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 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 Tourism and 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.
Other Possible Earths
 Image courtesy NASA In surveying potential candidates for "new Earths," astronomers |
Few planets have been found in the Goldilocks Zone, but in April 2007, European astronomers announced the discovery of one. It was also, at that point, the most Earth-like planet ever found. The planet, called Gilese 581c, is 12,000 miles in diameter, or not much larger than Earth (8,000-mile diameter). It orbits a massive red star called Gilese 581, located in the Libra constellation, 20.5 light years from Earth. Gilese 581c orbits its star very closely, completing an orbit in just 13 Earth-days. This short orbit would make a planet too hot for life, except that Gilese 581's surface temperature is 1/50th that of our sun.
Because it lies in the Goldilocks Zone, Gilese 581c's surface temperature ranges from an estimated 32 degrees Fahrenheit to 102 degrees Fahrenheit. The research team that discovered it believes it has a developed atmosphere. The planet might not only have water -- it might be entirely covered by oceans.
Gilese 581c does have some things working against it. Its gravity is about twice as strong as Earth's, and it receives significant doses of radiation from its star. Both could inhibit life from developing. Even so, Gilese 581c is exciting not only for its Earth-like conditions, but also because of its relative proximity to Earth and its location in the elusive Goldilocks Zone.
As more powerful and precise telescopes go into space, future efforts will involve examining exoplanets' atmospheres for traces of oxygen and methane and looking for rocky planets that lie in the Goldilocks Zone. Scientists are also increasing their use of automated telescopes that are programmed to look for minuscule variations in a star's brightness caused by an orbiting planet passing in front of it. With a rapidly increasing pace of discovery of exoplanets and a practically infinite number of stars in the universe, many other exciting discoveries are ahead of us.
The ideal discovery would be a planet similar in composition to Earth that lies within the Goldilocks Zone and orbits a stable star. But it's important to keep in mind that popular depictions of extraterrestrial life are likely wrong. Some life forms may be no more advanced than bacteria. Others may be highly advanced but unrecognizable, a thought that has caused some scientists to advocate the search for so-called weird life.
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