A New Earth?

An international team of astronomers announced this week that they've found an exoplanet that's only about 3 times as massive as Earth. "Great," you say. "What's an exoplanet?" It's a world orbiting a star other than our sun.

Dubbed MOA-2007-BLG-192Lb, this new exoplanet orbits a tiny star some 3,000 light-years away. Based on what they know about planet formation, the scientists think it may be made of rock, like Earth. They also speculate that it could have a surface ocean, like Earth.

But it's very hard to tell. Spotting even giant exoplanets is tough. Finding ones that are almost as small as Earth is nearly impossible--though all that may be about to change.

Seeing "Jupiters"

Until 15 years ago, there were no confirmed exoplanet detections. Now there are nearly 300 of them. And the planet hunters are just getting started. Says one expert, "we're just now getting to the point where, if we were observing our own solar system from afar, we would be seeing Jupiter."

Jupiter is a gaseous giant. It's about 2.5 times more massive than all of the other planets in our solar system combined--and about 318 times more massive than Earth alone.

Most of the exoplanets spotted so far are probably big balls of gas, too. But as technologies improve, astronomers expect to detect many smaller planets like MOA-2007-BLG-192Lb--including some that are lukewarm and wet, like Earth.

Seeing "Earths," Too

Spotting any exoplanet is no mean feat. They're almost always too far away--and too washed out by light from the stars they orbit--to be seen directly. Astronomers generally search for them by looking for "wobbles" they cause in the light coming from their parent stars. (See how this works.)

Still, they've already found a few that they think are far Earthier than Jupiter. Along with MOA-2007-BLG-192Lb, there's Gliese 436 b, which appears to be a Neptune-sized planet with a rocky core covered in water (although the water on Gliese 436 b is compressed into solid form).

There's also the Gliese 581 system, a group of at least three planets circling a red dwarf star 20 light-years away. Recently, astronomers have debated whether two of the Gliese 581 planets might harbor liquid water, a key ingredient of life, at least as we know it.

Seeing Stars, Too Many to Count

Even if none of the known exoplanets turns out to be Earthy, planet hunters won't despair. Experts estimate that 10 percent of the Milky Way's 200 to 400 billion stars have planetary systems, and that 30 percent of the stars with planetary systems have more than one planet. So the 300 exoplanets detected so far are a drop in the galactic bucket.

And that's just the Milky Way bucket. As we mentioned earlier this week, some estimates say a universal census would count at least 100 billion galaxies. Those billions of galaxies include billions of billions of stars. Even if only a fraction of those have planetary systems, the number of exoplanets is downright astronomical. Clearly, the effort to explore strange new worlds has only just begun.