Dust to Dust: The Death of an Exoplanet
NASA’s space-based Kepler space telescope is a planet hunter, and it has found an especially unusual potential exoplanet near a star dubbed KIC 12557548, some 1,500 light years away.
It’s small, with one of the shortest planetary orbits yet observed, circling its star every 15 hours. And it seems to be disintegrating to dust.
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Kepler looks for planet-like objects by detecting tiny, but periodic, dips in a star’s brightness. That periodicity is a clue that there might be a planet orbiting the star. Whenever the planet passes in front of the star, it blocks the same small amount of light.
But in a new paper in the Astrophysical Journal, co-author Saul Rappaport, a professor emeritus at MIT, and his colleagues report an unusual pattern of light from KIC 12557548. Its light dropped by different intensities every 15 hours.
So some object was blocking light from the star quite regularly, but to varying degrees. And that’s just, well, kind of weird.
They considered that it might be a binary planet system: two planets orbiting each other that also orbited the star, similar to how the Earth and the moon orbit each other as well as the sun. But that short 15-hour orbit simply wouldn’t allow enough time for two orbiting planetary bodies.
So the team reached outside the proverbial box and came up with a genuinely novel hypothesis: perhaps that periodic but varying intensity in the starlight was due to an orbiting object that was more of a shape-shifting body. And the most obvious explanation for something like that would be a stream of dusty debris emitting off the planet, just like a comet’s tail.
Given its short, tight orbital period, the planet must be quite close to its star; Rappoport et al. estimate its temperature at around 3600 degrees Fahrenheit (2255 Kelvin). That is sufficient to pretty much melt rocky stuff on the planet’s surface, and the resulting wind would carry the dust into space, forming a tail of debris as it orbits its star.
To test their hypothesis, the team built a model of the planet and star system, including its long dusty tail. The model showed the dust should be densest surrounding the planet and thin out as it trailed away into space. They also ran computer simulations of the star’s variations in brightness as their model planet passed by. The resulting light patterns matches the same irregular patterns detected by Kepler.
It’s a tiny planet, roughly the size of Mercury, and given the rate at which its shedding dust, Rappoport et al. estimate the little exoplanet will disintegrate entirely in about 100 million years. That’s oodles of time compared to the average human lifespan, but it’s a blink of an eye for the cosmos.
“This might be another way in which planets are eventually doomed,” Dan Fabryky, another member of the Kepler Observatory team, cheerily noted. “A lot of research has come to the conclusion that planets are not eternal objects, they can die extraordinary deaths, and this might be a case where the planet might evaporate entirely in the future.”
Ah, poor little exoplanet circling KIC 12557548, we barely knew you.
Images: (top) Artist’s conception of the comet-like tail of a possible disintegrating super Mercury-size planet candidate as it transits its parent star named KIC 12557548. NASA/JPL/Caltech. (bottom) Artist’s rendering of the Kepler mission. NASA/Public domain.