Kepler-10, formerly known as KOI-72, is a Sun-like star in the constellation of Draco that lies 607 light-years (186 parsecs) from Earth.[5][6] Kepler-10 was targeted by NASA's Kepler space telescope, as it was seen as the first star identified by the Kepler mission that could be a possible host to a small, transiting exoplanet.[7] The star is slightly less massive, slightly larger, and slightly cooler than the Sun; at an estimated 11.9 billion years in age, Kepler-10 is 2.3 times the age of the Sun.[2]
Kepler-10 is host to a planetary system made up of at least three planets. Kepler-10b, the first undeniably rocky planet,[7] was discovered in its orbit after eight months of observation and announced on January 10, 2011. The planet orbits its star closely, completing an orbit every 0.8 days,[8] and has a density similar to that of iron.[7] The second planet, Kepler-10c, was confirmed on May 23, 2011, based on follow-up observations by the Spitzer Space Telescope. The data shows it has an orbital period of 42.3 days and has a radius more than double that of Earth, but it was initially thought to have a higher density, making it the largest and most massive rocky planet discovered as of June 2014.[2][9][10] However, refined mass measurements have shown it to be a more typical volatile-rich planet.[11] A third planet, Kepler-10d, was discovered in 2023 by radial velocity observations.[4]
Nomenclature and history
Kepler-10 was named because it was the tenth planetary system observed by the Kepler spacecraft, a NASA satellite designed to search for Earth-like planets that transit, or cross in front of, their host stars with respect to Earth. The transit slightly dims the host star; this periodic dimming effect is then noted by Kepler.[12] After eight months of observation ranging from May 2009 to January 2010, the Kepler team established Kepler-10b as the first rocky exoplanet discovered by the Kepler satellite. Kepler-10 was the first Kepler-targeted star suspected of having a small planet in orbit. Because of that, verifying Kepler's discovery was prioritized by telescopes at the W.M. Keck Observatory in Hawaii. The discovery was successfully verified.[7] Although there had been many potentially rocky exoplanets discovered in the past, Kepler-10b was the first definitively rocky planet to have been discovered.[13]
The discovery of Kepler-10b was announced to the public at a winter meeting of the American Astronomical Society on January 10, 2011 in Seattle.[13] On May 23, 2011, the existence of Kepler-10c was confirmed at the 218th AAS meeting in Boston.[14]
Characteristics
Kepler-10 is a G-type star, like the Sun. With a mass of 0.895 (± 0.06) Msun and a radius of 1.056 (± 0.021) Rsun, the star is approximately 10% less massive than and 5% wider than the Sun. The metallicity of Kepler-10, as measured in [Fe/H] (the amount of iron in the star), is -0.15 (± 0.04); this means that Kepler-10 is about 70% as metal-rich as the Sun. Metallicity tends to play a large role in the formation of planets, determining if they form, and what kind of planet they will form.[15] In addition, Kepler-10 is estimated to be 11.9 billion years old and to have an effective temperature of 5627 (± 44) K;[5] To compare, the Sun is younger and hotter, with an age of 4.6 billion years[16] and an effective temperature of 5778 K.[17]
Kepler-10 is located at a distance of 186 parsecs from the Earth, which equates to approximately 607 light years. Also, Kepler-10's apparent magnitude, or brightness as seen from Earth, is 10.96; it therefore cannot be seen with the naked eye.[5]
Per the usual exoplanet nomenclature, the first planet discovered to be orbiting Kepler-10 is called Kepler-10b. Announced in 2011, it was the first definitely rocky planet identified outside the Solar system. The planet has a mass that is 3.33±0.49 times that of Earth's and a radius that is 1.47+0.03 −0.02 times that of Earth.[2] The planet orbits Kepler-10 at a distance of 0.01684 AU every 0.8375 days; this can be compared to the orbit and orbital period of planet Mercury, which circles the Sun at a distance of 0.3871 AU every 87.97 days.[18] Because the planet orbits so closely to its star, its eccentricity is virtually zero. It, thus, has an extremely circular orbit.[8]
Kepler-10c[9] was also discovered by NASA's Kepler Mission,[19] the second exoplanet found to orbit Kepler-10. Radial-velocity measurements of the body initially suggested that it has a mass of 17.2±1.9 Earth masses and a radius of 2.35 Earth radii, which would have made it the largest known rocky planet as of 2014[update]. Kepler-10c would orbit Kepler-10 at a distance of 0.24 AU every 45.29 days.[2] However, in July 2017, more careful analysis of HARPS-N and HIRES data showed that Kepler-10c was much less massive than originally thought, instead around 7.37+1.32 −1.19ME with a mean density of 3.14 g/cm3. Instead of a primarily rocky composition, the more accurately determined mass of Kepler-10c suggests a world made almost entirely of volatiles, mainly water.[11]
A candidate third planet with an orbital period of about 102 days, given the provisional designation KOI-72.X, was identified in 2016 based on transit-timing variations.[20][11] In 2023, the presence of a third planet, Kepler-10d, was confirmed by radial velocity observations. It has an orbital period of 151 days and a minimum mass about 13 times that of Earth.[4]
↑ All masses, eccentricies, Kepler-10d's orbital period and Kepler-10c's inclination are from from Bonomo et al. (2025),'"`UNIQ--ref-00000023-QINU`"' the remaining from Bonomo et al. (2023).'"`UNIQ--ref-00000024-QINU`"'
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