There’s a large difference between a planet and a star, but some planets can be significantly larger than anything we find in our own Solar System. Image credit: ATG Medialab / ESA.

What’s the largest planet in the Universe?

You might think Jupiter is large, but you’ll be surprised at what happens if you try and make it larger!

“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

In our Solar System, Jupiter is the largest planet we have, but what’s the upper limit to planetary size?

Jupiter may be the largest and most massive planet in the Solar System, but adding more mass to it would only make it smaller. Image credit: Lunar and Planetary Institute.

If you get too much mass together in a single object, its core will fuse lighter elements into heavier ones.

It takes about 75–80 times as much mass as Jupiter to initiate hydrogen burning in the core of an object, but the line between a planet and a star is not so simple. Image credit: NASA, ESA, and G. Bacon (STScI).

At about eighty times the mass of Jupiter, you’ll have a true star, burning hydrogen into helium.

Brown dwarfs, between about 13–80 solar masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining at the same approximate size as Jupiter but achieving much greater masses. Note the Sun is not to scale and would be many times larger. Image credit: NASA/JPL-Caltech/UCB.