There’s something strange obscuring the light from a cool, low-mass star observed by NASA’s Kepler mission. Every 15.685 Earth days, 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.

evaporating HD 209458b

The first exoplanet ever discovered to transit in front of its star, the gas giant HD 209458b also wowed astronomers when they found evidence that it was evaporating, sending a tail of material into space. Now astronomers wonder if they may have found a small, rocky exoplanet dissipating away in the same fashion.

ESA / Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS, France)

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 dematerializing.

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 vaporized. 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 vaporize.


Image of Jon Jenkins

Jon Jenkins

January 20, 2012 at 9:12 am

The period is 15.685 hours, not 15.685 days.

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Image of Rod


January 21, 2012 at 8:26 pm

Data like this points to problems with the conventional model for explaining the origin of various solar systems and exoplanets via dust disks and accretion of tiny dust grains growing into planets over millions of years. Another group that does not get the press attention can be found at 47 are listed with semi-major axis >=5 AU, the average is close to 236 AU and max is 2500 AU. The average mass is close to 11 Jupiters for this group and max is 31 Jupiters. Again here is data pointing to conflicts with current stellar evolution model explanations for planetary formation from tiny dust grains growing over millions of years. If evaporating exoplanets and the large exoplanets at great distances from their parent stars formed in situ, the conventional model is overthrown.

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