Sub-Neptunes, super-Earths and the radius valley

7 problems

• Does the radius valley form in 100 Myr or over billions of years?

  Open

  Photoevaporation should finish stripping planets within about 100 million years, while core-powered loss takes about a billion. Comparing young and old planet populations can separate them.

• How do magma oceans change sub-Neptune atmospheres?

  Open

  Under a thick hydrogen layer, the rocky surface of a sub-Neptune may be molten, and hydrogen can dissolve into it and react with the rock to make water and other gases. This would change what the planet's atmosphere looks like from outside.

• What empties the hot Neptune desert?

  Open

  Planets the size and mass of Neptune are rare on orbits shorter than a few days, even though both smaller and larger planets are found there. Starlight stripping their gas, and the way giant planets arrive in such close orbits, are proposed causes.

• What produces the gap between super-Earths and sub-Neptunes?

  Contested

  Close-in small planets come in two sizes with a gap between them. Starlight boiling away thin hydrogen envelopes, heat from the planet's own core doing the same, or differences in how planets formed could each explain it.

• Why do planet numbers drop steeply above three Earth radii?

  Open

  Planets just under three Earth radii are very common, but slightly larger ones are much rarer. Something must stop envelopes from growing, possibly hydrogen dissolving into molten rock.

• Are sub-Neptunes rocky cores with hydrogen or water worlds?

  Open

  A planet two or three times Earth's size can be a rocky ball with a thin hydrogen layer or a world with a large fraction of water, and mass and radius alone cannot tell which. Their atmospheres carry clues.

• Do sub-Neptunes with more than 10 percent water exist?

  Open

  Planets that formed beyond the line where water freezes and then moved inward could be up to half water by mass. Whether any such water worlds exist among close-in sub-Neptunes has not been shown.