“A few centuries ago, the pioneer navigators learnt the size and shape of our Earth, and the layout of the continents. We are now just learning the dimensions and ingredients of our entire cosmos, and can at last make some sense of our cosmic habitat.” -Martin Rees
You might think that Jupiter is the largest planet in the Solar System because it’s the most massive, but that’s not quite right. If you kept adding mass to Saturn, it would get larger in size, but if you kept adding mass to Jupiter, it would shrink! For a given set of elements that your planet is made out of, there’s a maximum size it can reach, that’s somewhere in between the mass of Saturn and Jupiter in general.
Planetary size peaks at a mass between that of Saturn and Jupiter, with heavier and heavier worlds getting smaller until true nuclear fusion ignites and a star is born. Image credit: Chen and Kipping, 2016, via https://arxiv.org/pdf/1603.08614v2.pdf.
Our Solar System is on the dense side of things, meaning that we’ve discovered a large number of exoplanets out there that are approximately twice the physical size of Jupiter without becoming brown dwarfs or hydrogen-fusing stars. ...