Astronomers at the Harvard-Smithsonian Center for Astrophysics participating in NASA’s Kepler Space Telescope’s search for exoplanets — planets in solar systems other than out own — are puzzled by the discovery of a planet named Kepler-78b whose existence should be a physical impossibility.
Not only is Kepler-78b an astronomical enigma, but it’s also the first planet discovered, courtesy of the Kepler program, that’s of both similar size and density to our own Earth.
Kepler-78b orbits a star, classed as G-Type, a yellow dwarf star similar to our Sun, about 400 light years from Earth in the constellation Cygnus.
But the newly discovered world is a searing hot lava planet that whizzes round its parent star in just 8.5 hours. The distance between Kepler-78b and its own sun is a mere one million miles. That’s only slightly more than four times the distance between the moon and the Earth.
Kepler-78b has one of the tightest known orbits. Current theories of planet formation dictate Kepler-78b couldn't possibly have formed so close to its star, nor could it simply have arrived in its very close orbit to its sun.
Astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA) commented, "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."
Latham’s view was endorsed by fellow CfA astronomer Dimitar Sasselov, who predicted, “Kepler-78b is going to end up in the star very soon, astronomically speaking."
A planetary enigma
Kepler-78b has caught astronomers’ attention as it’s the first known Earth-sized planet with an Earth-like density. At 9,200 miles diameter, Kepler-78b is about 20 percent larger than the Earth, and weighs almost twice as much. That means its density is similar to that of Earth suggesting an iron and rock composition, again similar to Earth.
According to Latham, Kepler-78b is the first planet in a new class of small planets, roughly the same size as the Earth, to have its mass calibrated. Members of this planetary class all take less than 12 hours to orbit their parent stars.
And it’s Kepler-78b’s tight orbit that has astronomers stumped. When the planetary system, of which Kepler-78b forms part, was forming, the nascent star was larger than now. With Kepler-78b lying just 1.7 stellar radii from the surface of the star as it is today, the planet would have been inside the swollen star.
As Sasselov explained, "It couldn't have formed in place because you can't form a planet inside a star. 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."
But Kepler-78b’s fate is not in doubt, say Harvard-Smithsonian astrophysicists. Gravitational forces will draw the planet ever closer to its parent star. Ultimately, gravity will sunder Kepler-78b, causing it to vanish within 3 billion years, predict planetary theorists.
Could our own solar system once have had its own Kepler-78b equivalent? Maybe, but it would have been destroyed eons ago leaving no trace of its existence.
The Harvard-Smithsonian team studied Kepler-78b using a newly commissioned, high-precision spectrograph known as HARPS-North, at the Roque de los Muchachos Observatory on La Palma in the Canary Islands. Work concerning Kepler-78b was coordinated with a second, independent team using the HIRES spectrograph at the Keck Observatory in Hawaii. Both teams' measurements agreed with each other, giving researchers high confidence in the results of their analysis.