Can ‘mini’ gas giants exist?

Yes, mini gas giants exist. A gas giant is a planet with most of its atmosphere composed of hydrogen and helium, sitting on top of a solid core that is several times larger than Earth. Within our Solar System, there are four gas giants: Jupiter, Saturn, Uranus and Neptune. However, we have also discovered more than 4,500 planets orbiting other stars.

While some of these ‘exoplanets’ are the size of our gas giants, a surprising finding is that a significant number of other planetary systems seem to have

‘mini’ gas giants that are somewhere between Earth and Neptune size, with less gaseous atmospheres – but a lot more than Earth – than our own giant planets. We call them ‘mini-Neptunes’ or ‘sub-Neptunes’. Why is this surprising? Because even though these planets appear to be everywhere, our own Solar System doesn’t have a single ‘mini’ gas giant! We either have gas giants, or we have small, rocky planets. This might be a result of different planet formation processes, and we will know a lot more about the atmospheres of these ‘mini gas giants’ when NASA launches the James

Webb Space Telescope.

Right: Neptune is the Solar System’s smallest gas planet, but is still quite large