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Astronomers now know that planets between the size of Earth and Neptune are the most common type in our galaxy. These objects, called super‑Earths and sub‑Neptunes, do not exist in our own Solar System. A team working within the Swiss research network NCCR PlanetS has reviewed progress from 2014 to 2025 in finding and understanding these planets.
The researchers combined space telescopes that watch stars for tiny dips in light when a planet passes in front (transits) with ground instruments that measure the star’s motion caused by an orbiting planet (radial velocity). Key space missions included Kepler, the repurposed K2 mission, and the all‑sky survey TESS, plus CHEOPS for refined radius measurements. Ground facilities and spectrographs named in the review include HARPS, ESPRESSO, NIRPS, CORALIE and several dedicated telescopes such as NGTS, ExTrA, SAINT‑EX and EulerCam. The result has been a big jump in well‑measured small planets: the catalog discussed rose from 43 transiting planets with radius below 4 Earth radii in 2014 to 225 in 2025.
One clear pattern from those data is a split in planet sizes. The distribution shows two peaks, near 1.3 and 2.4 times Earth’s radius, with a relative dip or “radius valley” at about 1.8 Earth radii. Planets near the smaller peak are often called super‑Earths, while those near the larger peak are called sub‑Neptunes. That valley shifts with orbital period and with the mass of the host star. Interpreting a planet’s nature remains hard because measurements of mass and radius can match very different interiors. A sub‑Neptune can be a rocky core wrapped in a thin hydrogen‑helium envelope (a “gas dwarf”) or it could hold lots of water as a thick interior or steam atmosphere (a “water world”).
These findings matter because they test and guide theories of how planets form and evolve. Better counts and better mass and radius measurements help rule out some ideas and support others. New instruments make a difference: for example, the ESPRESSO spectrograph can reach radial velocity precision near 20 centimetres per second, opening the way to measure masses of small, potentially habitable planets. The review also notes that the James Webb Space Telescope has started to probe atmospheres of individual planets, which can add key chemical information beyond bulk density.
Important caveats remain. The link between a planet’s mass and radius is degenerate: different mixes of rock, water and gas can give the same measured values. Multiple theories exist to explain the radius valley, and more data are needed to decide between them. Early missions such as Kepler found many planets around faint stars that were hard to follow up, and only with later surveys and new ground instruments did the sample become much easier to characterize. The chapter summarizes work done within NCCR PlanetS but is not an exhaustive account of the whole field, and the picture will keep evolving as new observations arrive.