In the relatively new science of planet hunting, no find is more prized than finding a planet like ours, one that could support life. As of late August 2007, almost 250 exoplanets -- planets orbiting stars besides our sun -- had been found [source: BBC News]. The announcement of new planets has become almost routine; some don't even make it into the news. But we do periodically hear about exoplanets that seem similar to Earth or that scientists speculate may hold liquid water, one of the key ingredients for carbon-based life. How many of these Earth-like planets are out there, and are they really like Earth, or do we just hope they are? In this article, we'll take a look at some potential Earths and what they may tell us about the future of planet hunting.
Space Exploration Image Gallery
 Image courtesy NASA This artist's depiction shows a possible Earth-like planet |
In August 2007, scientists announced the discovery of a star which might have once had an Earth-like planet orbiting it. The star was a white dwarf called GD 362, and it's 150 light years from Earth, still within our galaxy. While no Earth-like planets appear to orbit the star now, the presence of asteroid debris can tell us something about a planet that likely once orbited the star.
The debris, which came from an asteroid that was once 125 miles long, showed little carbon and high levels of calcium and iron. That means the rocky material is much like the moon and the rock that makes up the Earth. The presence of this familiar material, scientists say, implies that an Earth-like planet may have orbited the star millions of years ago, before it became a white dwarf. The star also has rings similar to Saturn's, and some of the ring material may be from planets and other objects that were torn apart by the white dwarf's gravity.
GD 362's diameter is about half of the Earth's, but its mass is about that of the sun, making GD 362 far more dense than our planet. However, GD 362 started out as a star like the sun in our solar system. But when the star used up its fuel, it swelled up into a red giant and then ejected its outer shell. The center of that star then transformed into a white dwarf, at once very hot (more than 100,000 Kelvin) and very small. A white dwarf retains about half of its mass but becomes incredibly dense because of its small size. Our sun should become a white dwarf in about five billion years. The process will destroy Mercury, Venus and possibly Earth.
So how many other Earth-like planets are out there (or were out there)? No one knows, but many scientists believe that it's inevitable that other Earths will be found. One NASA scientist told BBC News that some scientists believe that nearly every star has Earth-like planets orbiting it [source: BBC News]. Of course, excitement around finding these other Earths is based on the idea that they may contain alien life or even, centuries from now, allow for far-flung human space colonies -- before our star explodes and destroys the Earth.
On the next page, we'll take a look at the most Earth-like planet ever found and something called the Goldilocks Zone.
