Kepler-78b is more like Earth than any of the hundreds of planets scientists have discovered. It shares Earth's size and composition, so why is our planet a green and blue paradise, while Kepler-78b is a lava-covered hellscape incapable of supporting life? Location, location, location.
Earth is 93 million miles from our sun, putting us solidly in the "just right" zone for life – not too hot, not too cold. Kepler-78b is barely 1 million miles from its sun. That's about as close as Comet ISON will get to the sun next month – so close that scientists are still placing bets as to whether the comet will survive or be ripped apart by the sun's gravitational pull.
So how can a planet exist so close to its sun? Scientists are baffled.
"This planet is a complete mystery," says David Latham, an astronomer with the Harvard-Smithsonian Center for Astrophysics (CfA). "We don't know how it formed or how it got to where it is today," if it moved there from some safer distance.
At just 1 million miles out, Kepler-78b has an incredibly tight orbit. It whirls around its star every 8 and a half hours. Earth's orbit around the sun takes about 1,000 times longer (8,742 hours). Kepler-78b is only 20 percent larger than Earth, with 1.8 times the mass (because volume increases with the cube of the radius), which means that its density is almost identical to ours. That suggests that Kepler-78b is made of the same mix of rock and iron as Earth.
The Kepler space telescope has discovered many Earth-size, sun-hugging planets, with orbits of 12 hours or less, though Kepler-78b is the first whose density has been calculated.
"Kepler-78b is the poster child for this new class of planets," says Dr. Latham.
These new planets create serious challenges to existing models of how solar systems form. Most theories say that suns initially burn hotter – and bigger – and then cool and shrink over time. Under those theories, this new class of planets would be inside their swollen stars.
"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," says CfA astronomer Dimitar Sasselov.
It's also doomed, say the astronomers. Gravity will tug it ever closer to its sun, until it gets so close that the star's gravity will rip the world apart. They estimate that this will happen within three billion years.
"Kepler-78b is going to end up in the star very soon, astronomically speaking," says Dr. Sasselov.
The Goldilocks problem
In the fairy tale, Goldilocks bypassed beds and chairs that were too hard or too soft, or porridge that was too hot or too cold, always looking for "just right" conditions. Evolution faces the same problem, since most lifeforms need liquid water to evolve.
If a planet is too far away from its sun, it gets too cold, and water freezes. If it's too close, the water boils off. That "just right" distance – not too far, not too close, not too hot, not too cold – is what scientists call the circumstellar habitable zone (CHZ).
Scientists debate whether Venus or Mars are or could ever have been within our sun's CHZ. If our sun used to be bigger and hotter, then the CHZ would have reached further out, perhaps embracing Mars. Some argue that Venus is still within the habitable zone, and that if its atmosphere were thinner and less poisonous, Venus might be hospitable to life.
But there's no question that a planet a scant million miles from a big, hot star couldn't possibly harbor life (or at least, anything we would recognize as life). In fact, scientists estimate that temperatures on Kepler-78b's surface could exceed 5,000 degrees Fahrenheit.
The only way such a small orbit could be within the habitable zone is if the star it orbits is very, very cold. Scientists are on the lookout for sun-hugging planets orbiting brown dwarfs, mini-stars that are too small to ignite fusion reactions.
So despite Kepler-78b having the right size and density to be Earth's twin, its location plays the trump card. Like Goldilocks's first bowl of porridge, Kepler-78b is too hot – much, much too hot.