What is the largest planet among all those we know?

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 gather too much mass into a single object, its core will fuse lighter elements into heavier ones.

It takes about 75 to 80 times more mass than Jupiter to initiate hydrogen combustion in an object’s core, but the line between a planet and a star isn’t that simple. (Credit: NASA/ESA/G. Bacon (STScI))

At about eighty times the mass of Jupiter, you’ll achieve “true star” status, burning hydrogen into helium.

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

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

Gliese 229 is a red dwarf star, and orbits Gliese 229b, a brown dwarf, which only fuses deuterium. Although Gliese 229b has 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.

super-earth

When we classify the known exoplanets by both mass and radius, the data indicates that there are only three classes of planets: terrestrial/rocky, with an envelope of volatile gas but no self-compression, and with a volatile envelope and with self-compression. Anything above is a star. Planetary size peaks at a mass between that of Saturn and Jupiter, with worlds getting heavier and smaller until true nuclear fusion ignites and a star is born. (Credit: J. Chen and D. Kipping, ApJ, 2017)

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

the largest planet

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

Above a certain mass, the atoms inside large planets will start to compress so tightly 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 these claims, including around the star HD 131399, as shown here. (Credit: ESO/L. Calçada/University of Arizona)

This happens in our solar system, explaining why Jupiter is three times the mass of Saturn, but only slightly larger physically.

A section of the interior of Jupiter. If all atmospheric layers were removed, the core would appear as 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 puffy planets made of much lighter elements, without big rocky cores inside.

the largest planet

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% the mass. Many other “bloated” planets are relatively large, but none are significantly larger yet. (Credit: ESA/Hubble and NASA)

Larger planets, like WASP-17b, can be up to twice the size of Jupiter before becoming stars.

Ethan is on vacation. Please enjoy this older article from the Starts With A Bang archives!

Mostly Mute Monday tells the scientific story of an astronomical object or phenomenon in pictures, visuals and no more than 200 words.

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