Man is curious to know whether there is extra-terrestrial life. Today we assume that life is possible only on an earth-like-planet of a sun-like-star placed at more or less the same distance as the earth is from the sun. There are billions of galaxies each having billions of stars. There has to be lots of planets and astronomers are almost certain that there would be thousands of earth-like planets in the universe.The difficulty lies in locating them.
But whether there is a planet at all around a star was a big question until one was discovered in 1995.
During the last decade and a half scientists have been studying whatever stars they could survey, for signs of planets. They have discovered 500 or so gas giant planets of very large stars millions of kilometers away. But they are massive, some of them much bigger than even Jupiter, and none earth sized . They are either too hot (close to the star) or too cold (far away from the star) and as they lack solid or liquid surfaces they cannot harbor life as we know it. The few rocky worlds (solid planets) discovered so far also do not fall into the ‘habitable zone’ and therefore cannot support life. Locating one for a sun-like-star, approximately at the same distance as earth is from the sun, (so that the temperature is neither too cold nor too hot) looks to be really a tough job. Remember we took thousands of years just to identify the other planets that circle our own sun.
Finding gas giants is all interesting but the truth is that it is not just very useful to us. Discovering earth –like planets might give us hints as to whether we are alone or not. But there has been very little luck in locating planets of the size of earth or even close to it so far.
Steven Vogt, Paul Butler and other astronomers found out a new planet, Gliese 581g, around the star Gliese 581 raising hopes. Detected from the minuscule amount of gravitational influence it exerts on its star, the planet was a mere 20 light-years away in the constellation Libra. The planet is the sixth world discovered around its sun and the fourth most distant. It was classified as a super-Earth, a category which incorporates planets exceeding the mass of Earth but smaller than 10 Earth masses. On April 21, 2009, another smaller planet orbiting the same star was announced. Things looked very bright.
But later studies have made scientists doubt whether such planets exist at all! The planet hunting game is not easy as we are dealing with light-year-distances. (Light travels 300,000 kms in a second and hence it goes 300,000x60x60x24x365 kms in one year!) This is a huge distance and hence the work becomes harder.
Even the largest planet of another star cannot be seen with any telescope in the world. Scientists have to assume the presence of a planet from the ‘wobble’ of a star. Two heavenly bodies, say, like a star and one of its planets, will have to revolve around their common center of gravity to keep equilibrium. If one of them (obviously the planet) is very small, the common center of gravity will lie within the star—separate from its own center of gravity--and the planet will be revolving around this point. The star will be thus just wobbling and not revolving as that point is well within it. By analyzing this back and forth movement we can infer the size of the planet. Scientists also theorize the presence of a planet by studying the light obstructed by the planet. The intensity of the dimming throws light on the size of the body. Astronomers have another way too: they study the bending of the light emanating from the star as it passes a planet. All these methods imply arduous work and long observations, analysis and calculations.
The search for an earth like planet is reaching a feverish pitch. The huge Kepler telescope, which has been orbiting the sun since it was launched in January 2009, keeps gazing more than 10,000 stars. It studies the wobbles of the stars, the bending and dimming of rays, and provides valuable data for the scientists. The Astronomers from around the world have revealed data regarding the planets so far found. More than 100 planets of similar size to earth have been reported to have been discovered. The innumerable number of stars out there makes it almost inevitable the presence of an earth-like-planet. But the difficulty is in accurately locating the same. But if astronomers succeed in identifying one, the real search for earth-like-life will begin.
A planetary system 1200 light years from Earth was thought to contain two worlds entirely covered by global oceans. Kepler 62e and 62f are in the habitable zone of a roughly Sun-like star. Each planet is slightly larger than Earth. But nothing came out of this hypothesis too
Aastronomers have recently discovered the first Earth-size planet orbiting a star in the "habitable zone" -- the range of distance from a star where liquid water might pool on the surface of an orbiting planet through Nasa’s Kepler Space telescope. The discovery of Kepler-186f confirms that planets of the size of Earth exist in the habitable zone of stars other than our sun. While planets previously found in the habitable zone, are all at least 40 percent larger in size than Earth and understanding their makeup is challenging. Kepler-186f is more reminiscent of Earth. Although the size of Kepler-186f is known, its mass and composition are not.
As of now we know of just one planet where life exists. None has been located that mimic earth.