The massive Kepler survey found a treasure trove of exoplanets. But in all that wealth they found three anomalies: what appeared to be rings of dust surrounding stars where planets should be. They were rocky planets in the process of being obliterated. And a team of astronomers that found a way to use these gory sites to understand some of the most mysterious and hard to detect planets in the universe.
We currently know of about 5,000 exoplanets in the galaxy. This represents only a small fraction of the estimated 1 trillion worlds within the Milky Way. But even though we’ve made great strides, we have exceptional difficulty finding one particular class of exoplanet: the small, rocky ones. Our techniques rely on transits. When an exoplanet crosses in front of the face of the star, it causes a small dip in brightness from our point of view. But if the planet is too small, the change in brightness isn’t large enough for us to detect, and so the small planets, roughly the size of the Earth and smaller, remain hidden from us.
But recently a team of researchers recently pointed out that some anomalies in the Kepler data may be a blessing in disguise. Among the data returned from Kepler includes what appears to be rings of dust and debris surrounding a star. Previous researchers had concluded that these are rocky planets in the process of obliteration. They are worlds that got too close to their parent star, and the heat of that star is boiling them alive.
The team of researchers released a paper detailing simulations of how this process could unfold. They found that these small worlds are caught between two extremes. Because they orbit closely to their parent star, they are almost certainly tidally locked, which means that only one side of the planet faces the star at all times. The other side is permanently locked in night. The day side gets blasted to such a degree that instead of a crust it just has a thin shell of pure magma. But the other side is so cold that the rocky crust remains in place.
The night side cools down the planet while the day side heats it up. The astronomers found that there is only a very narrow window where we can observe such situations. If a planet is too big or the star is not bright enough, then it does not evaporate enough material for us to detect it in something like Kepler. However if the planet is too small or the star is too intense, the entire planet obliterates in a short enough time that we are unlikely to see it in a random sample of stars.
Only certain special cases can lead to a ring of debris large enough and visible enough for us to see it. Going from this the astronomers estimate that for every star in the galaxy there is roughly one planet the size of the Earth or smaller.
Additionally, the astronomers found that these debris trails can give us very important clues as to rocky planetary formation. We don’t normally get to crack open planets and see what’s inside of them. But in these cases the parent star is doing the job for us. They advocate for a follow-up observations with the James Webb Space telescope to study these systems in detail to understand what these rocky planets were made of.