Astronomers spot the smallest potentially-habitable planets yet

I love astronomy. It's one of the few topics that never fails to excite me, and the most exciting part of following astronomy news in recent years has been tracking the discovery of planets orbiting around other stars. Now, the Kepler team at NASA has announced that they have discovered three new planets in the habitable zones of their stars that are potentially the most Earth-like exoplanets we know of so far.

From left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth

Two of these planets (the two closer to Earth in the above image), called Kepler-62e and Kepler-62f, are part of the same five-planet system orbiting Kepler-62, a cool orange K-type star about 1,200 light years away, in the constellation Lyra.

A comparison diagram between the Kepler-62 system and our inner solar systemKepler-62e is a 'super-Earth' roughly 1.6 times the size of Earth, and it takes just over 122 days to circle its star. At the distance it orbits, it's still in the 'goldilocks zone' — the band around the star where liquid water can exist on the planet's surface — but it receives about 20 per cent more solar radiation than Earth does, and based on its size, astronomers believe that it may be a 'water world'.

Kepler-62f is perhaps the more Earth-like of the two. It's about 1.4 times the size of Earth, still making it a 'super-Earth', and it orbits roughly in the middle of Kepler-62's habitable zone. It takes roughly 267.3 days to complete one orbit, and at its distance, it receives about half of the solar radiation that Earth does. This planet's size and orbital distance make it a prime candidate, in the eyes of astronomers, for it to be a rocky world that could possibly support life. Although it it would be a cooler world than Earth, just based on the amount of radiation it receives, the planet is large enough that it could 'hold on' to heavier gases in its atmosphere — greenhouse gases — that could keep the planet warmer.

A comparison diagram between the Kepler-69 system and our inner solar systemIn another system, Kepler-69, roughly 2,700 light years away, the astronomers have found two planets orbiting the star. The innermost of the two, Kepler-69b, is twice as big as Earth, orbits around its star every 13 days, and would be very hot.

However, the other planet, Kepler-69c, is only around 1.7 times the size of Earth. It orbits along the inner edge of Kepler-69's habitable zone, taking about 242 days to go around the star, and given its size, the astronomers believe that this might be another 'water world'.

The first exoplanet the Kepler mission discovered in the habitable zone of its star was Kepler-22b (shown on the left in the top image for comparison). It orbits a Sun-like star about 600 light years away in the constellation Cygnus, and is about 2.4 times the size of Earth.

The astronomers still need to determine the mass of these newly discovered planets, which will give them a better idea of their composition, but they use computer models based on what we do know about the planets in our own solar system and those we have found orbiting other stars, to figure out the potential conditions for the exoplanets we find. These worlds, especially Kepler-62f, look like prime candidates for followup studies to determine if they could support life.

"The Kepler spacecraft has certainly turned out to be a rock star of science," said John Grunsfeld, Associate Administrator of the Science Mission Directorate at NASA Headquarters in Washington, according to a statement. "The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."

(Images courtesy: NASA Ames/JPL-Caltech)

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