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Kepler data reveals Earth-like planet in star’s habitable zone

Artist’s rendering of Kepler-1649c, which orbits in the habitable zone of a red dwarf star. Image Credit: NASA/Ames Research Center/Daniel Rutter

Scientists who took a second look at data returned by NASA’s Kepler space telescope have discovered an Earth-sized exoplanet orbiting in the habitable zone of a red dwarf star.

A star’s habitable zone is the region where temperatures allow liquid water to exist on a planet’s surface. Red dwarf stars are smaller and less massive than the Sun, so their habitable zones are closer than those of Sun-like stars.

Kepler, which operated between 2009 and 2018, used the transit method to detect exoplanets. This involves looking for periodic dimming of stars caused by planets passing in front of them.

The newly-discovered planet, Kepler-1649c, evaded discovery during initial reviews of Kepler data via a computer algorithm. Now, a group of researchers taking a second look at the data found the planet, which had been mislabeled when the data was first reviewed by the algorithm, which is known as Robovetter.  

Kepler-1649c is located approximately 300 light years from Earth. It is just 1.06 times larger than Earth and receives about 75 percent as much starlight as Earth receives from the Sun. Its size and estimated temperature makes it the most Earth-like exoplanet found in a star’s habitable zone.

The exoplanet orbits its star once every 19.5 Earth days.

“This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found. The data gathered by missions like Kepler and our Transiting Exoplanet Survey Satellite (TESS) will continue to yield amazing discoveries as the science community refines its abilities to look for promising planets year after year,” noted Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, based in Washington, DC.

Another, smaller rocky planet, Kepler-1649b, also orbits the star, but it does so at about half the distance that the newly-discovered planet does,  This puts it out of the star’s habitable zone.

“Out of all the mislabeled planets we’ve recovered, this one’s particularly exciting–not just because it’s in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet,” explained Andrew Vanderburg of the University of Texas at Austin, lead author of a paper on the discovery published in The Astrophysical Journal Letters. “If we hadn’t looked over the algorithm’s work by hand, we would have missed it.”

In an unusual configuration, Kepler-1649c orbits the red dwarf four times for every nine orbits of Kepler-1649b. While this indicates the planetary system is stable and likely long-lived, it also suggests the possible presence of a third planet orbiting between the two.

Scientists have combed Kepler data looking for a possible third planet but so far have come up empty. This does not mean the planet isn’t there. It could be very small and/or have an eccentric orbit, both of which would make it impossible to discover using the transit method.

Current calculations of Kepler-1649c’s size are an estimate and could have a margin of error. Additionally, researchers know almost nothing about its atmosphere, which directly affects its temperatures.

Red dwarf stars are the most common types of stars in the universe. However, they are often very active and undergo frequent stellar flares that bombard orbiting planets with radiation, possibly making them uninhabitable in spite of their orbiting in habitable zones.

Other habitable zone exoplanets that are roughly Earth-size include TRAPPIST-1f and Teegarden c. Exoplanets in habitable zones with temperatures closest to that of Earth include TRAPPIST-1d and TOI 700D.

Launched in 2018, TESS, which also uses the transit method, has identified 1766 planet candidates and 43 confirmed planets in an area 400 times bigger than that surveyed by Kepler.

NASA’ James Webb Space Telescope (JWST), scheduled for launch in 2021, will observe in infrared wavelengths, enabling it to penetrate dust and identify the composition of exoplanet atmospheres.

“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-size exoplanets are common around these kinds of stars,” Vanderburg said regarding red dwarfs. “With red dwarfs almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them, the chance one of them isn’t too different than our Earth looks a bit brighter.”




Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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