New-found Earth-size Exoplanet Doomed

Artist's impression of the planet Kepler-78b and its host star. Art by Karen Teramura (UHIfA)
Artist’s impression of the planet Kepler-78b and its host star. Art by Karen Teramura (UHIfA)

Astronomers announced this week that they have spotted a rocky Earth-size planet beyond our solar system, the smallest alien world accurately sized by observers to date. However, the super-hot planet is no second Earth, and according to theories, the distant world some 700 light-years away from Earth shouldn’t exist.

The planet, Kepler-78b, was first discovered by its namesake NASA space telescope. The planet is about 20 percent larger than the Earth, with a diameter of 9,200 miles, and it weighs almost twice as much.

Using the world’s largest ground-based telescopes, two independent research teams have now confirmed the planet’s mass and density by measuring “wobbles” of its sun-like host star, seen as the exoplanet orbits around it. They report the confirmations in the journal Nature.

Unfortunately, Kepler-78b is not Earth 2.0, however, because it turns out that it circles its star at a scorching distance of one million miles. A year on this fast-paced little world lasts only 8.5 hours.

“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,” says co-author Josh Winn, an astronomer at MIT.

“It’s a step along the way of studying truly Earth-like planets.”

The problem astronomers have with their finding is that according to what we understand about planet formation, this hot lava world couldn’t have formed so close to its star, nor could it have moved there.

“This planet is a complete mystery,” says study co-author David Latham, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “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.”

Astronomers have a quandary on their hands.  They don’t believe the planet could have formed in its current orbit because then Kepler-78b would have been inside the much larger, younger star. At the same time, it couldn’t have formed farther out and migrated inward, because it should have been drawn on a swirling kamikaze dive straight into the star.

“How it came to reside in its current 8.5-hour orbit is uncertain,” says planetary scientist Drake Deming of the University of Maryland in a commentary accompanying the studies. “Among the more exotic possibilities is that it is the remnant core of a disrupted gas giant,” he writes.

Because it has the tightest orbit around a star ever seen, one thing researchers know for sure is that Kepler-78b’s days are numbered. The extreme gravitational pull from its star will draw it ever closer in, ripping the entire planet apart in about three billion years.


Follow Andrew Fazekas, the Night Sky Guy, on Twitter and Facebook.

Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.

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