What is the largest planet known to us?

What is the largest planet known to us?

In our solar system, Jupiter is the largest planet we have, but what is the upper limit of planet size?

Jupiter may be the largest and most massive planet in the solar system, but adding more mass to it only makes it smaller. The sizes of the planets are drawn to scale, but the distances between them are not. (Source: NASA/Lunar and Planetary Institute)

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

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

About eighty times the mass of Jupiter, they achieve “true star” status, burning hydrogen to form helium.

Brown dwarfs with a mass between about 13 and 80 solar masses will fuse deuterium + deuterium into helium-3 or tritium, being 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. (Source: 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 creates its own energy.

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

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

If we classify the known exoplanets together by mass and radius, the data shows that there are only three classes of planets: terrestrial/rocky, with a volatile shell of gas but not self-compressing, and with a volatile shell and with self-compression. Anything above that is a star. Planet 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. (Source: J. Chen and D. Kipping, ApJ, 2017)

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

Although there are more than 4,000 confirmed exoplanets known, more than half of which were discovered by Kepler, the largest planets out there are only about twice the radius of Jupiter: at around 25 Earth radii. (Source: NASA/Ames/Jessie Dotson and Wendy Stenzel; commented by E. Siegel)

Above a certain mass, the atoms in large planets begin to compress so much that adding more mass actually shrinks your planet.

Although there have been many claims of “super Jupiter” planets that were between 4 and 7 times the radius of Jupiter, subsequent observations have disproved all of these claims, including around the star HD 131399, as shown here. (Source: ESO/L. Calçada/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 section of Jupiter’s interior. If all atmospheric layers were removed, the core would appear like a rocky super-Earth. Planets formed with lighter elements can be much larger and less dense than Jupiter. (Source: 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% the mass. Many other “swollen” planets are comparable in size, but none are significantly larger. (Source: ESA/Hubble & NASA)

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

This article was reprinted with permission from Big Think, where it was originally published.

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