Exoplanet Kepler 78b Has Earth-Like Mass And Size

Brett Smith for redOrbit.com – Your Universe Online

In August, an international team of researchers announced the discovery of a planet outside our solar system – also referred to as an exoplanet – that orbits its star in just 8.5 hours and is roughly the same size as Earth.

Now, the same research team members that found the planet, dubbed Kepler 78b, have made another startling discovery: the planet’s mass is roughly the same as our own.

“This planet is a complete mystery,” said astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA). “We don’t know how it formed or how it got to where it is today. What we do know is that it’s not going to last forever.”

“Kepler-78b is going to end up in the star very soon, astronomically speaking,” agrees CfA astronomer Dimitar Sasselov.

According to the Harvard scientists involved with the discovery, Kepler 78b is the first known Earth-sized planet that also shares an Earth-like density. While the exoplanet is only about 20 percent larger than Earth in terms of size and volume, it weighs almost twice as much – suggesting an Earth-like composition of iron and rock.

“It’s Earth-like in the sense that it’s about the same size and mass, but of course it’s extremely unlike the Earth in that it’s at least 2,000 degrees hotter,” said team member Josh Winn, an associate professor of physics at MIT. “It’s a step along the way of studying truly Earth-like planets.”

The determination of the planet’s orbit and size was made by analyzing the light given off by its host star as the planet passes in front of it in a phenomenon known as a transit. The researchers recorded a transit each time the star’s light dipped, and calculated the amount of dimming to determine the planet’s size.

To determine the planet’s weight, the researchers followed the motion of the star itself. Because a planet can exert a gravitational tug on its star based on the planet’s mass, the star can be seen as having a slight wobble, known as a Doppler shift.

By determining its star’s Doppler shift, the team discovered Kepler 78b’s mass is about 1.7 times that of Earth. Winn said the result was somewhat expected, considering the planet’s extremely close distance to its star. A gaseous planet, for example, would break apart in such a close orbit.

According to Artie Hatzes, a professor of astronomy at the Institute of Thuringer Landessternwarte in Germany, the planet’s short orbital period helped in the investigation of Kepler 78b’s mass. “This is a tricky measurement to make because the star is very active and it has starspots,” said Hatzes, who did not participate in the research.

“These create a false Doppler signal often referred to as ‘activity jitter.’ You can use special tricks to disentangle the Doppler wobble due to the planet, and the Doppler velocity variation caused by the spots on the star. If the planet had a much longer orbital period, it would be much more difficult to do so.”

While the planet’s initial discovery was reported in The Astrophysical Journal, the new study on Kepler 78b’s mass was published in the journal Nature.