According to astronomers reporting in the journal Nature, Kepler-78b is the first Earth-sized exoplanet found to have an Earth-like density.
This image shows Kepler-78b and Earth. Kepler-78b is about 20 percent larger than Earth and weighs 1.8 times as much as our planet. Image credit: David A. Aguilar / Harvard-Smithsonian Center for Astrophysics.
Kepler-78b orbits the Sun-like G-type star Kepler 78 in just 8.5 hours at a distance of less than 1 million miles. The star, also known as KIC 8435766, is located in the constellation Cygnus about 400 light-years away.
In August 2013, Dr Roberto Sanchis-Ojeda from Massachusetts Institute of Technology and his colleagues discovered the planet and determined that it is 1.2 times Earth’s size.
Now, they have found that Kepler 78b shares another characteristic with Earth – its mass.
They determined that the planet is about 20 percent larger than the Earth, with a diameter of 9,200 miles, and weighs almost twice as much.
As a result it has a density (5.3 grams per cubic centimeter) similar to Earth’s (5.5 grams per cubic centimeter), which suggests an Earth-like composition of iron and rock.
“This planet is a complete mystery. 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,” said co-author Dr David Latham of the Harvard-Smithsonian Center for Astrophysics.
The tight orbit of the planet poses a challenge to astronomers. When this planetary system was forming, the young star was larger than it is now. As a result, the current orbit of Kepler-78b would have been inside the swollen star.
“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. It’s a step along the way of studying truly Earth-like planets,” said co-author Dr Josh Winn from Massachusetts Institute of Technology and the Kavli Institute for Astrophysics and Space Research.
Kepler-78b is a doomed world. Gravitational tides will draw it even closer to its star. Eventually it will move so close that the star’s gravity will rip the world apart.
Scientists predict that the planet will vanish within 3 billion years.
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Bibliographic information: Howard AW et al. A rocky composition for an Earth-sized exoplanet. Nature, published online October 30, 2013; doi: 10.1038/nature12767
