Exoplanets, or worlds orbiting stars beyond our Sun, present an astonishing variety of sizes, densities, and orbital patterns. These distant bodies range from rocky spheres smaller than Earth to gas giants far larger than Jupiter, the biggest planet in our solar system. As astronomers discover more of these cosmic extremes, the question arises: what is the absolute largest planet found to date? The answer involves understanding the complex boundary that separates true planets from failed stars.
The Current Largest Exoplanet
The largest known exoplanet by radius is HAT-P-67b, an enormous world discovered in 2017. This gas giant orbits a star roughly 1,200 light-years away in the constellation Hercules, completing a very close orbit in less than five days. Its proximity to the host star causes it to be intensely heated, which contributes significantly to its massive size. The intense stellar heat causes its outer atmosphere to balloon outward, giving it an exceptionally low density.
HAT-P-67b is classified as a “hot Jupiter” and has a radius that is approximately 2.08 to 2.14 times the radius of Jupiter. This enormous volume means the planet has a density lower than that of cork. Despite its immense size, its mass is only about one-third to one-half that of Jupiter. The host star, HAT-P-67, is an F-type subgiant that is hotter, larger, and more luminous than our Sun.
Defining the Upper Limit of Planetary Size
The search for the largest planet encounters a scientific challenge: determining where a planet ends and a star begins. The boundary that separates a giant planet from a brown dwarf, or “failed star,” is based on mass, not radius. This dividing line is set at roughly 13 times the mass of Jupiter. Objects exceeding this mass possess enough gravitational pressure and core temperature to initiate and sustain the fusion of deuterium, a heavy isotope of hydrogen.
An object that undergoes deuterium fusion, even briefly, is classified as a brown dwarf, distinguishing it from a true planet, which cannot sustain nuclear fusion. This 13-Jupiter-mass threshold is not an absolute, fixed number, as the exact mass required can vary depending on the object’s composition. However, the 13 Jupiter-mass limit serves as the widely adopted rule for distinguishing between the two classes of celestial bodies. The concept of “biggest” by radius becomes ambiguous near this boundary because a brown dwarf can be smaller than an extremely inflated gas giant, like HAT-P-67b, despite having more mass.
Determining the Size of Distant Worlds
Astronomers rely on two complementary techniques to accurately measure the size and mass of distant worlds.
Transit Method
The most effective method for determining a planet’s radius is the transit method. This technique measures the slight dip in a star’s brightness as an orbiting planet passes directly in front of it from our perspective. The amount of light blocked is proportional to the size of the planet relative to the star, allowing scientists to calculate the planet’s diameter once the star’s size is known. This technique favors the discovery of large planets with short orbital periods.
Radial Velocity Method
To determine the mass of the transiting planet, scientists use the radial velocity method, which is based on the gravitational tug the planet exerts on its host star. This gravitational interaction causes the star to subtly wobble, a motion detected by observing shifts in the star’s light spectrum due to the Doppler effect. By combining the radius data from the transit method with the mass data, astronomers can calculate the planet’s density. This density measurement confirms the planet’s composition and validates its classification as a gas giant or a brown dwarf.