This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
A significant number of exoplanets orbit their parent stars far more closely than anything does in our solar system. From hot Jupiters to hot-Neptunes, and hot super-Earths, there is quite an array of worlds in devilishly close proximity to the blazing radiation of a star. In some cases we have been able to measure the emission temperature of these worlds, and not surprisingly they are enormously hot (at least on their daysides). This could boil away atmospheres from giant planets, or melt the solid mineral surfaces of smaller ones.
Now it appears that intense stellar irradiation may also cause worlds to turn to dust.
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A new study by Rappaport et al. reports the investigation of a rather peculiar observation in the Kepler data archive. The target system, designated as KIC (Kepler Input Catalog) 12557548, is a star a little less massive than our Sun, and consequently a little cooler. Roughly every 15 hours something moves between us and this star, blocking a tiny fraction of its light. This on its own would not be cause for that much excitement, it could be a companion star, or it could be yet another of the over 2,300 planetary candidatesKepler has currently turned up. But the way in which the starlight seems to be blocked is very unusual, imprinting a shape in the timeline of light (the lightcurve) that's at odds with what we'd expect for a planet (as illustrated here):
Something's askew. On the left, a 'normal' planetary transit lightcurve (WASP-10) showing a simple, symmetrical, dip in starlight as the planet transits. To the right, the lightcurve from KIC 12557548 (Credit: Rappaport et al.)
So what's going on? Rappaport and colleagues have tested various possibilities, from a peculiar binary giant planet to a triple stellar system in very specific geometrical configurations, but none seem to reproduce what is seen very well. Instead, the model that appears to best describe what Kepler is seeing is that of a Mercury-sized planet (KIC 1255b, about 1/10th the size of Earth) whose solid surface is literally being turned to gas and dust by the extreme temperature this close to the star. This dust may be forming a comet-like tail streaming behind the planet, that also blocks the starlight, like a dreadful plume of industrial pollution.
Other, probably rocky, planets have been found before orbiting extremely close to their parent stars, but these have been quite massive worlds. That larger mass, and increased gravitational pull, makes it easier for them to hold onto evaporated material. Whereas the proposed diminutive Mercury-sized KIC 1255b cannot hang onto much, and will spew gas and dusty grains of material like pyroxene or aluminum oxide out into space - driven by the fearsome thermal environment.
It is also possible that KIC 1255b is volcanically active, perhaps tormented by tidal forces this close to the star, and as heavy elements boil into its skies some will condense into dust grains, only to be lofted to escape velocity by gases heated at the surface to close to 2,000 Kelvin. At this rate it may take only about 200 million years more for this world to be whittled away to nothing.
Future observations may help determine what exactly is going on in this strangely transiting system, spectroscopy might be able to spot the presence of photoionized elements like magnesium, silicon, and even iron, to help pin down the nature of KIC 1255b. But it does seem that planets are not always in for an easy ride in life, born out of dust and turned back to dust, just another tiny pinch of elements adrift in the cosmos.
