“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.

But lower than that, at about 14 times the mass of Jupiter, you’ll initiate deuterium fusion, where leftover fuel from the Big Bang slowly self-generates its own energy.

Gliese 229 is a red dwarf star, and is orbited by Gliese 229b, a brown dwarf, that fuses deuterium only. Although Gliese 229b is about 20 times the mass of Jupiter, it’s only about 47% of its radius. Image credit: T. Nakajima and S. Kulkarni (CalTech), S. Durrance and D. Golimowski (JHU), NASA.

This line — between a gas giant and a brown dwarf — defines the most massive planet.

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.

In terms of physical size, however, brown dwarfs are actually smaller than the largest gas giants.

Jupiter may only be about 12 times Earth’s diameter, but the largest planets of all are actually less massive than Jupiter, with more massive ones shrinking as more mass is added. Image credit: NASA Ames / W. Stenzel; Princeton University / T. Morton.

Above a certain mass, the atoms inside large planets will begin to compress so severely that adding more mass will actually shrink your planet.

The exoplanet Kepler-39b is one of the most massive ones known, at 18 times the mass of Jupiter, placing it right on the border between planet and brown dwarf. In terms of radius, however, it’s only 22% larger than Jupiter. Image credit: Wikimedia Commons user MarioProtIV.

This happens in our Solar System, explaining why Jupiter is three times Saturn’s mass, but only 20% physically larger.

A cutaway of Jupiter’s interior. If all the atmospheric layers were stripped away, the core would appear to be a rocky Super-Earth. Planets that formed with fewer heavy elements can be a lot larger and less dense than Jupiter. Image credit: Wikimedia Commons user Kelvinsong.

But many solar systems have planets made out of much lighter elements, without large, rocky cores inside.

WASP-17b is one of the largest planets confirmed not to be a brown dwarf. Discovered in 2009, it is twice the radius of Jupiter, but only 48.6% of the mass. Many other ‘puffy’ planets are comparably large, but none are yet significantly larger. Image credit: ESA/Hubble & NASA.

As a result, the largest planets can be up to twice as big as Jupiter before becoming stars.