Astrophile is Joshua Sokol 's monthly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

An artist’s impression of a Mercury-sized exoplanet crossing its parent star, KIC 12557548 NASA/JPL-Caltech

If you wanted to see something that looks like an evaporating exoplanet close to home, you might watch a sun-grazing comet. After drifting for aeons in the outer solar system, these comets fall inwards, skimming over the sun’s surface – where many of them blow apart like dandelions. For the comet, it’s a violent trauma; but to the sun, it barely takes a puff of air.

In contrast, the exoplanet known as KIC 12557548b doesn’t just occasionally venture within spitting distance of its star. It lives there all the time, where magnetic hotspots could focus death rays at its surface.

KIC 12557548b orbits a star a little smaller and redder than our own. Seen from Earth, that star dims when the planet crosses in front of it, like thousands of other planet-hosting stars discovered by NASA’s Kepler observatory.


Unlike those other planets, however, no two crossings of this particular alien world look the same: the star appears to flicker erratically. That’s how we know something strange is going on.

Zooming in

Since KIC 12557548b was discovered in 2012 by a team led by Saul Rappaport at Massachusetts Institute of Technology, follow-up observations and careful inferences have gone a long way towards explaining what’s happening in this system.

Let’s zoom in about 1500 light years, towards the constellation Cygnus – where the planet is located.

Picture a Mercury-sized ball of rock swerving around its star like a roller coaster going round a tight turn. A single orbit takes just 15.7 hours, one of the shortest “years” of any planet found by Kepler.

An enormous tail of dust comes off the planet like a supersized comet, trailing behind and shrouding much of the star’s surface. One theory about where this tail comes from is that the planet is spewing out its own insides through volcanoes, similar to what we see on Jupiter’s moon Io.

But the leading theory is that the star is baking the planet to death and scattering its ashes. At the planet’s surface, radiation from the nearby star is hot enough to vaporise layers of exposed rock, after which a wind of charged particles – also from the star – blows that vapour into space.

Magnetic rays of death

Star spots may increase the devastation. These dark patches, where the star’s magnetic field is concentrated, may act as death rays, blasting out more material when the planet passes in front of them. This could happen either because these patches emit high-energy radiation that can better chip into the rock, or because the swirling magnetic fields around them help yank the vapour into orbit.

When it reaches space, the rock vapour cools, leaving a trail of dust grains behind the planet smaller than the smoke particles from a wood fire. Exactly how small these are is an open question – and if we knew the answer, we would know how much material the star is grinding off the planet, and thus how long KIC 12557548b has left to live.

Current estimates give it anywhere from under 100 million to a billion years.

Either way, we’ll have plenty of time before the end to watch layers of the planet peel away like an onion. Maybe KIC 12557548b was originally whittled down from a rocky world like Earth, or even a gas giant the size of Jupiter. But we may now be seeing straight down to what makes up a planet’s naked core – something we can only otherwise guess at.

And KIC 12557548b is just one of several planets being vaporised by their stars, meaning we may soon be seeing the insides of more than one planet. This system’s cousin is KIC 8462852, the famous so-called “alien megastructures” star that similarly exhibits variable dips in brightness – a pattern probably also caused by orbiting natural debris (and not aliens). There are a few other oddballs like it too, but KIC 12557548b was the first one we discovered.

Consider it the hipster’s alien megastructure: we knew it was blinking erratically before blinking erratically was cool.