Identifying the smallest planet in the universe is impossible due to the unobservable vastness of space; astronomers have explored only a tiny fraction of existing galaxies and stars. Therefore, the most accurate answer requires separating known worlds into two categories: the smallest confirmed planet within our Solar System and the smallest planet detected orbiting a distant star. Examining these two record holders provides a clearer perspective on the range of sizes that qualify a celestial body as a planet and highlights the rules scientists use for categorization.
The Smallest Planet in Our Solar System
The smallest of the eight official planets orbiting our Sun is Mercury, the scorching hot world closest to the star. With a mean radius of approximately 2,440 kilometers, Mercury is about 38% the size of Earth, making it only slightly larger than Earth’s moon. Its mass is about 3.30 x 10^23 kilograms, roughly 5.5% of Earth’s total mass.
Mercury is actually smaller in radius than two of the Solar System’s largest moons, Jupiter’s Ganymede and Saturn’s Titan. Although physically smaller than these satellites, Mercury is significantly denser and more massive, as it is a terrestrial planet primarily composed of silicate rock and metal.
Its high density, second only to Earth’s, is explained by its unusually large metallic core, which makes up an estimated 42% of its volume. This core is thought to be the remnant of a colossal impact early in the Solar System’s history that stripped away most of Mercury’s original, lighter mantle material. Its proximity to the Sun, orbiting at an average distance of 58 million kilometers, contributes to extremely high surface temperatures exceeding 400 degrees Celsius.
The Smallest Known Exoplanets
To find the smallest planet beyond our Solar System, we examine exoplanets, which are worlds orbiting stars other than the Sun. The current record holder for the smallest confirmed exoplanet orbiting a main-sequence star is Kepler-37b, discovered by the retired Kepler space telescope. This tiny world is located approximately 215 light-years away in the constellation Lyra.
Kepler-37b is smaller than any planet in our Solar System, possessing a radius of just 0.31 times that of Earth, or a diameter of about 3,900 kilometers. This makes it only slightly larger than our Moon, but definitively smaller than Mercury. Due to its small size and extremely close orbit—completing a revolution in just 13 days—Kepler-37b is believed to be a hot, rocky world with no atmosphere.
The size of distant planets is primarily determined using the transit method, where scientists measure the minute dip in a star’s light as a planet passes in front of it. The amount of light blocked corresponds to the planet’s size relative to its star. For Kepler-37b, the precise size measurement was achieved using asteroseismology, a technique that studies the star’s internal structure by observing its natural oscillations.
Kepler-37b is simply the smallest known exoplanet, a distinction limited by current technology. Smaller planets likely exist throughout the galaxy but are currently undetectable because they block too little starlight or have orbital periods too long for observation. Future, more sensitive telescopes will undoubtedly uncover even smaller worlds, redefining the lower limit of planetary size.
What Defines a Planet
Planets like Mercury or Kepler-37b hold the “smallest” title, even though many smaller celestial bodies exist, due to a formal classification system. The size of an object alone is not the sole factor in determining its planetary status. The International Astronomical Union (IAU) established three specific criteria in 2006 to define a planet within our Solar System.
The first criterion is that a body must be in orbit around the Sun. Second, it must have sufficient mass for its own gravity to pull it into a nearly round shape, a state known as hydrostatic equilibrium. This requirement ensures that only objects large enough to overcome their own rigid body forces are considered.
The third and most distinguishing criterion is that the object must have “cleared the neighborhood” around its orbit. This means the planet must be gravitationally dominant in its orbital path, having either incorporated or ejected most other comparably sized objects over time. This dynamic requirement is the reason Pluto was reclassified as a dwarf planet, despite satisfying the first two conditions.
Pluto shares its orbital zone with many other large, icy objects in the Kuiper Belt, meaning it has not achieved gravitational dominance. Mercury, despite being small, easily satisfies this third rule because its mass is overwhelmingly greater than the combined mass of any other bodies that cross its path. This framework explains why a small, dense world like Mercury qualifies as a planet, while many larger, non-dominant worlds do not.