One of the main goals of planet hunting has been to locate an Earth 2.0 - a planet of a similar size to our own that orbits its star at a distance that enables liquid water to exist on its surface.
Scientists this week announced that they have discovered what appears to be a planet that meets these criteria, a rocky Earth-sized world called Kepler-186f that sits right inside its star's habitable zone.
Unlike most exoplanet discoveries that tend to be worlds either too hot, too cold, too close or too distant from their parent star, this newly identified world seems to be just right. Astronomers are now hailing it as the single most promising candidate for an Earth-like planet elsewhere in the universe.
"We're always trying to look for Earth analogues, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun," said astronomer Stephen Kane.
"Some people call these habitable planets, which of course we have no idea if they are. We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets."
This artist illustration shows what it might be like to stand on the surface of the planet Kepler-186f, the first-ever Earth-size planet to be found in the habitable zone of its star.
A planet similar in size to Earth with surface temperatures suitable for water to exist in liquid form has been discovered orbiting a distant star in what is the strongest candidate yet for a habitable world outside the Solar System.
Scientists said last night that the planet, called Kepler-186f, is the smallest “exoplanet” so far discovered and is highly likely to be a rocky planet like Earth, with its own atmosphere and solid surface where liquid water and possibly life can exist.
Astronomers have now confirmed the existence of nearly 1,000 planets beyond the Solar System using highly sophisticated techniques based on measuring minute changes in the intensity of starlight as objects pass in front of their own sun. Hundreds of other planetary candidates are awaiting analysis.
However, most of the confirmed exoplanets are gaseous giant planets like Jupiter with no solid surfaces, potentially toxic atmospheres, and that are either too hot or too cold for liquid water and hence life.
Kepler-186f, meanwhile, fits the description of a rocky planet with a benign atmosphere that lies within the water-friendly “Goldilocks zone” which is neither too hot, nor too cold for life. It comes the closest yet to being “Earth 2.0”, astronomers said.
“This is the first definitive Earth-sized planet found in the habitable zone around another star,” said Elisa Quintana of the SETI Institute at Nasa Ames Research Centre in Moffett Field, California, and lead author of the study published in the journal Science.
Finding Earth-like habitable planets outside the Solar System was the main purpose of the Kepler space telescope and the discovery of such a planet around a red dwarf star, Kepler 186, suggests there are many more to be found given that this type of star comprises about 70 per cent of the hundreds of billions of stars in the Milky Way galaxy, Dr Quintana said.
The size of the planet, estimated to be 1.1 times the radius of Earth, suggests that it is likely to have a rocky surface rather than being a large gas giant planet, although this can only be confirmed after scientists have calculated its mass and hence density.
The red dwarf star Kepler 186 is 500 light years away and the planet’s orbit is similar to that of Mercury’s which is too hot for water. However, because red dwarfs are smaller and cooler than the Sun the planet lies just on the outer edge of its sun’s habitable zone where liquid water can exist – although if water does exist some of it is likely to be frozen for at least part of the time.
This artist illustration shows the planet Kepler-186f, the first Earth-size alien planet discovered in the habitable zone of its star.
“We’re always trying to look for Earth analogues, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun,” said Stephen Kane, an astronomer at San Francisco State University who chairs the habitable zone working group of the Kepler mission.
“Some people call these habitable planets, which of course we have no idea if they are. We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets,” Professor Kane said.
Planets similar to Venus orbit a little too close to the Sun and so are in danger of losing their water and being cloaked in carbon dioxide, whereas planets like Mars orbit a little too far away, which means any surface water is locked away as ice.
Even though the orbit of Kepler 186f takes it on the extreme cold side of its habitable zone, there is a possibility that it slightly larger size compared to Earth means that it has pulled in a thicker atmosphere, making it warmer than it otherwise might be due to an enhanced greenhouse effect, Professor Kane said.
The planet is the fifth to be discovered orbiting the same red dwarf star and Dr Quintana and her colleagues detected its faint signature when analysing the minute fluctuations in the brightness of the distant star as the planet passed between the star and Earth. Calculations showed that its orbit of 130 days takes it within the star’s vital habitable zone.
“We are not saying that there’s water on the surface. All we know is that the surface has the right temperature that water could exist there in a liquid state,” Steve Howell, Kepler’s project scientist at Nasa Ames, told Science.
The rocky alien planet Kepler 186f is an Earth-size world that could have liquid water on its surface, and possibly even life. It orbits a star 490 light-years away. See the full details of alien planet Kepler-186f in this Space.com infographic.
The edge of habitability
Kepler-186f actually lies at the edge of the Kepler-186 star's habitable zone, meaning that liquid water on the planet's surface could freeze, according to study co-author Stephen Kane of San Francisco State University.
Because of its position in the outer part of the habitable zone, the planet's larger size could actually help keep its water liquid, Kane said in a statement. Since it is slightly bigger than Earth, Kepler-186f could have a thicker atmosphere, which would insulate the planet and potentially keep its water in liquid form, Kane added.
"It [Kepler-186f] goes around its star over 130 days, but because its star is a lower mass than our sun, the planet orbits slightly inner of where Mercury orbits in our own solar system," Barclay said. "It's on the cooler edge of the habitable zone. It's still well within it, but it receives less energy than Earth receives. So, if you're on this planet [Kepler-186f], the star would appear dimmer."
Exoplanet hunting in the future
Kepler-186f could be too dim for follow-up studies that would probe the planet's atmosphere. NASA's James Webb Space Telescope — Hubble's successor, expected to launch to space in 2018 — is designed to image planets around relatively nearby stars; however, the Kepler-186 system might be too far off for the powerful telescope to investigate, Barclay said.
Scientists using the Kepler telescope discovered Kepler-186f using the transit method: When the planet moved across the face of its star from the telescope's perspective, Kepler recorded a slight dip in the star's brightness, allowing researchers to learn more about the planet itself. Kepler suffered a major malfunction last year and is no longer working in the same fashion, but scientists are still going through the spacecraft's trove of data searching for new alien worlds.
This diagram shows the position of Kepler-186f in relation to Earth.
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