How many habitable planets have been discovered

Once the realm of supposition and science-fiction, alien worlds orbiting other stars are now indisputable fact. Astronomers call them exoplanets , and the number they have found is in the thousands and rising fast. To detect them, astronomers use two main methods.

The first is called transit photometry and involves pointing a telescope at a star and looking for the regular, slight dimming in its light caused by a planet orbiting in front of it.

The other is the radial velocity technique. This relies on the gravitational tug of an exoplanet making its host star wobble slightly. Because … it's not deliberately aimed at them," Shostak said. The only directed signal was sent out in by the now defunct Arecibo telescope in Puerto Rico. But it was sent to a star cluster, M13, that lies more than 21, light-years away.

But so far, it's been quiet. The closest we ever came to a potential signal was in , often referred to as the "Wow! Since then, there has been much skepticism about it, with one paper claiming that it was the result of passing comets.

Nick Cowan, an astronomer and associate professor at McGill University's department of Earth and planetary sciences who was not involved in the study, said that there are many scenarios that may account for the silence.

This leads to the Fermi Paradox, named after physicist Enrico Fermi who is said to have asked, "Where is everybody? The idea is, if there are so many stars, there should be millions or billions of planets, and potentially intelligent life that we should be able to detect.

But so far, we have been greeted by a great silence. It's like, 'Hey, are you out there? Nicole has an avid interest in all things science. Only one candidate, KOI The 24 possible planets are all more than light-years away, and some are probably too far to study right now even with the strongest telescopes. Keplerc , for example, is more than 2, light-years away, meaning astronomers probably won't be able to examine it more closely for signs of life anytime soon. However, pinpointing what makes a planet "superhabitable" is important, Schulze-Makuch and his colleagues wrote, because it's possible that one of these planets will be discovered within light-years.

If so, they wrote, that planet should be the first place Earthlings turn to find out if there is other life in the universe. Stephanie Pappas is a contributing writer for Live Science covering topics from geoscience to archaeology to the human brain and behavior.

A freelancer based in Denver, Colorado, she also regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.

Live Science. Stephanie Pappas. See all comments Not, short of a few billion years of evolution. To develop a reasonable estimate, the researchers looked at exoplanets similar in size to Earth and thus most likely to be rocky planets.

They also looked at so-called Sun-like stars, around the same age as our Sun and approximately the same temperature. Another consideration for habitability is whether the planet could have the conditions necessary to support liquid water.