“Be still when you have nothing to say; when genuine passion moves you, say what you’ve got to say, and say it hot.” –D.H. Lawrence
Here in our Solar System, the planet closest to our Sun is Mercury, speeding around in a complete orbit once every 88 days, with daytime temperatures reaching a sweltering 400° C, or around 800° F.
With its size, temperatures, and distance from the Sun, Mercury has had its entire atmosphere stripped away by the Sun. And while these temperatures might seem hot, in comparison with what we experience on Earth, remember that Mercury is still a whopping 46 million kilometers away from the Sun. And that’s at its closest! Compared to the diameter of the Sun itself, the size of Mercury’s orbit is more than thirty times as large.
But not every solar system in the Universe is like ours. In fact, a select few of them have planets so close to their parent stars that they’re only a couple of solar radii away. Fortunately for us, planets like this are the easiest type to find, because they both cause their parent stars to wobble the most, and also because they transit in front of their parent stars most frequently, blocking a small portion of the light.
But while Mercury reaches high enough temperatures that its atmosphere acquires so much kinetic energy from the Sun that the individual atoms cannot stay gravitationally bound to the planet, at least the surface of Mercury is safe.
It seems unfathomable that a planet could be so close to its parent star that its very surface would be in jeopardy, but let’s remember what we ought to see when a planet transits in front of a star.
What ought to happen is that a small — but increasing — fraction of the star’s light should be blocked as the planet moves across its parent star, hitting a maximum when the planet crosses the Sun’s center, and then decreasing in a symmetric fashion as the planet moves off of the Sun once again. And this pattern should repeat, over and over again, as the planet continues to orbit its star.
But this isn’t what they found, not for the star KIC 12557548, not according to Kepler mission scientist S. Rappaport and his team. In fact, from the team’s paper, their transit curve looks like this:
You’ll notice that this curve is not symmetric. It rises, as expected, but it tails off much more slowly and gradually both than you’d expect and than it rose by. This can, as far as we understand it, only mean one thing: this planet has a tail!
“Tails are for comets, not planets,” you’re probably thinking. How could a planet — a planet that probably lacks an atmosphere, no less — possibly have a tail?
With an orbital period of just fifteen hours, temperatures on the surface of this world easily approach 2,000° C (3,600° F), temperatures high enough, in the vacuum of space, to literally boil a planet! In other words, this planet gets so hot that its surface is literally evaporating!
And based on the rate of evaporation, the entire planet will likely be gone in about 100 million years. That may seem like a long time, but it’s only about 2% as long as our Solar System’s been around! According to Kepler scientist Dan Fabrycky,
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.
Welcome to the first star — KIC 12557548 — with a planet so close that it’s going to die slowly and painfully, literally boiled away atom-by-atom. And from over a thousand light-years away, here on Earth, we’re watching it happen.