The extrasolar planet-search sweepstakes crossed a threshold early this week with the widely publicized report of the first bona fide rocky world orbiting a star 158 light-years away.
It’s a foregone conclusion among astronomers that rocky planets are abundant in the galaxy, but seeing — er measuring — is beleiving.
But once again the dog-eared phrase “Earthlike planet” hit the press. “Planet Discovery: One Step Closer to a Second Earth,” reported one magazine headline. Another headline: “NASA Finds Smallest Earthlike Planet Outside Solar System.”
But wait, just four months ago a headline announced “Found: An Earthlike Planet, at Last,” referring to the 7-Earth mass world Gliese 581g, merely 20 light-years away. This was reported as the first planet identified in a star’s habitable zone (because the detection has not been confirmed by other astronomers, its reality is now in doubt.)
Yes, Kepler-10b is the smallest companion yet found around a normal star but the size of a planet alone doesn’t make it “Earth-like.”
Exoplanet habitability depends on a long list of at least 50 other variables: the planet’s atmosphere, geology, evolutionary history, influence of the other planetary systems bodies, etc. And, whether a rocky planet is one, three or five Earth-masses could be largely irrelevant. We simply don’t know yet.
Because of the close proximity to its sun-like star, we can deduce that Kepler-10b is a very hot rock. It’s not impossible that the roasting world might have an oxygen atmosphere that is being cooked out of the surface rocks. But that’s where any remote similarity to Earth ends.
Kepler-10b is so dense it is similar to Mercury, which means its could be a ball of iron with a thin rocky crust. But the craggy sun-baked planet Mercury looks comparatively mild-mannered because Kepler-10b is 20 times closer to its star than Mercury is to the sun.
The planet must also be gravitationally tide-locked — just as the moon keeps one face toward Earth. Kepler-10b may have a 2,500 degree Fahrenheit ocean of liquid magma on its sunlit side. Depending on how cool the shadowed far-side crust is, sluggish waves of lava would slosh up to frozen basaltic shores near the planet’s terminator.
The planet’s core of radioactive elements could be even be bigger than Earth’s. Therefore, aside from being baked, the planet would have a lot of trapped heat to get rid of too. Magellan spacecraft radar mapping of cloudy Venus gives us a preview of what the dark side of the planet might look like. Imagine molten rock pouring from immense but shallow volcanic calderas, creeping across the nighttime side surface like a luminous monster stretching its tentacles. There could also be oozing pancake domes; lave channels, sinuous rills, and rolling volcanic plains.
The planet may be so hot it is a ball of mush. The dark-side crust could be floating on molten rock. This would be analogous to the “ice rafts” floating on Europa’s subsurface ocean. These rafts may continuously drift into the planet’s sunlit side and melt like pats of butter left on a sunny picnic table. New material volcanically upwelling on the far-side of the planet might cool and make new crust, and keep the conveyor belt going.
What also makes Kepler-10b even more unearthly is that could be the stripped down “naked core” of a gas giant planet that migrated close to its star. The planet’s lighter elements boiled away under torrential heat and stellar winds.
On the other hand, we know so little about Earth’s origin, we may in fact be living on the residual core of a Neptune-sized world that originally condensed around the newborn sun. “Nobody really knows what Earth was like when it was born,” says exoplanet sleuth Geoff Marcy of the University of California at Berkeley.
All of these befuddling possibilities mean we must be very cautions even suggesting a substellar companion takes us closer to finding and Earth clone. As I’ve discussed in previous articles, this landmark discovery of anything out there even remotely resembling Earth is a long way off.