The search for planets outside our solar system, known as exoplanets, has introduced a vast array of worlds that challenge the traditional definitions of a planet. With thousands of confirmed discoveries, the sheer scale and diversity of these celestial bodies continue to expand our understanding of the cosmos. The title of the largest planet is not held by a static object, but constantly shifts as astronomers detect and confirm new, distant worlds. These massive gas giants force scientists to draw increasingly fine lines between what constitutes a planet and what classifies as a failed star.
Identifying the Largest Known Planet
The current record holder for the largest confirmed exoplanet by radius is HAT-P-67b, a colossal gas giant orbiting a star approximately 1,200 light-years from Earth. This planet possesses a radius measured at about 2.14 times that of Jupiter, making it one of the most physically inflated objects ever discovered. Despite its immense size, HAT-P-67b has a surprisingly low mass, estimated to be only about 0.45 times the mass of Jupiter.
The disparity between its physical size and its relatively low mass results in a low density, comparable to that of cork or even a marshmallow. HAT-P-67b is classified as an ultra-hot Jupiter, orbiting its host star, HAT-P-67, every 4.8 days. The intense heat and stellar radiation from its F-type host star, which is larger and hotter than our Sun, causes the planet’s atmosphere to “puff up” dramatically, creating its enormous, distended size.
Understanding Planetary Scale
To grasp the magnitude of HAT-P-67b, it helps to use our own solar system’s largest planet, Jupiter, as a measuring stick. Jupiter’s radius is about 11 times that of Earth. HAT-P-67b’s radius is more than double Jupiter’s, meaning it is over 23 times wider than Earth. If Earth were a small marble, Jupiter would be the size of a basketball, and HAT-P-67b would be a sphere roughly the size of a beach ball.
The Line Between Planet and Star
The existence of massive exoplanets, often referred to as “Super-Jupiters,” brings to light the distinction between a large planet and a brown dwarf, which is often called a failed star. The physical criterion that separates these two types of celestial bodies is the ability to sustain nuclear fusion in their core. While true stars ignite hydrogen fusion, the lower mass threshold for a brown dwarf is defined by its ability to fuse deuterium, a heavier isotope of hydrogen.
The approximate mass limit for deuterium fusion to occur is 13 times the mass of Jupiter (\(13 M_{Jup}\)). An object above this limit is classified as a brown dwarf because it generates its own heat and light from this fusion process. Objects below this mass limit, like HAT-P-67b, are considered planets because they are not massive enough to trigger sustained fusion. This 13-Jupiter-mass boundary is a theoretical approximation that can vary based on a body’s initial composition and metallicity. The measurement of a body’s mass against this deuterium fusion limit remains the most consistent way to classify a large world.
Discovery Methods for Giant Worlds
The gargantuan size of worlds like HAT-P-67b makes them easier to detect than smaller, rocky planets using two primary methods. The first technique is the Transit Method, which involves continuously monitoring a star’s brightness and looking for periodic dips in light. When a planet passes directly between its star and our telescopes, it blocks starlight, and the amount of light blocked is directly proportional to the planet’s radius. A large planet will block significantly more light than an Earth-sized world, making the transit signal much clearer and easier to confirm.
The second technique is the Radial Velocity Method, which measures the minuscule wobble of a star caused by the gravitational tug of an orbiting planet. Large, massive planets in close orbits exert a stronger gravitational force, causing a more pronounced and detectable wobble in the star’s light spectrum. The detection of these massive, close-orbiting gas giants, often termed “hot Jupiters,” is favored by both methods, giving them a detection bias. The large radius provides a strong signal for the Transit Method, while the high mass and close orbit produce a clear Doppler shift for the Radial Velocity Method. These complementary measurements allow astronomers to determine both the planet’s radius and its true mass, which are essential for calculating its density and confirming its planetary status.