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 are extra solar planets out there and some of them seemingly Earthlike. Planets and stars move along their common center of mass. Planets have a wide berth to their orbits, naturally, but even stars have their own much smaller orbit around their own center of mass which is detected as a wobble. It's like watching a person use a hula hoop. The hoop represents the planet's wide swinging orbit around the star and the person swinging the hoop around their hips represents the star slightly moving within its own center of mass. This is one way astronomers began to detect that there were other planets outside of our solar system. Another way is the Doppler Shift or Doppler Effect, studying the frequency emitted from waves, in this case, from stars and planets. Like soundwaves, lightwaves appear to shift in frequency from the objects emitting them depending on whether they were going forwards or backwards. As a star wobbles, the lightwaves coming from a celestial body appraoching us seem blue while the lightwaves shifting away appear red. by using these techniques and their knowledge of the Doppler Effect, astronomers can tell whether there is a planet moving around a star very far away and where that planetary body is moving in relation to its star.
What we have learned about our own system and other star systems are very surprising and have challenged astronomer's perceptions of what they know about how our solar system works. First, most extra solar planets that have been found do not orbit anywhere near the habitable zones of their stars. Also, the phenomenon of "roasters" or hot Jupiters as they have been termed, gas giants like Jupiter, that orbit extremely close to their stars have challenged long held assumptions about how gas giants form and where they are usually located in a solar system. Old assumptions are now falling by the wayside. It was thought that gas giants were only able to form outside of the snow zone, far away from the star they orbit. Another very interesting thing that surprised scientists are the elliptical orbits of most planets everywhere, except in our own solar system. In our sytem circular orbits are the norm and elliptical orbits are considered odd. Because Pluto's orbit is elliptical it was knocked off of its "major planet" pedastal and has been demoted to dwarf planet status, or even simply a Kuiper Belt object. Comets have elliptical orbits, it was thought, not true planetary bodies. Astronomers were wrong about planetary orbits after finding alien planets. They have found that our solar system is unique in many respects and the manner of planetary orbits is one of them. The study of alien worlds and stars are affirming in the minds of many that our planet is far more unique than once thought. It's not that other planets outside our system don't exist; they are quite abundant, but are they anything like Earth is the question and that is a far harder thing to know.
Other facts found about these extra solar planets are fascinating: some extra solar planets are bigger than Jupiter and some have storm winds up to 6000 mph! Extra solar planets and their formation has taught astronomers that we are not unique in ways we thougt we were previously and that we are unique in other ways. There are many planets out there but planets that can sustain complex life like Earth are rare, if they exist. So what are Earth like planets? They have to meet certain requirements. Water is needed in some form, the right atmospheric conditions, the planet needs to be rocky and it needs to have a stable, circular orbit, it needs the right kind of moon(s) and it needs to be within it's star's habitable zone for starters and that's not all. Astronomers have found a few planets that might fit the bill of sustaining life in the future: Gliese 436b and Gliese 876d. Gliese 436b is an interesting one because it is an ocean planet. In fact, it has no surface, much like what Earth used to be like billions of years ago. However, the water on Gliese 436b may not be anything like what we know it. Astronomers say that the water molecules are so dense that the structure of it is more like ice, except that it isn't cold. Why are they so dense? Most liekly because water is so abundant on the planet that all of its surface is completely covered. Then there's Gliese 876d which is 6-7 times Earth's mass. It was once thought to be a rocky planet, like Earth. It sits just on the edge of its star's habitable zone. It may have a rocky surface or not. Some astronomers now think that it too may be a water planet completely devoid of a surface. As far as anyone knows, neither of these Earthlike planets can hold complex carbon based life forms even though they may seem similar to Earth. The issue becomes complex because there are many factors that go into creasting the right conditions for life and so far the Earth like planets don't have not enough to support life. Like Mars, Venus and a few moons in the outer solar system like Europa, Io they have some features that make them seem likely candidates but not enought to be true sister planets to Earth. They are more like distant cousins of Earth.
So the search goes on for Earthlike alien planets. It is fascinating and has earned a respected place in astronomy that reveals more and more about the wonders of the universe. It also reveals what we know already about our place in the universe and reveals what we don't yet know. Perhaps one day we can travel closer to these planets for better study and exploration and continue our pursuit of knowledge of other planets and their stars.