The question of the universe’s biggest planet opens a door to the vast scale of extrasolar worlds, known as exoplanets. Our current understanding is constantly evolving, as new detections often challenge the boundaries of what a planet can be. The sheer size of the universe suggests that the definitive record-holder for “biggest” is likely still undiscovered. For now, the largest examples we have found are exclusively exoplanets, which dwarf the largest planets in our own solar system.
Defining Planetary Size Metrics
Determining the “biggest” planet requires defining how size is measured, as astronomers use two distinct metrics: radius and mass. Radius refers to the physical volume or diameter of the object. Mass, conversely, is a measure of the planet’s weight or the amount of matter it contains.
These two measurements do not always align, creating a complex picture of planetary size. Astronomers often use Jupiter as the standard unit of measure, expressing the size of exoplanets in terms of Jupiter radii (\(R_J\)) and Jupiter masses (\(M_J\)). A planet that is large by radius may be surprisingly light, while a planet with a relatively small radius can be extremely massive.
Gas giants are composed mostly of hydrogen and helium gas. Unlike rocky planets, adding mass to a gas giant does not cause its volume to increase indefinitely. Increased mass results in stronger gravity, which simply compresses the gas, often leading to a denser, but not necessarily larger, object.
The Largest Known Planets by Volume
The current record-holders for planetary volume are often low-density gas giants. These worlds are physically bloated far beyond what their mass would typically allow, making them the largest in terms of sheer size. The most famous example is WASP-17b, which holds the title for one of the largest known planets by radius.
WASP-17b is a peculiar world, boasting a radius nearly twice that of Jupiter, or approximately 1.99 Jupiter radii. Despite this immense volume, the planet has only about half the mass of Jupiter, giving it an incredibly low density.
Another notable example of a bloated giant is TrES-4b, which measures about 1.8 times the radius of Jupiter. The reason for this extreme inflation in both WASP-17b and TrES-4b is their close proximity to their parent stars.
These planets orbit their stars in just a few Earth days, making them a subtype of exoplanet known as “Hot Jupiters.” The intense stellar radiation heats the planets’ upper atmospheres, causing the gas to expand dramatically, a phenomenon known as tidal heating. The energy absorbed from the star prevents the planet’s atmosphere from contracting under its own gravity, maintaining its inflated state. This process creates a planet that is physically enormous but has an average density much lower than water.
The Most Massive Planets and the Brown Dwarf Limit
When considering mass, the concept of a “planet” reaches a strict upper limit defined by a particular nuclear reaction. This boundary distinguishes a true planet from a brown dwarf. The threshold for this transition is set at approximately 13 times the mass of Jupiter.
Once an object’s mass exceeds 13 \(M_J\), the pressure and temperature in its core become sufficient to ignite a process called deuterium fusion. This fusion reaction, even if brief, is the physical determinant that reclassifies the object as a brown dwarf.
The 13 Jupiter mass limit is not an absolute, fixed number but rather a rule of thumb, as the exact mass required for deuterium fusion depends on the object’s chemical composition. For objects with different compositions, the boundary can range from about 11 to 16 Jupiter masses. This ambiguity means astronomers still debate the classification of some massive worlds.
Exoplanets that push this boundary, such as those with masses around 11 to 12 \(M_J\), are sometimes called super-Jupiters. The most massive confirmed planets are therefore those that fall just below the 13 \(M_J\) threshold, such as Gaia-4b at 11.8 \(M_J\), or those whose classification remains uncertain.
Scale and Perspective Comparing Giants to Earth and Jupiter
To place the sizes of these giant exoplanets into perspective, a comparison with our own solar system is necessary. Earth is a tiny, rocky world next to its gas giant neighbor, Jupiter, which is approximately 318 times more massive than Earth. In terms of volume, Jupiter is so immense that over 1,300 Earths could fit inside it.
The largest exoplanets, such as WASP-17b, push this scale even further. Given that WASP-17b has a radius nearly double that of Jupiter, its total volume is roughly eight times greater than Jupiter’s. This means that a single WASP-17b could contain over 10,000 Earths within its bloated volume.
The density contrast is equally dramatic, highlighting the fundamental difference between rocky and gaseous worlds. Earth has a high density. Jupiter, by comparison, is a much less dense gas giant. The puffy exoplanets like WASP-17b are so diffuse that their density can be less than one-tenth that of Jupiter, making them extremely light for their enormous size.