Kepler-78b: An Earth-sized Inferno

Even though NASA's Kepler spacecraft is likely no longer able to continue its search for exoplanets due to the failure of two of its reaction wheels (which help keep its optics pointing steadfastly at the same region of sky), it has accumulated a truly vast mountain of data containing 3277 potential-planet signals over its 3.5 year mission. That's enough to keep mission scientists busy confirming these signals for years to come, despite the probable end of Kepler's main science mission. Recently, a team of researchers at MIT working with Kepler data discovered a small planet with a radius of just 1.16 Earth-radii orbiting a sun-like star. However, the planet has one of the shortest orbital period of any planet ever discovered. This world, called Kepler-78b, whips around its parent star in just 8.5 hours. 

A single year on this planet takes about as long as a good night's sleep.

Of course, orbiting that close to its sun has some pretty extreme consequences. The surface temperature of Kepler-78b is estimated to be 2300-3000 K, which is far in excess of the melting points of most metals and silicate minerals, meaning that the entire surface of the planet is a roiling ocean of lava. If the planet has any kind of atmosphere, it would be made of rock or metal vapor. The size of its orbit (just 3 stellar radii across) also means that Kepler-78b is tidally locked to its star, always showing the same side to the star like our Moon does to the Earth.

The light-curve data for Kepler-78, from Sanchis-Ojeda et.al.

What's more interesting is what the light-curve reveals. A light-curve is a plot of a star's brightness over time. The Kepler spacecraft uses them to search for exoplanets by looking for tiny periodic changes in the star's brightness as a planet transits across the face of the star. The minimum point on the curve (the dips) represent when the planet is directly between us and the star. But what's interesting here is the smaller dip between the two transit events, starting at around the 3.5-4 hour mark. What's going on here is that as the planet is going around the star, the angle at which we view it is changing. We're actually seeing the change in illumination on the surface of the planet as it goes around the star. We can actually see the phases of this planet! This is a direct measurement of starlight reflected off the planet's surface. That can tell us a lot about the kind of surface we're looking at, especially things like composition.

But you also must remember that this is still a very small change in brightness; around 10 parts per million. That small amount of reflected light can still be detected through the glare of the host star! That's pretty amazing!

For all that, we don't really know that much about this strange new world. The transit method can only tell us so much. We have its radius and orbital period; 1.16 Earth-radii and 8.5 days, respectively. From the orbital period, we can calculate the average distance from the star with Kepler's Third Law; P^2 = A^3, where P is the period of the planet in years and A is the distance in astronomical units, or AU. This gives an average distance of just 0.010 AU, or just 1.5 million kilometers. We can also estimate the surface temperature based on the brightness of the star and the distance of the planet, which yields the 2300-3000 K estimate. And that's really about it. The MIT paper posits an upper mass bound of 8 Earth-masses, but the actual value is most likely far lower.

While this planet is far from being habitable, the fact that it and several other exoplanets have been detected with diameters about the same size as Earth means we now have the ability to detect planets in the same size range as our own. It is only a matter of time before we find one at the right distance from its star for liquid water to be stable. And there very well might be one sitting in that vast mountain of planet candidates, just waiting to be uncovered.