A planet is a celestial body that meets three specific criteria: it orbits the Sun, it has enough mass for gravity to pull it into a roughly spherical shape, and it has cleared smaller debris from its orbital path. These three rules were formally established by the International Astronomical Union in 2006, reducing our solar system from nine recognized planets to eight and famously demoting Pluto to a “dwarf planet.”
The Three Official Criteria
The IAU’s definition, passed as Resolution B5 at its 2006 General Assembly, requires all three conditions to be met simultaneously. Missing even one disqualifies a body from full planet status.
First, the object must orbit the Sun. This sounds obvious, but it excludes moons (which orbit planets rather than the Sun directly) and rogue objects drifting through interstellar space. Second, the body must have enough mass for its own gravity to overcome the rigidity of its material, pulling it into a nearly round shape. Astronomers call this hydrostatic equilibrium. Small, irregularly shaped objects like asteroids don’t have the gravitational pull to reshape themselves. Third, the body must have “cleared the neighborhood” around its orbit, meaning it gravitationally dominates its orbital zone. It has either absorbed, ejected, or captured the vast majority of smaller debris sharing its path around the Sun.
What “Nearly Round” Actually Means
Every object in space has gravity, but only bodies above a certain mass generate enough gravitational force to compress themselves into a sphere. Below that threshold, a body’s internal rigidity wins out and the object keeps whatever lumpy, irregular shape it formed with. Think of the potato-shaped asteroids that populate the belt between Mars and Jupiter.
The transition happens at roughly 400 to 600 kilometers in diameter for rocky objects, though the exact cutoff depends on composition. Icy bodies reach roundness at somewhat smaller sizes because ice deforms more easily than rock under pressure. Once a body crosses this line, gravity smooths it into something close to a ball, though rotation can still cause a noticeable bulge at the equator. Saturn, for example, is visibly wider across its middle than from pole to pole because it completes a full rotation in just 10 hours.
What “Clearing the Orbit” Means
This criterion is what separates planets from dwarf planets, and it’s the rule that cost Pluto its status. A planet doesn’t need a perfectly empty orbital lane, but it does need to be the gravitationally dominant body in its zone. Physicist Steven Soter proposed a simple way to measure this: divide the mass of the candidate planet by the total mass of all other objects sharing its orbital region. He called this the “planetary discriminant.” If that ratio exceeds 100, the body qualifies as having cleared its neighborhood.
By this measure, Earth’s ratio is enormous. Our planet vastly outweighs every asteroid and speck of dust near its orbit. Jupiter’s ratio is even more extreme. Pluto, on the other hand, shares the Kuiper Belt with a huge population of icy objects and accounts for only a small fraction of the total mass in its orbital zone. It simply doesn’t dominate its surroundings the way the eight planets do.
Another approach, developed by Alan Stern and Harold Levison, calculates a value based on a body’s mass and its distance from the Sun. Objects closer to the Sun need less mass to clear their orbits because there’s less debris and the orbital speeds are higher, making gravitational interactions more frequent. By either metric, there’s a clean gap of several orders of magnitude between the eight planets and everything else in the solar system. The distinction isn’t a borderline call.
Where Dwarf Planets Fall Short
A dwarf planet meets two of the three criteria. It orbits the Sun and has enough mass to be round, but it has not cleared its orbital neighborhood. It also can’t be a moon. The solar system currently has five officially recognized dwarf planets: Pluto, Eris, Haumea, Makemake, and Ceres. Ceres sits in the asteroid belt between Mars and Jupiter, while the other four reside in the Kuiper Belt beyond Neptune.
Pluto’s reclassification in 2006 was controversial, but the underlying reason is straightforward. Pluto is one relatively small body among thousands of similar icy objects in the Kuiper Belt. It hasn’t swept its region clean the way Neptune, its nearest planetary neighbor, has.
How Planets Form in the First Place
Planets begin as dust and gas swirling in a disk around a young star. Tiny grains of rock, metal, and ice collide and stick together, gradually building larger clumps. Once these clumps reach roughly boulder size, their mutual gravity starts pulling in even more material, a process called accretion. Over millions of years, some of these growing bodies reach a size where they dominate their local region of the disk, sweeping up or scattering smaller competitors.
For rocky planets like Earth, the process largely stops there. The solid core grows through countless collisions until it consumes most of the available material nearby. Gas giants follow a different path. Once a rocky core reaches several times Earth’s mass, it becomes gravitationally powerful enough to capture enormous volumes of hydrogen and helium gas from the surrounding disk. This has to happen quickly, because the gas disk dissipates within a few million years. If the core grows too slowly, the gas disappears before a giant planet can form. Research in planetary formation models shows that smaller planetesimals, whose motion is more easily slowed by gas drag, are accreted more efficiently, favoring faster core growth and making gas giant formation more likely.
Types of Planets
The eight planets in our solar system fall into three broad categories based on composition. Mercury, Venus, Earth, and Mars are terrestrial planets, built primarily from rock and metal with solid surfaces you could theoretically stand on. They’re relatively small, dense, and close to the Sun.
Jupiter and Saturn are gas giants. Their atmospheres are mostly hydrogen and helium, and there’s no clear boundary between atmosphere and interior. Deep inside, immense pressure compresses hydrogen into a liquid metallic state that conducts electricity. Both likely have a rocky or metallic core buried at the center, but the vast majority of their mass is gas or compressed liquid. Jupiter and Saturn spin fast (Saturn completes a rotation roughly every 10 hours), which drives powerful wind bands visible as east-west stripes across their surfaces. Both have extensive ring systems and large families of moons.
Uranus and Neptune are sometimes called ice giants. Though they also contain hydrogen and helium, a much larger fraction of their mass consists of heavier compounds: water, methane, and ammonia compressed into hot, dense fluid layers beneath their outer atmospheres. Their bluish coloring comes from methane in their upper atmospheres absorbing red light.
The Upper Limit: Planet vs. Brown Dwarf
There’s also a ceiling on how massive a planet can be. The accepted upper boundary is about 13 times the mass of Jupiter. Beyond that mass, the object’s interior becomes hot and dense enough to fuse deuterium (a heavier form of hydrogen) in its core. Objects that burn deuterium but not regular hydrogen are classified as brown dwarfs, a category that sits between planets and true stars. Full hydrogen fusion, which powers stars like the Sun, kicks in at roughly 80 Jupiter masses.
This 13-Jupiter-mass cutoff applies specifically to the definition of exoplanets (planets orbiting other stars). The IAU’s 2006 resolution technically only defines planets within our own solar system, but the deuterium-burning threshold has become the widely used dividing line for planets discovered around other stars. The definition also requires that the object’s mass be no more than about 1/25th the mass of whatever it orbits, ensuring that it’s genuinely a companion rather than one member of a binary pair.