Feb 12 2024
The Exoplanet Radius Gap
As of this writing, there are 5,573 confirmed exoplanets in 4,146 planetary systems. That is enough exoplanets, planets around stars other than our own sun, that we can do some statistics to describe what’s out there. One curious pattern that has emerged is a relative gap in the radii of exoplanets between 1.5 and 2.0 Earth radii. What is the significance, if any, of this gap?
First we have to consider if this is an artifact of our detection methods. The most common method astronomers use to detect exoplanets is the transit method – carefully observe a star over time precisely measuring its brightness. If a planet moves in front of the star, the brightness will dip, remain low while the planet transits, and then return to its baseline brightness. This produces a classic light curve that astronomers recognize as a planet orbiting that start in the plane of observation from the Earth. The first time such a dip is observed that is a suspected exoplanet, and if the same dip is seen again that confirms it. This also gives us the orbital period. This method is biased toward exoplanets with short periods, because they are easier to confirm. If an exoplanet has a period of 60 years, that would take 60 years to confirm, so we haven’t confirmed a lot of those.
There is also the wobble method. We can observe the path that a star takes through the sky. If that path wobbles in a regular pattern that is likely due to the gravitational tug from a large planet or other dark companion that is orbiting it. This method favors more massive planets closer to their parent star. Sometimes we can also directly observe exoplanets by blocking out their parent star and seeing the tiny bit of reflected light from the planet. This method favors large planets distant from their parent star. There are also a small number of exoplanets discovered through gravitational microlensing, and effect of general relativity.
None of these methods, however, explain the 1.5 to 2.0 radii gap. It’s also likely not a statistical fluke given the number of exoplanets we have discovered. Therefore it may be telling us something about planetary evolution. But there are lots of variables that determine the size of an exoplanet, so it can be difficult to pin down a single explanation.