The two planets originally discovered by the Kepler mission may not be what we thought. Based on the initial characterization, it was thought that these planets were rocky bodies slightly larger than Earth. But continued monitoring has produced data indicating that the planets are much less dense than we originally thought. And the only realistic way to get the kind of densities they now seem to have is for a significant amount of their volume to be occupied by water or a similar liquid.
We have bodies like this in our solar system – most notably the moon Europa, which has a rocky core surrounded by an ice-covered water crust. But these new planets are much closer to their host star, which means their surfaces are probably a hazy boundary between a vast ocean and a steamy atmosphere.
Let’s revisit that
There are two main ways to find an exoplanet. The first is to observe dips in their star’s light, which are caused by planets with an orbit that takes them between the star and Earth. The second is to track whether the star’s light periodically shifts to redder or bluer wavelengths, as the star moves due to the gravity of the planets orbiting it.
Either of these two methods can tell us whether or not a planet exists. But having both gives us a lot more information about the planet. The amount of light a planet blocks can give us an estimate of its size. The amount of red and blue shifting of a star’s light can indicate a planet’s mass. With both of them, we can find out its density. And density limits the types of materials it can be composed of — low density means gas-rich, and high density means rock with a mineral-rich core.
This is exactly what we have been able to do in the Kepler-138 system. Data from these two methods indicate that the system contains three planets. Kepler-138b appears to be a small, rocky body the size of Mars. Kepler-138c and Kepler-138d both fall into the super-Earth category: rocky planets that were somewhat larger than Earth and had much more mass. All were orbiting close to Kepler-138a, a red dwarf star, with the farthest (Kepler-138d) orbiting around 0.15 astronomical units (AU is the typical Earth-Sun distance).
In the grand scheme of things, there was nothing unusual about this system that required a second look. But the researchers thought it was a good candidate for studies of the planet’s atmosphere. While the planet blocks all light as it transits in front of its host star, a small amount of light will pass through the atmosphere on its way to Earth. And the molecules in that atmosphere will absorb certain specific wavelengths, allowing us to discern their presence.
To conduct this study, a team of researchers obtained data from the Hubble and Spitzer space telescopes, timed when Kepler-138d was passing in front of the star. That’s when things started to get weird.
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