While thousands of exoplanets have been discovered in the past two decades, all observation techniques have found no more than a few Earth-like candidates. Astronomers aren’t sure what properties would define another earth. Do we need to find a planet that is exactly the same as Earth? The same size and mass as the earth? This can be difficult and may not be important from a quality of life perspective. After all, we have no reason to believe that life on Earth could not have originated if our planet had been a little smaller or bigger, and remember that a planet’s habitability depends as much on its distance from its star & nature of his atmosphere. The greenhouse effect can heat some planets.
We can ask other questions that we don’t yet know the answers to. Does this Earth “twin” have to orbit a sun-like star, or can we consider the many exoplanets orbiting K- and M-class stars as candidates? (In the summer of 2016, astronomers reported the discovery of a planet at least 1.3 times the mass of the Earth around the nearest star, Proxima Centauri, which is spectral-type M and 4.2 light-years away from us) We have a particular interest in finding planets that can host life like ours. In this case, we need to find planets within the habitable zone of their star, where surface temperatures coincide with liquid water on the surface. This is probably the most important defining characteristic of an Earth-analog exoplanet.
The search for potentially habitable worlds will be one of the main drivers in exoplanet research over the next decade. Astronomers are beginning to develop realistic plans for new instruments that can search for signs of life even in distant worlds (examining their atmospheres for gases associated with life .) , for example). If we need telescopes in space to find such worlds, we have to recognize that it takes years to plan, build and launch such space observatories. The discovery of exoplanets and the knowledge that most stars have planetary systems are changing the way we think about life beyond Earth. We are closer than ever to knowing if habitable (and inhabited) planets are common. This work gives a new optimism to the search for life elsewhere. to which we will return in life in the universe.