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Exoplanet Comparison Kepler-78 b.png
Size comparison of Kepler-78 b with Earth
Discovery dateNovember 5, 2013[1]
Transit and reflection/emission modulations (Kepler Mission)
Orbital characteristics
0.01 AU (1,500,000 km)
0.355007 d
Inclination81 +6
Physical characteristics
Mean radius
1.12 R
Mass1.69-1.85 M
Mean density
5.3-5.6 g cm−3

Kepler-78b (formerly known as KIC 8435766 b) is an exoplanet orbiting around the star Kepler-78.[2][3][4] At the time of its discovery, it was the known exoplanet most similar to Earth in terms of mass, radius and mean density.[5]


The planet was discovered in 2013 by analyzing data from the Kepler space telescope. The planet was found not only transiting the star, but its occultation and reflected light from the parent star due to orbital phases were also detected. It did not have Kepler object of interest designation at first as the automatic data analysis missed this planet due to its short period.[6]


Size, mass and composition[edit]

Kepler-78b is approximately 1.69 times the mass of the Earth and 1.2 times the radius.[7] The acceleration due to gravity on the planet's surface is about 11 m/s2, slightly greater than the surface gravity of Earth.[7]Two independent teams were involved in pioneering work to estimate the mass of the planet. The estimates were made possible because Kepler-78b's gravity causes a "wobble" in the orbit of the host star. While the method has been used to characterize gas giants, ordinarily it is hard to estimate the mass of Earth-sized exoplanets because their gravity is too weak to produce a visible influence on their stars. In this case, the planet's orbit is so close to its star that its gravitational influence has a detectable effect.[8]

One team, led by Francesco Pepe, used the High Accuracy Radial Velocity Planet Searcher-North (HARPS-N) spectrograph at the Telescopio Nazionale Galileo in the Canary Islands[9] to estimate that the planet has a mass 1.86 times that of the Earth and a radius 1.16 times. The other, led by Andrew Howard of the University of Hawaii at Manoa, used data from the High Resolution Eschelle Spectrometer on the Keck 1 Telescope at the W.M. Keck Observatory in Hawaii to estimate the mass as 1.69 times that of the Earth and the radius as 1.2 times. Both estimates put the planet's density at about 5.5 grams per cubic centimetre, equivalent to that of the Earth and possibly indicative of a rock-iron composition like Earth's.[8][10] The iron core could build up to 40% of the planet mass.[5]

Kepler-78b is most similar to larger high-density, hot exoplanets like Kepler-10b, Kepler-36b and CoRoT-7b.[5]


Kepler-78b orbits around the parent star in an 8.5-hour orbit. It reflects 20% to 60% of the starlight it receives.[6] Due to its extremely close solar orbit, about 40 times closer to its star than Mercury is to the Sun, the planet's surface is estimated to be at a temperature of 2,300 K (2,030 °C; 3,680 °F) to 3,100 K (2,830 °C; 5,120 °F).[7] This temperature is high enough to have stripped the planet of any stable atmosphere, but the liquid and solid portions of the planet should be stable.[7] According to Francesco Pepe, one of the astronomers involved with the discovery, the planet may be Earth-sized but "it can be imagined like a lava planet rather than an Earth-like planet."[8]


According to CFA astronomer Dimitar Sasselov, "this lava world is an abomination. There’s no physical way a small world, only 20 percent larger than Earth, could have evolved in that location and there’s no known mechanism that could have transported it there. But one thing that is certain, it can’t stay roasting in that hellish orbit for long; it’s destined to get swallowed by its star very soon".[11] It is estimated that the planet will be swallowed by its parent star in about three billion years.[12][13]


  1. ^
  2. ^ Transits and occultations of an Earth-sized planet in an 8.5-hour orbit
  3. ^
  4. ^ Chang, Kenneth (30 October 2013). "Astronomers Find Earthlike Planet, but It's Infernally Hot". New York Times. Retrieved 31 October 2013.
  5. ^ a b c Pepe, F.; Cameron, A. C.; Latham, D. W.; Molinari, E.; Udry, S. P.; Bonomo, A. S.; Buchhave, L. A.; Charbonneau, D.; Cosentino, R.; Dressing, C. D.; Dumusque, X.; Figueira, P.; Fiorenzano, A. F. M.; Gettel, S.; Harutyunyan, A.; Haywood, R. L. D.; Horne, K.; Lopez-Morales, M.; Lovis, C.; Malavolta, L.; Mayor, M.; Micela, G.; Motalebi, F.; Nascimbeni, V.; Phillips, D.; Piotto, G.; Pollacco, D.; Queloz, D.; Rice, K.; Sasselov, D. (2013). "An Earth-sized planet with an Earth-like density". Nature. 503 (7476): 377–380. arXiv:1310.7987. Bibcode:2013Natur.503..377P. doi:10.1038/nature12768. PMID 24172902.
  6. ^ a b Transits and occultation of an Earth-sized planet in an 8.5-hour orbit
  7. ^ a b c d Howard, A. W.; Sanchis-Ojeda, R.; Marcy, G. W.; Johnson, J. A.; Winn, J. N.; Isaacson, H.; Fischer, D. A.; Fulton, B. J.; Sinukoff, E.; Fortney, J. J. (2013). "A rocky composition for an Earth-sized exoplanet". Nature. 503 (7476): 381–384. arXiv:1310.7988. Bibcode:2013Natur.503..381H. doi:10.1038/nature12767. PMID 24172898.
  8. ^ a b c Gibney, Elizabeth (30 October 2013). "Exoplanet is built like Earth but much, much hotter". Nature. Retrieved 30 October 2013.
  9. ^ "An Earth-like planet characterized by HARPS-N at the TNG".
  10. ^ Grossman, Lisa (30 October 2013). "Astrophile: Evil twin planet makes other Earths likely". New Scientist. Retrieved 30 October 2013.
  11. ^
  12. ^
  13. ^ Strange 'Lava World' Is Most Earthlike Alien Planet Yet

External links[edit]