Since astronomers first looked beyond the solar system three decades ago to find exoplanets, or exoplanets, we have known that the planets in the Milky Way – and possibly the wider universe – vary widely in width and mass.
But how big are these planets, and what are the largest planets that we know of?
Prior to 1992, when the first exoplanet, the gas giant, was discovered Jupiter, 11 times the width of Earth, it held 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 the radius) and its mass.
By analogy with the first, “The largest exoplanet has a planetary radius about twice that of Jupiter,” Solen Ulmer MolA postdoctoral exoplanet researcher at the University of Geneva, told Live Science in an email. “This is an extreme object that orbits very close to its parent star.”
A planet’s width and mass are related, but there isn’t always a direct correlation between them. This is because planets vary in density, meaning that some low-mass gas giants can “jump” to sizes larger than other heavy exoplanets.
For example, the gas giant HAT-P-67 b, which has a radius about twice that of Jupiter, is currently one of the largest planets known by latitude. However, the exoplanet, which is 1,200 light years from Earth, has a very low density, so it has only one third the mass of Jupiter, said Ulmer Mull.
WASP-17 b is almost twice as wide as Jupiter. The runner-up is KELT-9b. It adds that its radius is 1.84 times that of Jupiter.
Most of the rocky planets are not as close to the “super-Jupiters” mentioned above. The largest rocky planets, called super-Earths, are twice as wide as Earth. “For comparison, Wasp-17B’s radius is 22 times that of Earth,” said Ulmer Mull.
Although rocky planets are denser than gas giants, they are still not as heavy as gas giants. He said that as rocky planets grew, they accumulated gas, ice and water gradually turning them into gas giants with rocky centers.
The largest planet has about 13 times the mass of Jupiter. This includes 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 this planet really?
We didn’t expect to find a planet much larger than this giant Jupiter, because a planet becomes a “brown dwarf” once it reaches a certain size and mass.
Brown dwarfs are often referred to as “failed stars” because they are heavier than super-Jupiters but not massive enough to fuel normal hydrogen fusion in their cores. But there is still something burning in the heart of the brown dwarf.
Illustration of super-Jupiter orbiting a “failed” brown dwarf. (Image credit: NASA/STScI)
The main difference between brown dwarfs and planets is their mass and the burning of deuterium – heavy hydrogen. Nolan’s gravea postdoctoral researcher in the Department of Astronomy at the University of Geneva, Notify Live Science via email. “In a larger mass, the body has a high enough internal pressure and temperature to burn most of the deuterium that was originally in the body.”
The line between planets and brown dwarfs was drawn in the 1990s, with brown dwarfs being classified as objects that burn 50% or more primordial deuterium. This dividing line is believed to exist at an upper limit of 14 Jupiter masses, meaning that no planets can exist above this limit.
“There are planets that we have measured about 13 Jupiter masses in 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 so far is SDSS J0104+1535, located 750 light years from Earth at the edge of the Milky Way. It is 90 times larger 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.
2023-08-11 18:47:00
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