Scientists are busy looking for other planets like our planet. So far they have discovered some 889 planets. In addition, there are some 2712 “candidate planets.” These are cases where we have some data indicating that a planet probably exists at a particular place, but the data is not quite strong enough for scientists to conclude that the planet definitely exists.
Most of these planets have been discovered by the Kepler Space Telescope, which unfortunately has recently suffered a mechanical glitch which has put it out of service for the time being. But scientists will be busy for years analyzing the huge amount of data that Kepler has already collected. Even if the Kepler Space Telescope takes no more observations, it will be regarded as one of the greatest success stories in scientific history.
The Kepler Space Telescope mainly uses a technique called the transit technique. This technique is so simple that you could illustrate it to someone by using a light bulb and a rubber ball. Take the lampshade off a lamp, and tell someone the light bulb represents a distant star. Then revolve the rubber ball around the light bulb. Then ask the person, “How does your observation of the light bulb change when the ball passes in front of the light bulb?” The answer is that the observer sees less light from the bulb when the ball passes in front of it. The same thing happens (on a much different scale) to a telescope looking at a distant star when a planet passes in front of the star – for a short time a little less light is observed from the star. By taking repeated observations of a distant star over several years, and carefully analyzing dips in light from the star which occur at periodic intervals, scientists can detect planets around other stars. By exactly how much the light from the star dips, scientists can estimate the mass of these extrasolar planets.
What astronomers are most hoping to find are potentially habitable planets – planets that could support the existence of intelligent life. To be potentially habitable, a planet must not be many times smaller than our planet or many times larger than our planet. If a planet is too small, it won't have enough gravity to hold an atmosphere. If the planet is too many times larger than Earth, it would presumably be a planet similar to the gas giants in our solar system (Neptune, Uranus, Saturn and Jupiter), which don't even have solid surfaces. To be potentially habitable, a planet must also be in a zone that is neither too close to the star which it revolves around, nor too far away from the star. This zone where planets are not too hot and not too cold has been called the Goldilocks Zone (after the famous story for children in which a little girl rejects one bowl of porridge because it's too hot, another because it's too cold, and finally picks a third bowl of porridge which is just the right temperature).
The image below shows the current state of the search for habitable planets. So far astronomers have found 10 potentially habitable planets. Their relative sizes are shown below. All of these planets are in the Goldilocks Zones of the stars they resolve around.
The decimal number under each planet is an estimate of how similar to Earth the planets are. The closest matches (with a similarity index of .82) could well be very similar to Earth. We will apparently have to wait until 2017 for the next great breakthrough in finding potentially habitable planets. In that year NASA will launch the Transiting Exoplanet Survey Satellite (TESS). It will have capabilities much greater than that of the Kepler Space Telescope. Some predict that this satellite will be able to discover between 1000 and 10000 planets.