By Camille Carlisle

January 23: There’s something strange obscuring the light from a cool, low-mass star observed by NASA’s Kepler mission. Every 15.685 hours, KIC 12557548’s light dims for about 1.5 hours. The dips in starlight aren’t always the same — some events block more light than others — so the occultations don’t look like the regular blip caused by a planet passing in front of the star. After considering various options, an international team of astronomers reported recently that the signal might be from debris thrown off by a small rocky planet as it disintegrates under the star’s glare.

Astronomers found what looks like an evaporating gas giant in 2003, but if real KIC 12557548’s world would be the first solid exoplanet seen dematerialising.

So far there isn’t a lot of information to go on. The transits’ regularity argues against wildly off-kilter orbits, and observations seem to rule out anything larger than three Jupiter masses. The comet-like tail the researchers suggest as an explanation can’t be made of hydrogen, like the tail seen from the 2003 discovery HD 209458b, because the gas wouldn’t block enough light: particulates (like dust) are needed. The planet needs to be pretty small, too — the astronomers assume a little less than 2 times Mercury’s mass for their calculations — in order to not show up in the observed light dips. It also needs to be small enough that dust can overcome the body’s surface gravity and launch into space.

A super-Mercury like the one the astronomers propose would only survive about 200 million years before it vaporised. The authors note that “this is probably less than the age of the star, but not alarmingly so.” The “not alarmingly so” means that the timescales for the star’s age and for planet evaporation are about the same, which is good for the dissipating exoplanet hypothesis because it isn’t impossible for the planet to still be there. On the other hand, it’d be safer for the hypothesis if the disintegration time was significantly longer than the star's age.

Spectral observations may be able to determine if the planet and its tail are there and what the tail is made of. The astronomers put their bet on pyroxene, a silicate mineral found in Earth’s crust and mantle (and in meteorites) that should survive close proximity to KIC 12557548 long enough to block starlight before the grains vaporise.