The massive Kepler survey has found a treasure trove of exoplanets. But in all that richness, they found three anomalies: rings of dust surrounding stars where planets should be. They were rocky planets about to be wiped out. And a team of astronomers who have found a way to use these gory places to understand some of the most mysterious and elusive 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 while we’ve made great strides, we’re having exceptional difficulty finding a particular class of exoplanets: the small, rocky ones. Our techniques are based on transits. In our view, when an exoplanet crosses in front of the star’s face, it causes a small drop in brightness. But if the planet is too small, the change in brightness isn’t large enough for us to see it, and so the small planets, which are about the size of Earth and smaller, are hidden from us.
But recently, a team of researchers has suggested that some anomalies in the Kepler data could be a hidden boon. The data returned by Kepler appear to include rings of dust and debris surrounding a star. Previous researchers had concluded that they were rocky planets in the process of extinction. These are worlds that have come too close to their parent star, and the heat of that star is boiling them.
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The research team published a paper detailing simulations of how this process might evolve. They found that these small worlds are caught between two extremes. Because they orbit closely around their parent star, they are almost certainly tidally locked, meaning only one side of the planet faces the star at a time. The other side is permanently locked at night. The day side is blasted so far that instead of a crust it only has a thin shell of pure magma. But the other side is so cold that the rocky crust stays in place.
The night side cools the planet while the day side heats it up. The astronomers found that there is only a very narrow window in which we can observe such situations. If a planet is too big or the star isn’t bright enough, then it won’t evaporate enough material to detect it in something like Kepler. However, if the planet is too small or the star too intense, the entire planet will be obliterated in such a short time that we’re unlikely to see it in a random sample of stars.
Only certain special cases can result in a ring of debris large enough and visible enough for us to see. From there, astronomers estimate that for every star in the galaxy, there is roughly one planet the size of Earth or smaller.
In addition, the astronomers found that these debris tracks can give us very important clues about the formation of rocky planets. Normally we can’t break open planets and see what’s inside them. But in these cases, the parent star does the work for us. They advocate follow-up observations with the James Webb Space Telescope to study these systems in detail and understand what these rocky planets were made of.