Astronomers discover first Earth-sized planet in the habitable zone

Astronomers working with data from the Kepler Space Telescope have hit another milestone in the search for exoplanets as they've reported the very first confirmed Earth-sized planet discovered in the habitable zone of its star, making it a potential haven for both liquid water and alien life.

This newly-discovered exoplanet, named Kepler-186f, is just 10 per cent larger than Earth, meaning it's most likely a rocky world but more importantly, the planet orbits its star every 130 days or so. This puts Kepler-186f near the outer edge of the star's habitable zone — the ring-shaped region around a star where there's just enough heat from the star that a planet there could have liquid water on its surface.

This video animation shows the planet orbiting the star along with its sibling-worlds. The green band that circles the star represents the habitable zone.

"We're always trying to look for Earth analogs, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun," Stephen Kane, one of the study's co-authors who works with NASA's Exoplanet Science Institute at Caltech, said in a statement. "This situation is a little bit different, because the star is quite different from our sun."

According to the researchers, the star, Kepler-186, is a red dwarf about 500 light years away, in the constellation Cygnus. It's cooler than our sun, about about half the size and has a light spectrum shifted more towards the infrared. Kepler-186's planets orbit closer in than for larger stars, which gives them more of a chance of being tidally-locked, with one side always facing towards the star. Also, dwarf stars tend to be more active, blasting out powerful solar flares. This could all combine together to make Kepler-186f a cold, barren and radiation-blasted planet — perhaps something like a tidally-locked Mars.

However, there's a few points working in Kepler-186f's favour, to help it be more hospitable to life.

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First of all, the thickness of a planet's atmosphere usually depends on its size. The bigger it is, the more it can retain. Kepler-186f is just at the right size to retain enough of an atmosphere, but not too much that it ends up being like Neptune. Since the spectrum of light from a dwarf star is different than what we see from our sun, with more of the light in the longer wavelengths and peaking in the infrared, any atmosphere around Kepler-186f would absorb more radiation, as would any ice that might be on the planet's surface. The planet's size also means that it has a greater surface area to heat. All of this would help to keep the atmosphere warm, despite the planet's distance from the star.

As for radiation, most dwarf stars tend to be more active, but Kepler-186 is an older, calmer star, so it doesn't produce flares quite as powerful as younger ones. This would reduce the amount of damaging radiation the planet would be exposed to, especially at its distance from the star. Furthermore, this distance may actually help it, with respect to how much water it might have. According to the researchers, it's hard for a rocky Earth-sized world in the habitable zone of these M-dwarf stars to retain water, but Kepler-186f's position towards the outer edge of the habitable zone it actually makes it more likely than for a planet closer to the inner edge of the zone.

It's possible that Kepler-186f is tidally-locked with its star. That doesn't necessarily mean that it can't be habitable, as astronomers have speculated about 'eyeball Earths' for some time now. However, Kepler-186f's distance helps with this too, as the further the planet is from the star, the more likely it is to rotate. If the planet has a warm interior, this could give rise to a planetary magnetic field to protect the surface from damaging solar radiation.

Combine all this together and that's the recipe for a planet similar to Earth. It wouldn't be an Earth-twin, like we could expect to find in the habitable zone of a sun-like star, but Kepler-186f could be an Earth-cousin.

Kepler-186f is now the fifth planet discovered in this star system. Four others, labeled planets b through e, are also just slightly bigger than Earth (the biggest being about 30 per cent larger), and all four orbit much closer in to the dwarf star — the closest circling the star in just under four days and the farthest in just over 22 days. A comparison of this planetary system to our own puts Kepler-186f at the same distance from the centre of its star as Mercury is from our sun, but whereas Mercury is a barren blasted planet due to the intense heat and radiation from our sun, Kepler-186f is in the right place around its star to possibly support liquid water and life.

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Whether or not Kepler-186f actually is a habitable planet that harbours water and life is unknown. Just being in the habitable zone doesn't automatically make a planet habitable. Kepler-186f has potential, though, and is currently ranked as 17th on the Planetary Habitability Lab's Earth Similarity Index (12th if you only count confirmed worlds). Furthermore, the star that this planet orbits around is one of the most common type of stars, representing roughly 70 per cent of the stars in our galaxy. So, this confirmation that Earth-sized planets do form in the habitable zone of these dwarf stars is a big step towards confirming that this type of planet is also the most common in the galaxy.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, the director of NASA's Astrophysics Division, according to a statement. "Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds."

(Images courtesy: NASA Ames/SETI Institute/JPL-Caltech)

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