Fomalhaut's giant exoplanet may be small lava world

One of the first exoplanets to be imaged directly is not the plain Jane it seemed. Instead, it may be a rocky planet covered with lava.

Most planets are too small, dim and close to their parent stars to be photographed. But Hubble Space Telescope images from 2004 and 2006 revealed a Jupiter-like dot around the star Fomalhaut, which lies 25 light years away. The apparent planet, named Fomalhaut?b, appeared inside a gap in a dusty disc surrounding the star, suggesting it had carved out the space during its orbits.

Early on, though, the dot exhibited behaviour unbecoming of a planet, dropping in brightness by a factor of two between 2004 and 2006.

"It immediately struck me as weird," says Markus Janson of Princeton University. "I thought something more complex must be going on here than, 'There's light from a planet.'"

Now Janson and his colleagues have analysed new infrared observations ? the most sensitive yet ? made by NASA's Spitzer Space Telescope. They found nothing at the wavelengths where a Jupiter-sized planet should be brightest.

"There's really no wiggle room left for claiming that Fomalhaut?b is a directly imaged [Jupiter-sized] planet," says Ray Jayawardhana of the University of Toronto, Canada.

Spitzer is not sensitive enough to detect objects smaller than Jupiter, opening up a Pandora's box of new possibilities.

Small, small world?

Fomalhaut b's discoverer, Paul Kalas of the University of California, Berkeley, thinks it might be an exo-Saturn ? a ringed planet about a third the size of Jupiter. It would be too small for Spitzer to see, but would be brighter than expected for its size at visible wavelengths because its ring system would reflect its star's light.

"We have in our solar system an example of what we're proposing for Fomalhaut?b," Kalas says. "That's not proof of anything, but it's a plausibility argument."

Janson thinks the bright speck orbiting Fomalhaut is more likely to be a transient dust cloud kicked up by colliding asteroids. That would explain why its brightness has faded and suggests it will only get fainter with time.

However, there is another, more brutal possibility. Fomalhaut?b might be a rocky planet up to 10 times the mass of the Earth that is being beaten to a molten mess by incoming meteoroids, Janson says. The violence would make it bright at visible wavelengths, but as its surface cools, it should fade ? in line with the observations.

"You could have this super-Earth-type planet being very, very heavily pummelled, so hot that its surface melts, and it becomes basically a lava planet," Janson says. "In principle you could then get something small enough and hot enough [to] explain what you observe."

If that is borne out, rather than being the planet that wasn't, Fomalhaut?b could be the smallest exoplanet to have its picture taken.

Journal reference: arxiv.org/abs/1201.4388; the work will be published in a future edition of The Astrophysical Journal

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