What is the largest planet of all those we know of?

In our Solar System, Jupiter is the largest planet we have, but what is the upper limit of 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. The sizes of the planets are shown to scale, but the distances between them are not. (Credit: NASA/Lunar and Planetary Institute)

If you pack too much mass into a single object, its nucleus will fuse lighter elements into heavier ones.

It takes 75 to 80 times the mass of Jupiter to start burning hydrogen in an object’s core, but the line between a planet and a star is not that simple. (Credit: NASA/ESA/G. Bacon (STScI))

At about eighty times the mass of Jupiter, it will reach “true star” status, burning hydrogen into helium.

Brown dwarfs, between 13 and 80 solar masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining about the same size as Jupiter but reaching much larger masses. Note that the Sun is not to scale and would be many times larger. (Credit: NASA/JPL-Caltech/UCB)

But below that, at about 14 times the mass of Jupiter, deuterium fusion will begin, where the leftover fuel from the Big Bang slowly generates its own energy.

Gliese 229 is a red dwarf star, and is orbited by Gliese 229b, a brown dwarf, fusing only deuterium. Although Gliese 229b is about 20 times the mass of Jupiter, it is only about 47% of its radius. (Credit: S. Kulkarni (Caltech), D. Golimowski (JHU) and NASA/ESA)

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

When we classify known exoplanets by mass and radius together, the data indicates that there are only three classes of planets: terrestrial/rocky, with a volatile but non-self-compressing envelope of gas, and with a volatile and self-compressing envelope. . Anything above that is a star. Planetary size peaks at a mass between that of Saturn and Jupiter, with increasingly heavier worlds getting smaller until true nuclear fusion ignites and a star is born. (Credit: J. Chen and D. Kipping, ApJ, 2017)

However, in terms of physical size, brown dwarfs are actually minor than the largest gas giants.

Although more than 4,000 confirmed exoplanets are known, with more than half of them discovered by Kepler, the largest extant planets are only about twice the radius of Jupiter: about ~25 Earth radii. (Credit: NASA/Ames/Jessie Dotson and Wendy Stenzel; annotated by E. Siegel)

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

Although there have been many claims of “super-Jupiter” planets that were between 4 and 7 times the radius of Jupiter, follow-up observations have invalidated all of those claims, even around the star HD 131399, as illustrated here. (Credit: IS SUN. Sidewalk/University of Arizona)

This happens in our Solar System, which explains why Jupiter is three times the mass of Saturn, but only slightly larger physically.

A slice of the interior of Jupiter. If all atmospheric layers were removed, the core would appear to be a rocky Super-Earth. Planets that formed with fewer heavy elements can be much larger and less dense than Jupiter. (Credit:Kelvinsong/Wikimedia Commons)

But many solar systems have bloated planets made 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 ‘bloated’ planets are comparably large, but none are significantly larger. (Credit: ESA/Hubble and NASA)

The largest planets, like wasp-17bthey can be up to twice as big as Jupiter before they became stars.

This article was reprinted with permission from great thoughtWhere was originally posted.

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