Dwarf Planet
A dwarf planet is a celestial body orbiting the Sun that is massive enough for its self-gravity to achieve hydrostatic equilibrium (giving it a roughly spherical shape), but has not gravitationally cleared the neighbourhood around its orbit of other debris. The International Astronomical Union (IAU) formally defined this category in 2006, simultaneously reclassifying Pluto from a planet to a dwarf planet. Recognised dwarf planets include Pluto, Eris, Haumea, Makemake, and Ceres, with hundreds more suspected in the Kuiper Belt and scattered disc, making dwarf planets the most numerous planetary-class bodies in the Solar System.
Recognised dwarf planets of the Solar System
| Name | Location | Diameter (km) | Orbital Period (years) | Moons |
|---|---|---|---|---|
| Ceres | Asteroid Belt | 945 | 4.60 | 0 |
| Pluto | Kuiper Belt | 2,377 | 248 | 5 (incl. Charon) |
| Eris | Scattered Disc | 2,326 | 559 | 1 (Dysnomia) |
| Haumea | Kuiper Belt | ~1,560 × 996 | 285 | 2 |
| Makemake | Kuiper Belt | ~1,430 | 305 | 1 (MK 2) |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Outer Planet
Outer planets are the four large planets — Jupiter, Saturn, Uranus, and Neptune — that orbit beyond the asteroid belt at distances greater than 5 AU from the Sun. They are divided into gas giants (Jupiter and Saturn, composed mainly of hydrogen and helium) and ice giants (Uranus and Neptune, containing a higher proportion of icy materials like water, ammonia, and methane). Outer planets are characterised by massive sizes, strong magnetic fields, ring systems, numerous moons, and rapid rotation, and their gravitational influence shaped the architecture of the entire Solar System during its formation.
Inner Planet
Inner planets, also called terrestrial planets, are the four rocky planets of the Solar System — Mercury, Venus, Earth, and Mars — that orbit within the asteroid belt at distances less than about 1.5 AU from the Sun. They are characterised by solid, rocky surfaces with metallic iron cores, relatively small sizes and masses compared to the gas giants, and slower rotation in some cases due to tidal interactions with the Sun. Their proximity to the Sun means they experience intense solar radiation, shorter orbital periods, and in most cases lack large moons, making them distinct in composition and environment from the outer planets.
Kepler's Third Law
Kepler's Third Law states that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit around the Sun. This relationship, discovered by Johannes Kepler in 1619, applies to all objects orbiting the same central body and allows astronomers to calculate orbital periods or distances when one is known. It was later explained theoretically by Newton's law of universal gravitation and remains a foundational tool for planetary science and space mission planning.
The term "dwarf planet" was coined and formally adopted by the International Astronomical Union (IAU) at its 2006 General Assembly in Prague. "Dwarf" comes from Old English dweorg, meaning a small being; here used in the astronomical sense of a planet-like body of sub-planetary clearing capacity.