Oceans of lava might bubble on its surface. Hot pebbles may rain down from the sky. But the extrasolar planet CoRoT-7b is considered to be the most Earthlike world yet found outside our solar system.
A recent study, however, suggests that Earth might not be the best basis for comparison. Instead, the authors argue, CoRoT-7b is the first in a new class of exoplanets: a super-Io.
Like Jupiter's moon Io, CoRoT-7b could easily be in the right kind of orbit to experience what's known as tidal heating, according to study co-author Rory Barnes of the University of Washington in Seattle.
On Io, tidal heating is a result of the crust being constantly deformed by the push and pull of Jupiter's gravity. This action generates enough internal heat to drive hundreds of active volcanoes—and the same could be true for CoRoT-7b, Barnes said.
But unlike Io, CoRoT-7b closely orbits a star, not a planet, so tides aren't its only source of heat. Based on previous observations, astronomers know that CoRoT-7b's surface is between 1,832 and 2,732 degrees Fahrenheit (1,000 and 1,500 degrees Celsius).
That's hot enough for there to be "ponds or possibly even oceans of magma," Barnes said. Scientists also know that the planet is tidally locked, which means that only one side ever faces the star.
"There could be volcanism on the back side of the planet," Barnes said. "It could be that on one side the surface is molten, and on the other side there's raging volcanoes."
CoRoT-7b was found using the French-led planet-hunting mission CoRoT, which looks for periodic dips in starlight caused by orbiting bodies passing in front of—transiting—a star, as seen from Earth.
When CoRoT-7b's discovery was announced in February 2009, astronomers hailed the world as the smallest exoplanet yet found orbiting a sunlike star.
From CoRoT-7b's transits, astronomers could tell that the planet is about twice the size of Earth, which is approximately 7,920 miles (12,760 kilometers) wide. Io measures roughly 2,260 miles (3,630 kilometers) across.
Later studies measured CoRoT-7b's mass and density and confirmed that the planet is rocky. Based on these characteristics, CoRoT-7b was dubbed a super-Earth.
The term is one of a handful—such as "hot Jupiter" and "super-Neptune"—being used to informally classify exoplanets based on how closely they resemble worlds in our solar system.
In the recent study, presented last month at a meeting of the American Astronomical Society, Barnes and colleagues looked at the possible orbits for CoRoT-7b based on its size and mass, its proximity to its star, and its interactions with a nearby sister planet, CoRoT-7c.
The researchers found that even a slight eccentricity in CoRoT-7b's orbit would generate enough tidal heating to spawn bunches of volcanoes, making the planet much more Io-like than Earthlike.
For starters, just as Io circles close to massive Jupiter, CoRoT-7b orbits very close to its host star, so the influence of gravity is especially strong, Barnes said.
What's more, both Io and CoRoT-7b are tidally locked. In Io's case, this means that one side always faces Jupiter. That side of the moon is being tugged so much harder by gravity that the otherwise round world becomes slightly elongated, with a bulge around the middle.
"Earth does this—we have a tidal bulge due to interactions with the sun and our moon," Barnes noted. "The ocean tides are the result of [gravitational] tides, but rock is also distorted due to tidal effects."
In addition, Io maintains an irregular, elliptical orbit due to interactions with other Jovian moons close by, so its distance to Jupiter changes over time. As Io gets closer to Jupiter, it becomes more elongated, and as it moves away it becomes more spherical.
"If you had a tennis ball and you kept squeezing it, you would get heat from friction," Barnes said. "For Io it's like that, except you're doing it to a planet."
For now, CoRoT-7b is too distant to allow for current techniques and telescopes to accurately trace the planet's orbit, so whether the world truly resembles Io remains a mystery.
But "I think they have a pretty good case," said Rosaly Lopes, a planetary scientist with NASA's Jet Propulsion Laboratory in Pasadena, California.
After all, volcanism on Io had been predicted shortly before the Voyager spacecraft actually spotted the moon's volcanic plumes in 1979, she said. (See a picture of a giant plume on Io.)
"Stan Peale and colleagues ... analyzed the orbit of Io and said it would have tidal heating," Lopes said. The very next week, pictures from Voyager revealed about a dozen plumes, and later images from the Galileo spacecraft found more than 170 active volcanoes.
Peale, now a professor emeritus at the University of California, Santa Barbara, agreed that "the authors' conclusions are viable," noting that the presence of a second planet near CoRoT-7b means that the planet's orbit could vary "sufficiently to heat the interior to a super-Io state, leading to Io-like volcanic surface activity."
According to JPL's Lopes, "what's interesting here is that they're showing worlds like Io probably exist in other solar systems. But whether [CoRoT-7b] actually has active volcanism at the moment is going to be very difficult to prove."
It's possible spacecraft such as the Spitzer Space Telescope could see gases coming from CoRoT-7b's volcanoes, study author Barnes said.
"It might have a huge cloud of volcanic gases orbiting the planet. We could maybe see it in [light signatures] using Spitzer, but that would be very hard, because the planet is so far and so faint."
Overall, Barnes thinks similarly hot, rocky worlds will start turning up in the tens of hundreds as current planet-hunting missions such as CoRoT and the recently launched Kepler spacecraft peer deeper into the sky.
"I think of Kepler as a super-Io detector," Barnes said. And each super-Io found will be "a stepping stone to finding real super-Earths."
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