A great quest is underway to discover Earth-size worlds in their stars’ habitable zones. Along the way, astronomers have been surprised to learn that the most typical size of planet in our galaxy is one with no counterpart in our own solar system.

These planets are called super-Earths or mini-Neptunes. They populate an unfamiliar regime of worlds. They are larger than Earth, the Sun’s biggest rocky world, yet smaller than Neptune or Uranus, which are about 4 Earths in diameter. In fact, an exoplanet detected as it transits across the disk of its host star is 4 times more likely to have a size in the super-Earth/mini-Neptune regime than to be bigger than Neptune. These discovery statistics are telling us something—but what?

The ice giants of our solar system, Uranus and Neptune (see “The Realm of the Ice Giants,” page 7), represent a transition from a hydrogen-dominated mass, such as that seen in Jupiter and Saturn, to a mass defined more by solid or liquid ices deeper in the atmosphere formed of “heavy” elements (atoms more massive than helium). Such planets still have a hydrogen-helium outer envelope, but their mass is dominated by a deep, fluid ocean made of heavy, icy material and potentially a rocky core. By contrast, terrestrial planets have rock and metal compositions and relatively thin atmospheres containing little to no hydrogen or helium.

At what point does a planet stop being mostly rock and transition to one that’s mostly hydrogen and helium? For now, the answer is unknown, but we are trying to figure it out.

Very interesting article about Super Earths / Mini Neptunes. (But please, let’s try to avoid saying they are smaller than Uranus.)