Universe Today
Sub-Neptune
The most common kind of planet in the galaxy, and there is not one in our solar system.
Sub-Neptunes are planets between Earth and Neptune in size — roughly two to four times Earth's diameter — usually with a rocky or icy core under a thick hydrogen and helium envelope. They are the most common planets Kepler found, and the solar system has none, which is the single strongest hint that our arrangement of planets is not typical. There is a curious gap in the population around 1.8 Earth radii, the "radius valley", thought to be where a planet's atmosphere gets stripped away by its star's radiation, leaving a bare core behind. Several are now being examined by Webb for signs of what their air is made of.
Where it has come up lately
Related terms
Transit method
Finding planets by watching a star dim slightly as one passes in front.
Radial velocity
Finding planets by the wobble they induce in their star's motion.
Habitable zone
The orbital band where a planet could hold liquid water — no more than that.
Hot Jupiter
A gas giant orbiting closer to its star than Mercury does to the Sun.
Questions
What is Sub-Neptune?
The most common kind of planet in the galaxy, and there is not one in our solar system.
Why does it matter?
There is a curious gap in the population around 1.8 Earth radii, the "radius valley", thought to be where a planet's atmosphere gets stripped away by its star's radiation, leaving a bare core behind. Several are now being examined by Webb for signs of what their air is made of.