File:NASA's James Webb Space Telescope Primed to Lift the Haze Surrounding Sub-Neptunes (51685676567).png

llustration comparing the sizes of sub-Neptune exoplanets TOI-421 b and GJ 1214 b to Earth and Neptune. Both TOI-421 b and GJ 1214 b are in between Earth and Neptune in terms of radius, mass, and density. The low densities of the two exoplanets indicates that they must have thick atmospheres. The planets are arranged from left to right in order of increasing radius and mass:

   Image of Earth from the Deep Space Climate Observatory: Earth is a rocky planet with an average radius of roughly 6,370 kilometers, a mass of about 6 billion trillion metric tons, and a density 5.5 times that of water.
   Illustration of TOI-421 b: TOI-421 b is a hot sub-Neptune exoplanet with a radius 2.68 times Earth, a mass 7.2 times Earth, and a density 2.05 times water.
   Illustration of GJ 1214 b: GJ 1214 b is a warm sub-Neptune exoplanet with a radius 2.74 times Earth, a mass 8.2 times Earth, and a density 2.2 times water.
   Image of Neptune from Voyager 2: Neptune is an ice giant with a radius 3.88 times that of Earth (giving it a volume nearly 58 times Earth), a mass 17 times Earth, and a density of only 1.6 times water.

The illustration shows the planets to scale in terms of radius, but not location in space or distance from their stars. While Earth and Neptune orbit the Sun, TOI-421 b orbits a sun-like star roughly 244 light-years away, and GJ 1214 b orbits a small red dwarf star about 48 light-years away. Credits: NASA, ESA, CSA, and D. Player (STScI)