杏吧原创

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聽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.

鈥淚t immediately struck me as weird,鈥� says Markus Janson of Princeton University. 鈥淚 thought something more complex must be going on here than, 鈥楾here鈥檚 light from a planet.'鈥�

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

鈥淭here鈥檚 really no wiggle room left for claiming that Fomalhaut聽b is a directly imaged [Jupiter-sized] planet,鈥� says of the University of Toronto, Canada.

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

Small, small world?

Fomalhaut b鈥檚 discoverer, of the University of California, Berkeley, thinks it might be an exo-Saturn 鈥� 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鈥檚 light.

鈥淲e have in our solar system an example of what we鈥檙e proposing for Fomalhaut聽b,鈥� Kalas says. 鈥淭hat鈥檚 not proof of anything, but it鈥檚 a plausibility argument.鈥�

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聽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 鈥� in line with the observations.

鈥淵ou 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,鈥� Janson says. 鈥淚n principle you could then get something small enough and hot enough [to] explain what you observe.鈥�

If that is borne out, rather than being the planet that wasn鈥檛, Fomalhaut聽b could be the smallest exoplanet to have its picture taken.

Journal reference: ; the work will be published in a future edition of