this planet has the right temperature for liquid water to exist.

But I wouldn't want to live there, even if I could breath whatever atmosphere might exist. Red dwarf stars give off most of their radiation in the form of infrared light -- basically heat, like what you see with night vision goggles -- and almost no visible light. So it might be warm, but it would always be dark to human eyes. Finally, although it is among the 100 nearest stars to the Sun, even the fastest spacecraft ever built by humans would take more than 90,000 years to get there. So don't count on this planet offering us a safe haven in case we wreck our own.

It's an exciting discovery -- and one that we will undoubtedly see more of in the near future. As technology improves and allows astronomers to find ever-smaller planets around other stars, we will eventually find a true Sister Earth, and maybe even life. But we're not there yet.