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Kepler data reveals 715 more exoplanets

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On Feb. 26, scientists with NASA's Kepler mission announced the discovery of 715 more exoplanets in 305 different solar systems.

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"The Kepler team continues to amaze and excite us with their planet hunting results," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington. "That these new planets and solar systems look somewhat like our own, portends a great future when we have the James Webb Space Telescope in space to characterize the new worlds.”

The discovery was made using a new statistical technique called verification by multiplicity. Kepler collected data from about 150,000 stars, and a few thousand had planet candidates. Several hundred of these had multiple candidates, and these were studied more carefully to reveal the 715 newly confirmed planets. This data came from observations between May 2009 and March 2011, meaning that another similar windfall can come from data taken between March 2011 and May 2013, when the Kepler probe lost its ability to accurately observe other star systems. All of the systems studied are single-star systems, as systems with multiple stars are too chaotic to produce the kind of data needed for this method.

“Four years ago, Kepler began a string of announcements of first hundreds, then thousands, of planet candidates --but they were only candidate worlds,” said Jack Lissauer, planetary scientist at NASA's Ames Research Center in Moffett Field, Calif., who co-led the research team that performed the analysis. “We've now developed a process to verify multiple planet candidates in bulk to deliver planets wholesale, and have used it to unveil a veritable bonanza of new worlds.”

The new method appears to be best at finding smaller planets, as almost 95 percent of the newly discovered planets are Neptune-sized or smaller. The number of Earth-sized planets has increased fivefold, the number of planets larger than Earth but smaller than Neptune (known as super-Earths) has increased sevenfold, the number of Neptune-sized planets has increased threefold, but the number of Jupiter-sized planets has only risen by 2 percent.

Four planets in particular are of special interest: Kepler-174d, Kepler-296f, Kepler-298d and Kepler-309c. Unlike the other 711 planets, these planets all orbit in the habitable zones of their stars.

Kepler-174 is a star in the constellation Lyra. It has 0.602 times the mass of the Sun and 0.620 times the radius of the Sun. Its mean surface temperature is estimated to be 4,880 K, while the mean surface temperature of the Sun is 5,778 K. Kepler-174 is estimated to have an age of 7±4 billion years, compared to 4.57 billion years for the Sun. The spectral classification of Kepler-174 is K3V, which is significantly dimmer and redder than the G2V spectral classification of the Sun. Its planet of interest, Kepler-174d, has an undetermined mass and an estimated radius of 2.19 Earth radii. It is 0.677 AU away from its star, and completes an orbit of Kepler-174 once every 247.35373 Earth days. For comparison, Venus is 0.723 AU away from the Sun on average, and orbits the Sun once every 224.701 Earth days. The black-body temperature of Kepler-174d is 195 K or -78°C, which is only slightly warmer than the coldest air recorded on Earth. The actual temperature of any planet will be warmer than its black-body temperature due to albedo, atmosphere, and internal heating effects.

Kepler-296 is a star in the constellation Draco. It has 0.516 times the mass of the Sun and 0.560 times the radius of the Sun. Its mean surface temperature is estimated to be 4,249 K. The spectral classification of Kepler-296 is K6V, which is significantly dimmer and redder than the spectral classification of the Sun. Its planet of interest, Kepler-296f, has an undetermined mass and an estimated radius of 1.79 Earth radii. It is 0.263 AU away from its star, and completes an orbit of Kepler-296 once every 63.335879 Earth days. For comparison, Mercury is 0.387 AU away from the Sun on average, and orbits the Sun once every 87.969 Earth days. The black-body temperature of Kepler-174d is 241 K or -32°C, which is close to Earth's black-body temperature of 254 K or -19°C.

Kepler-298 is a star in the constellation Draco. It has 0.650 times the mass of the Sun and 0.580 times the radius of the Sun. Its mean surface temperature is estimated to be 4,465 K. The spectral classification of Kepler-298 is K4V, which is significantly dimmer and redder than the spectral classification of the Sun. Its planet of interest, Kepler-298d, has an undetermined mass and an estimated radius of 2.5 Earth radii. It is 0.305 AU away from its star, and completes an orbit of Kepler-298 once every 77.473633 Earth days. The black-body temperature of Kepler-298d is 294 K or 21°C, which is somewhat warmer than Earth.

Kepler-309 is a star in the constellation Lyra. It has 0.666 times the mass of the Sun and 0.720 times the radius of the Sun. Its mean surface temperature is estimated to be 4,713 K. The spectral classification of Kepler-309 is K3V, which is significantly dimmer and redder than the spectral classification of the Sun. Its planet of interest, Kepler-309c, has an undetermined mass and an estimated radius of 2.51 Earth radii. It is 0.401 AU away from its star, and completes an orbit of Kepler-309 once every 105.356383 Earth days. The black-body temperature of Kepler-309c is 268 K or -5°C, which is slightly warmer than Earth.

“From this study we learn planets in these multi-systems are small and their orbits are flat and circular ― resembling pancakes ― not your classical view of an atom,” said Jason Rowe, research scientist at the SETI Institute in Mountain View, Calif., and co-leader of the research. “The more we explore the more we find familiar traces of ourselves amongst the stars that remind us of home.”

This discovery brings the Kepler confirmed count of planets up to 961, and the total count to nearly 1,700. The number of planet candidates is now 3,845. The findings papers will be published on March 10 in The Astrophysical Journal.

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