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Since astronomers first looked beyond the solar system three decades ago to discover exoplanets, or outer planetsWe’ve known that planets in the Milky Way – and perhaps the wider universe – come in a wide range of widths and masses.
But how big can these planets get, and what is the largest planet we know of?
Before 1992, when the first exoplanet, the gas giant, was discovered JupiterAt 11 times the width of Earth, it holds the title of the largest known planet. But Jupiter is a dot squeaker compared to some of the monster worlds we’ve explored since.
Related: 10 extreme exoplanets that are out of this world
There are two metrics to consider when determining the size of a planet: its width (twice its radius) and its mass.
By analogy with the first, “The largest exoplanets have a planetary radius about twice that of Jupiter,” Solen Ulmer MolAn exoplanet postdoctoral researcher at the University of Geneva, told Live Science via email. “These are extreme objects orbiting very close to their host star.”
The width of a planet and its mass are related, but there is not always a direct correlation between them. This is because planets vary in their densities, which means that some low-mass gas giants can “bulge” to larger sizes than other heavy exoplanets.
For example, the gas giant HAT-P-67 b, which has a radius about twice that of Jupiter, is currently among the largest known planets by latitude. However, the exoplanet, which is 1,200 light-years from Earth, has a very low density, so it only has a third of the mass of Jupiter, says Ulmer Mull.
WASP-17 b is nearly twice as wide as Jupiter. Runner-up is KELT-9bIt added that its radius is 1.84 times that of Jupiter.
Most rocky planets are nowhere near as close as the aforementioned “super-Jupiter”. The largest of the rocky planets, called super-Earths, is twice as wide as Earth. “By comparison, the radius of Wasp-17B is 22 times the radius of Earth,” said Ulmer Mull.
Although the rocky planets are denser than the gas giants, they are still not as heavy as the gas giants. She said that because the rocky planets grow, they accumulate gas, ice and water that gradually turn them into gas giants with a rocky center.
The largest planets have about 13 times the mass of Jupiter. These include the gas giant HD 39091 b, which is located 60 light-years from Earth and has a mass about 12.3 times that of Jupiter.
How big is the planet actually?
We don’t expect to discover planets much larger than these giant Jupiters, because a planet becomes a “brown dwarf” once it reaches a certain size and mass.
Brown dwarfs are often referred to as “fail stars” because they are heavier than super-Jupiters but not massive enough to trigger normal hydrogen fusion in their cores. But something still burns in the hearts of brown dwarfs.
The main difference between brown dwarfs and planets is their mass and the occurrence of deuterium – heavy hydrogen – burning. Nolan Gravesa postdoctoral researcher in the Department of Astronomy at the University of Geneva, Tell Live Science via email. “In larger masses, the body has an internal pressure and temperature high enough to burn off most of the deuterium that was initially present in the body.”
The line between planets and brown dwarfs was drawn in the 1990s, with brown dwarfs classified as objects burning 50% or more of primordial deuterium. This dividing line is believed to exist at an upper limit of 14 times the mass of Jupiter, which means that no planets should exist above this cap.
“There are planets that we have measured to be about 13 Jupiter masses within measurement uncertainty, such as HD 39091 b and HD 106906 b, and they are arguably the largest known planets,” Graves concluded.
The most massive brown dwarf discovered to date is SDSS J0104+1535, located 750 light-years from Earth at the edge of the Milky Way. It is 90 times more massive than Jupiter, but its radius is between 0.7 and 1.4 times that of Jupiter. So in fact, the most massive brown dwarf may be younger than the largest planet in our solar system.
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