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Scientists Identify 20 Alien Worlds Most Likely to Be Like Earth
Aug 17, 2016

An international team of researchers has identified the 20 most Earth-like worlds among the more than 4,000 exoplanet candidates that NASA's Kepler space telescope has detected to date.

  • The 20 planets in the most restrictive category—rocky surface and a conservative habitable zone—are the most likely to be similar to Earth.
  • All 20 potential Second Earths lie within the habitable zones of their sun-like stars—meaning they should be able to harbour liquid water on their surfaces—and are likely rocky.
  • Identifying these Earth-like planets is important in the hunt for alien life.
  • Now scientists can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable
  • Team of the scientists sorted through the 216 habitable-zone Kepler planets and candidates found so far. (A ‘candidate’ is a world that has yet to be confirmed by follow-up observations or analysis.)
  • Kepler has found about 4,700 candidates to date, more than 2,300 of which have been confirmed; about 90 percent of all candidates should eventually turn out to be the real deal.
  • Second-Earth candidates had to be safely within the habitable zone. If a planet is too close to the inner edge, it could experience a runaway greenhouse effect like the one that occurred on Venus. And if it's too close to the outer edge, the planet could end up being a frigid world like Mars.
  • In addition to categorizing the planets by their place in the habitable zone, the team of the scientists also sorted them by size, ruling out worlds that were big enough to be gaseous.
  • There are five confirmed planets in the top 20: Kepler-186f, Kepler-62f, Kepler-283c, Kepler-296f and Kepler-442b. The other 15 are unconfirmed candidates.
  • Categorizations like this suggest that the universe is teeming with planets that could possibly harbour life.

The research also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of those outside of it—additional evidence that the universe is teeming with planets and moons where life could potentially exist.

Kepler Space Telescope

NASA's Kepler Space Telescope is an observatory in space dedicated to finding planets outside our solar system, particularly alien planets that are around the same size as Earth in the "habitable" regions of their parent star.

Since the launch of the observatory in 2009, astronomers have discovered hundreds of extra-solar planets, or exoplanets, through this telescope alone. Most of them are planets that are ranging between the size of Earth and Neptune (which itself is four times the size of Earth).

In the early years of exoplanet hunting, astronomers were best able to find huge gas giants—Jupiter's size and larger—that were lurking close to their parent star. The addition of Kepler (as well as more sophisticated planet-hunting from the ground) means that more "super-Earths" have been found, or planets that are just slightly larger than Earth but have a rocky surface.


Kepler's major achievement is showing the sheer variety of planetary systems that are available. Planet systems can exist in compact arrangements within the confines of the equivalent of Mercury's orbit. They can orbit around two stars. And in an exciting find for those seeking life beyond Earth, the telescope has revealed that small, rocky planets similar to Earth are more common than larger gas giants such as Jupiter.

Kepler's largest discovery in sheer volume occurred in February 2014, when astronomers unveiled 715 new worlds confirmed in one go. The single release of information nearly doubled the number of known planets to that point to almost 1,700.
Astronomers noted this find, using the verification by multiplicity technique, came out of the first two years of Kepler data. Two years of data remain to be studied, and investigators expect hundreds more could come out of examining that information.
Kepler was the first telescope to find a planet approximately the size of Earth in the habitable region of a star. Dubbed Kepler-69c, the exoplanet is about 2,700 light-years away and has a diameter about 1.5 times that of Earth.

The telescope also has the capability to find planets that are much smaller than Earth, such as Kepler-37b. The planet is considered to be close to Mercury's size and is likely rocky and airless, much like the planet in our own solar system.
Other weird worlds discovered by the telescope include Kepler-62e and Kepler-62f, two water worlds that likely have a global ocean—as opposed to Earth, which has a significant fraction of dry land. The planets are about 1,200 light-years away in the constellation Lyra and are close to the size of Earth.

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