Astronomers this week revealed that the mass of exoplanet Kepler 78b has been determined. The planet, located 400 light-year away in orbit around the star Kepler 78, has a mass of around 1.7 times that of Earth. The planet also has a radius just 1.2 times that of Earth and its density is also similar to Earth’s. The new research appeared this week in the journal Nature.

These measurements make Kepler 78b the smallest planet outside our solar system to have had both its mass and size measured. Researchers believe the new data suggests the planet is made of mostly rock and iron – a composition similar to that of the Earth’s.

Though Kepler 78b resembles Earth in many ways, there is no chance that humans would ever be able to live there. The planet is located very close to its host star, orbiting it once very 8.5 hours. At that range, Astronomers say, the planet is sure to have extremely hot temperatures unsuitable for life.

“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 Josh Winn, a co-author on the paper and an associate professor of physics at the Massachusetts Institute of Technology. “It’s a step along the way of studying truly Earth-like planets.”

Astronomers were able to measure the mass of Kepler 78b by measuring its Doppler effect it has on Kepler 78. For eight straight days a team led by astronomer Andrew Howard of the University of Hawaii used the school’s Keck Observatory to analyze the star. Their measurements are backed up by a second paper published in Nature this week by a separate group of astronomers.

(Image courtesy NASA/David A. Aguilar)