by Victoria Jeffrey

Created on: March 29, 2009

It is a question that has fascinated many for years - How many planets exist out there in the universe? Along with this comes a second, even more pertinent one: How many planets in space are like Earth? It used to be years ago that searching for extra-solar planets was considered fringe science at best, a useless endeavor at worst. It simply wasn't a respectable use of an astronomer's time, looking for planets outside of our solar system. But there was once a time when people assumed certain things about our own solar system and when those ideas were challenged, people thought those challenges were strange. As we can see today this has changed. it is no longer odd to look for or expect to find alien (extra solar) planets.

Scientists have found that extra solar planets are actually quite common outside of our solar system. Some say that there may be millions of planets in outer space. The real question then becomes how man of them are Earthlike? Are Earthlike planets common or rare? That is a complex question. There are certain requirements that scientists look for when searching for worlds that could hold life as we know it. The word "Earthlike" is itself relative - it could mean a whole range of things. Some may be water planets, others dry and rocky. Some planets may be ice worlds and still others may have hothouse atmospheres that choke out sunlight. So what do we mean when we say Earthlike? When people think of Earth, through time immemorial it has always meant verdant forests and jungles, oceans teeming with life and the right balance of chemical atmosphere, planetary orbit and rotation and also that it be situated in it's star's habitable zone. However, Earthlike in terms of reality when studying extra-solar planets has a far looser meaning.

Planet hunting started in the early 1980's back when it had no respectability and it was extremely difficult to find alien planets so scientists have had to find non-traditional ways. Finding stars only takes using the traditional telescopes but finding planets is much tougher. Unlike stars, planets do not give off their own light so other methods have to be used. One of those methods for looking for alien planets is tracking the orbits of alien suns. Astronomers had to look for certain kinds of movement in orbits of these stars, especially how stars wobble when being orbited by planets. By using what they know about the movement of our own star and the planets in our solar system they have been able to figure out there

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