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First Earth-sized rocky planet found in star's habitable zone

Paul Sutherland, Feature writer
Apr 17, 2014, 23:06 UTC

Sen—Astronomers using NASA’s Kepler space telescope have discovered the first alien planet that they believe to be rocky, of similar size to the Earth, and with conditions favourable for life.

The new world was spotted orbiting a red dwarf star called Kepler-186, 490 light-years away from us in the constellation of Cygnus, and is the fifth so-called exoplanet to be detected in that star’s own solar system.

It was detected by the tiny fade in the star’s light as the planet passed in front of it—something astronomers call a transit. From the amount that the star was observed to dim, scientists were able to work out its size.

Following the discovery, announced today, astronomers used two powerful telescopes on the ground, the W. M. Keck Observatory and the Gemini Observatory in Hawaii, to confirm that the planet really exists.

Because of its size, they say it is likely to be a rocky world, unlike the majority of the hundreds of giant gasballs previously found. For the first time, this is an Earth-sized planet in its star’s habitable zone, where water essential to life can exist as a liquid.

Since the first detection of the new planet, labelled Kepler-186f, alien-hunting scientists at the SETI Institute have been scanning the planet for any signs of extraterrestral intelligence using the giant radio dishes of the Allen Telescope Array, in California, but none has so far been heard.

Diagram of the red dwarf

A diagram comparing the position of inner planets in our own Solar System to those found orbiting Kepler-186, to the same scale. Image credit: NASA Ames/SETI Institute/JPL-CalTech

Nearly 1,800 exoplanets have been confirmed in the past 20 years, but the holy grail has always been to find one that could resemble the Earth. About 20 of the previous finds also orbit in the habitable zone, where it is not too hot and not too cold, but all are bigger than our planet and so their true nature is unknown.

Thomas Barclay, a NASA Kepler mission scientist, remarked that “theoretical models of how planets form suggest that those with diameters less than 1.5 times that of Earth are unlikely to be swathed in atmospheres of hydrogen and helium, the fate that’s befallen the gas giants of our own solar system. Consequently, Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars.”

The discovery, by a team led by Elisa Quintana of the SETI Institute at NASA Ames Research Center, is being announced in the journal Science. She said: “This is the first definitive Earth-sized planet found in the habitable zone around another star.

“Finding such planets is a primary goal of the Kepler space telescope. The star is a main-sequence M-dwarf, a very common type. More than 70 per cent of the hundreds of billions of stars in our galaxy are M-dwarfs.”

Although the Earth-sized planet’s star is smaller and cooler than the Sun, the scientists do not believe this would prevent Kepler-186f from having conditions suitable for life. Computer modelling suggests that winds or ocean currents could smooth out any extreme temperature variations.

Furthermore, the planet is far enough from the star that it is likely to be rotating like the Earth, rather than tidally locked like the Moon, and protected from flares that would be perilous to life.

Sen spoke to Professor Giovanna Tinetti, an expert in exoplanets, their atmospheres and chemistry, at University College London’s Department of Physics and Astronomy.

She told us: “In the past years a long list of planets has been found, but the fact that a planet as small as the Earth has at last been found shows how far we have progressed in finding them.

“Certainly we are all interested in habitable planets on top of just discovering exoplanets, and to have maybe a rocky planet more similar to Earth on top of that is a wonderful discovery.”

“Ideally we would love to see what the planet really looked like, and to check the properties of any atmosphere and its chemical composition. For that you would need to make measurements of the atmosphere with a spectroscope.

“For this particular planet, the star’s light is not bright enough to allow us to do those sort of measurements. But the fact that planets like this are now being discovered shows that we will very soon find similar planets that could be good candidates to look at spectroscopically.”

Planet-hunters believe that NASA’s upcoming James Webb Space Telescope (JWST), the successor to Hubble, will be powerful enough to image directly the planets orbiting nearby dwarf stars and to probe their atmospheres following its launch in 2018.


A rather fanciful depiction of the possible surface of Kepler-186f, based on what scientists know about the new planet. Image credit: Danielle Futselaar