Super-Earths & Sub-Neptunes
The galaxy's most common planets are sizes the Solar System skipped entirely — bigger than Earth, smaller than Neptune.
101 · zoom in
Our own system has a gap: nothing between Earth and Neptune, a factor of nearly four in size with no members. The galaxy, it turns out, loves that gap — planets in it are the single most common kind we've found.
They come in two flavours that blur together: rocky 'super-Earths' up to about 1.5 times Earth's size, and gas-shrouded 'sub-Neptunes' above roughly twice Earth's size. In between sits a real scarcity — the 'radius valley' — that hints at how these worlds lose or keep their atmospheres.
Because we have no local example, these worlds are genuinely alien territory: we're still learning what they're made of and whether any could be habitable.
Statistically, super-Earths and sub-Neptunes are the most abundant planets in the galaxy, yet the Solar System contains none. The 'radius valley' — a dip in the number of planets around 1.5–2 Earth radii — appears to divide bare rocky cores from worlds that have held onto a thick hydrogen-helium or steam envelope.
The dividing line is likely set by atmospheric escape: close-in planets can have their light envelopes stripped by stellar radiation, leaving a rocky super-Earth, while those that keep the envelope read as larger, lower-density sub-Neptunes.
Mass-and-radius measurements from paired transit and radial-velocity data — and increasingly atmospheric spectra from JWST — are what let us sort one from the other and probe a class of planet Earth has no cousin to.