Science Focus
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If you look at our Solar System, there's a sharp divide between the four rocky planets and the large "ice giants" of the outer Solar System, Uranus, and Neptune. The divide isn't only a matter of properties; there's a large size gap between Earth and Neptune, which has a radius four times the size of Earth's. There's also a matter of location, with the rocky planets all clustered in the inner Solar System, while the ice giants orbit on the outer edge.
This raises an obvious question: are any of these differences related and, if so, how? We might still be stumped on these questions if it weren't for the discovery of exoplanets. Much to our surprise, the majority of the planets we're discovering have a radius that's intermediate between the radiuses of Earth and Neptune. We've been able to get some understanding of the exoplanets' interiors even as we've struggled to get planet formation models to produce bodies of this size (previously, we didn’t know that any of these planets existed, so there was nothing for the models to explain). Now, a review of exoplanets suggests there's a sharp cutoff at about two times Earth's radius between what you might consider a super-Earth and a mini-Neptune.
Rocky or gassy?
The vast majority of exoplanet discoveries has come from the Kepler observatory, which tracks the dimming that exoplanets create as they pass between their host star and the Earth. That, combined with the orbital period, gives us a sense of a planet's radius. But it tells us nothing of its mass, which is needed if we're to determine the properties of the planet.
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