Universe's Largest Exoplanets: Super-Jupiters & Brown Dwarfs

For the past thirty years, astronomers have been observing exoplanets, planets located outside our solar system, discovering that planets within the Milky Way and likely beyond, exhibit a vast variety of sizes and weights. 

The question arises, what's the largest planet we've identified?

Before the detection of the first exoplanet in 1992, Jupiter, a gas giant roughly 11 times wider than Earth, was deemed the largest known planet. However, recent discoveries have unveiled planets far surpassing Jupiter in size.

There are two metrics to gauge a planet's size: its width (double its radius) and its mass. 

The most substantial exoplanets, as per the first parameter, have a radius roughly twice that of Jupiter, explains Solène Ulmer-Moll, an exoplanet researcher at the University of Geneva. These enormous bodies orbit extremely close to their host stars.

Although a planet's width and mass are related, there isn't always a direct correlation due to varying densities among planets. Consequently, some low-mass gas giants can inflate to sizes exceeding those of denser, heavier exoplanets.

For instance, gas giant HAT-P-67 b, one of the widest known planets with a radius twice that of Jupiter, has a very low density, giving it a mass that's only a third of Jupiter's. WASP-17 b is another example, nearly twice as wide as Jupiter, with KELT-9b following closely with a radius 1.84 times that of Jupiter.

Rocky planets, even the largest ones known as "super-Earths," don't come close to these "super-Jupiters." Super-Earths typically span twice the width of our home planet, whereas Wasp-17b's radius is equivalent to 22 times that of Earth.

Although rocky planets are denser than gas giants, they never reach the same mass. As they grow, they gather gas, ice, and water, gradually morphing into gas giants with a rocky core.

The heaviest known planets are approximately 13 times the mass of Jupiter, such as the gas giant HD 39091 b, situated 60 light-years from Earth, with a mass around 12.3 times that of Jupiter.

So, what's the limit to a planet's size?

We anticipate that planets won't significantly exceed the size of these super-Jupiters, given that they transform into a "brown dwarf" beyond a certain size and mass.

Brown dwarfs, often described as "failed stars," are heavier than super-Jupiters but lack sufficient mass to ignite typical hydrogen fusion at their cores. However, something still ignites within these brown dwarfs.

The primary distinction between brown dwarfs and planets lies in their mass and the burning of deuterium - heavy hydrogen, says Nolan Grieves, a postdoctoral researcher at the University of Geneva. With increased mass, an object can burn most of the original deuterium due to high internal pressure and temperature.

The demarcation line between planets and brown dwarfs was established in the 1990s, categorizing brown dwarfs as objects that burned 50% or more of their initial deuterium. It's believed this boundary exists at an upper limit of 14 times the mass of Jupiter, implying planets shouldn't surpass this threshold.

"There are planets that we've measured their mass to be approximately 13 Jupiter masses within the measurement uncertainty, such as HD 39091 b and HD 106906 b , and it could be argued they are the largest known planets," Grieves concluded.

The most massive brown dwarf yet discovered 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 has a radius between 0.7 and 1.4 times that of Jupiter. So in fact, the most massive brown dwarf might be smaller than the largest planet in our solar system.