Astronomers Found the First Earth-Sized Rocky Planet, Kepler-78b

November 3, 2013

Comparison between Earth and Kepler-78b.

Comparison between Earth and Kepler-78b. Credit: David A. Aguilar (CfA)

After many discovered planets that have either mass or size of the Earth, Kepler-78b is the first rocky exoplanet to have both mass and size similar to our planet's.

The Kepler's space telescope primary mission may have ended, but its legacy consists of more than 3200 exoplanet candidates awaiting confirmation from ground telescopes. The latest discovery is also the most Earth-like planet when it comes to both size and mass, as well as composition.

Named Kepler-78b, the planet orbits its star every 8.5 hours. The system is located some 400 light-years from Earth in constellation Cygnus. Kepler-78b is 1.2 times the size of Earth and 1.7 times more massive. This makes its density same as Earth's, meaning it is most likely a rocky planet with iron core. Its star is smaller and less massive than the Sun.

The Kepler space telescope detected potential planets by observing regular drops in stars' brightness caused by planets transiting in front of them. With the orbit of mere 8.5 hours, Kepler-78b was easier to detect than planets with longer orbit. By using this method, the Kepler space telescope determines planets' radius by the amount of blocked starlight.

Ground telescopes, however, use another method to detect planets. By employing the radial velocity method to measure how much a planet causes its host star to wobble, astronomers can determine the mass of the planet.

Two independent teams used ground telescopes and published two papers in the journal Nature. "The news arrived in grand style with the message: 'Kepler-10b has a baby brother,'" said Natalie Batalha. She is a scientist at NASA's Ames Research Center in Moffett Field, USA, who also led the team that discovered Kepler-10b, a larger but also rocky planet identified by the Kepler space telescope. "The message expresses the joy of knowing that Kepler's family of exoplanets is growing," she says. "It also speaks of progress. The Doppler teams are attaining higher precision, measuring masses of smaller planets at each turn."

The discovery is interesting in that Kepler-78b is the first planet with known both mass and size. Up until now, astronomers found planets with either the size or the mass similar to Earth's, never both. "This bodes well for the broader goal of one day finding evidence of life beyond Earth," Betalha adds.

It is worth noting that Kepler-78b isn't believed to support life. With orbit of just 8.5 hours and its distance of just 1.5 million kilometers from the host star, its surface should be extremely hot, up to 2000 degrees Celsius. When the planet and its host star were forming, the star was larger than it is now, meaning it extended beyond Kepler-78b's orbit.

"It couldn't have formed in its current place because you can't form a planet inside a star," said Dimitar Sasselov at Harvard-Smithsonian Center for Astrophysics, Cambridge, USA. He is a member of one of the teams discovering the planet. "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma."

No matter how it came to be, we know that Kepler-78b will in about 3 billion years when its host star tears it apart. "Kepler-78b is going to end up in the star very soon, astronomically speaking," Sasselov said.

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