Kepler-78b
The exoplanet most similar to Earth in mass, radius, and density at discovery.
Kepler-78b, originally designated KIC 8435766 b, orbits the star Kepler-78. When it was found, it was the exoplanet most like Earth in mass, radius, and average density. Discovered in 2013 using Kepler space telescope data, astronomers detected it by watching it cross its star's face, by the dimming when it moved behind the star, and by the starlight it reflected as it orbited.
The planet is about 1.69 times Earth's mass and 1.12 times its radius. Surface gravity is roughly 11 m/s², a bit stronger than Earth's. Two independent teams measured its mass by observing the wobble its gravity caused in its star's orbit—a technique usually applied to gas giants, but possible here because the planet orbits so closely. One team, using the HARPS-N spectrograph in the Canary Islands, estimated a mass 1.86 times Earth's and a radius 1.16 times larger. The other, using the Keck Observatory in Hawaii, found a mass 1.69 times Earth's and a radius 1.12 times larger. Both groups calculated a density around 5.5 g/cm³, matching Earth's, suggesting a rock-iron composition. The iron core could make up 40% of the planet's mass. Kepler-78b resembles other hot, dense exoplanets like Kepler-10b, Kepler-36b, and CoRoT-7b.
It completes an orbit every 8.5 hours and reflects 20% to 60% of the starlight it receives. Its orbit is about 40 times closer to its star than Mercury is to the Sun, giving a surface temperature of roughly 2200 K—hot enough to strip any stable atmosphere, though liquid and solid parts should remain. As one astronomer put it, it might be Earth-sized, but it is more like a lava planet than an Earth-like one.
Regarding its origin, one astronomer called it "an abomination," noting there is no known way a small world just 12% larger than Earth could have formed in that location, nor any known mechanism that could have moved it there. It cannot stay in that hellish orbit for long; it is destined to be swallowed by its star in about three billion years.
Quick Facts
- Extrasolarplanet
- yes
- Apsis
- astron
- Discovered
- November 5, 2013
- Discovery Method
- Transit and reflection/emission modulations (Kepler Mission)
- Eccentricity
- 0
- Period
- 0.35500745 · (8) days / (0.355007450 day hours)
- Star
- Kepler-78
Facts from the source article.
Lore & Background
Kepler-78b is the only planet found orbiting the star KIC 8435766, now known as Kepler-78. It was not initially designated as a Kepler object of interest because data analysis failed to identify it due to its short orbital period. Two independent teams estimated the planet's mass by detecting the gravitational 'wobble' it induces in its host star, a method difficult for Earth-sized exoplanets but made possible by the planet's extremely close orbit. Francesco Pepe's team used the HARPS-N spectrograph at the Telescopio Nazionale Galileo in the Canary Islands, estimating a mass 1.86 times Earth's and a radius 1.16 times Earth's. Andrew Howard's team used the High Resolution Eschelle Spectrometer at the W.M. Keck Observatory in Hawaii, estimating a mass 1.69 times Earth's and a radius 1.12 times Earth's. Both estimates yield a density of about 5.5 g/cm³, equivalent to Earth's density, possibly indicating a rock-iron composition with an iron core up to 40% of the planet's mass. Kepler-78b is most similar to larger high-density, hot exoplanets like Kepler-10b, Kepler-36b, and CoRoT-7b.
Reader's Guide
Kepler-78b's significance lies in being, at the time of its discovery, the exoplanet most similar to Earth in mass, radius, and mean density, suggesting a rock-iron composition like Earth's. Its detection via multiple methods—transit, occultation, and reflected light—and the successful mass measurement through radial velocity wobble for an Earth-sized world marked a milestone in exoplanet characterization. The planet orbits its star every 8.5 hours, about 40 times closer than Mercury is to the Sun, resulting in a surface temperature of 2200 K—hot enough to strip any stable atmosphere, leaving a lava planet with stable liquid and solid portions. Its origin is puzzling: according to astronomer Dimitar Sasselov, there is no known mechanism that could have formed or transported such a small world to that location, and it is destined to be swallowed by its star in about three billion years. Kepler-78b thus represents both a remarkable analog to Earth in composition and an extreme example of a doomed, hellish world.
Did You Know?
- The planet's surface temperature is estimated at 2200 K, high enough to strip any stable atmosphere.
- Two independent teams estimated its mass using the gravitational wobble method, a rare feat for an Earth-sized exoplanet.
- Kepler-78b is expected to be swallowed by its parent star in about three billion years.
More in Transiting Exoplanets, Part 2 1-24
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