Dwarf planet
A dwarf planet is a celestial body that orbits the Sun, has sufficient mass to assume a nearly round shape due to hydrostatic equilibrium, has not cleared the neighborhood around its orbit, and is neither a satellite nor a sub-satellite.[1] This definition distinguishes dwarf planets from the eight major planets in the Solar System, which meet all criteria except the last two but have gravitationally dominated and cleared their orbital paths.[2] The concept of dwarf planets was formalized by the International Astronomical Union (IAU) during its 2006 General Assembly in Prague, primarily in response to the discovery of large trans-Neptunian objects like Eris, which challenged the traditional planetary roster and led to Pluto's reclassification from a planet to a dwarf planet.[2] As of 2025, the IAU recognizes five dwarf planets: Ceres, located in the asteroid belt between Mars and Jupiter; Pluto, the prototype in the Kuiper Belt; Eris, the most massive known; Haumea, noted for its rapid rotation and elongated shape; and Makemake, a bright Kuiper Belt object.[3] These bodies are primarily found in the outer Solar System's Kuiper Belt and scattered disk, with Ceres as the sole exception in the inner system, and they share planetary-like characteristics such as potential atmospheres, moons, and geological activity despite their smaller sizes.[4] Dwarf planets play a crucial role in understanding Solar System formation, as their preservation of primordial materials offers insights into the early dynamics of planetary accretion and migration.[4] Missions like NASA's Dawn to Ceres and New Horizons to Pluto have revealed diverse surfaces, subsurface oceans, and organic compounds, highlighting their scientific significance beyond mere classification.[4] While the IAU's criteria have sparked debate among astronomers regarding the inclusion of additional candidates like Gonggong or Quaoar, the category continues to expand with ongoing discoveries in the distant reaches of the Solar System.[3]Definition and Criteria
IAU Official Definition
The International Astronomical Union (IAU) formally defined a dwarf planet in its Resolution B5, adopted on August 24, 2006, during the 26th General Assembly in Prague.[2] The resolution distinguishes dwarf planets from planets and other solar system bodies, emphasizing their role in the evolving understanding of planetary systems. The full text of the relevant section on dwarf planets states: "A 'dwarf planet' is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite."[2] This definition applies exclusively to objects within the Solar System and does not extend to exoplanets orbiting other stars.[2] The three core criteria for dwarf planets, supplemented by an exclusion, are as follows. First, the body must orbit the Sun directly, excluding satellites or moons that orbit planets.[2] Second, it must possess sufficient mass to achieve hydrostatic equilibrium, meaning its own gravity shapes it into a nearly spherical form, distinguishing it from irregularly shaped smaller bodies like asteroids or comets.[2] Third, unlike planets, it must not have cleared its orbital neighborhood of other debris, meaning it coexists with other objects in a similar orbital path without gravitationally dominating the region.[2] The exclusion ensures that moons, regardless of size or shape, are not classified as dwarf planets.[2] In contrast to the IAU's definition of a planet—which requires the same first two criteria but mandates clearing the orbital neighborhood—dwarf planets represent a separate category of substantial, rounded solar system objects that lack dynamical dominance.[2] This distinction highlights the ongoing process of planetary accretion and collision in the outer Solar System, where dwarf planets often reside among populations of similar-sized bodies.[2] The resolution's preamble underscores the need for such classifications amid new discoveries of trans-Neptunian objects, ensuring a consistent framework for solar system nomenclature.[2]Orbital Dominance
The orbital dominance criterion, as established by the International Astronomical Union (IAU), requires that a planet must have gravitationally cleared its orbital neighborhood, meaning it dominates the region around its orbit through gravitational perturbations that eject, capture, or incorporate other bodies into stable orbits around it.[2] Dwarf planets, by contrast, fail this test because they coexist with numerous comparable objects in their orbital zones without achieving such dynamical control.[2] Quantitatively, the extent of a body's potential orbital zone is approximated by its Hill sphere radius, which defines the region where the body's gravity dominates over that of the central star. The Hill radius is given byr_H = a \left( \frac{m}{3M} \right)^{1/3},
where a is the body's semi-major axis, m is its mass, and M is the mass of the central body (the Sun in the Solar System). For a body to clear its neighborhood, its mass must be sufficient to perturb objects within several Hill radii over the age of the system, often quantified by a planetary discriminant \Pi > 1, where \Pi compares the body's mass to a minimum orbit-clearing mass.[5] Dwarf planets typically have \Pi < 1, indicating insufficient dominance.[5] This dynamical shortfall is evident in recognized dwarf planets, such as Ceres, which orbits within the asteroid belt alongside millions of other bodies that it cannot perturb significantly, and Pluto, which shares the Kuiper belt with thousands of similar trans-Neptunian objects. The criterion's emphasis on gravitational interactions ensures that classification as a dwarf planet reflects not just physical properties but also the body's failure to shape its orbital environment, distinguishing it from planets even if it meets other requirements like hydrostatic equilibrium.[2]
Hydrostatic Equilibrium
Hydrostatic equilibrium refers to the physical state in which a celestial body's internal pressure balances its self-gravity, allowing it to relax into a nearly spherical or oblate spheroid shape over time. In the context of dwarf planets, this criterion, as incorporated into the International Astronomical Union (IAU) definition, requires sufficient mass for self-gravity to overcome rigid body forces, resulting in a hydrostatic equilibrium shape that is approximately round.[6] Achieving hydrostatic equilibrium depends on the body's mass, composition, and internal structure, with minimum thresholds varying by material properties. For icy bodies, such as those in the outer Solar System, a minimum mass of approximately $10^{20} kg—corresponding to a diameter of about 400–500 km—is typically required to drive this shape relaxation. Rocky bodies demand higher thresholds, around 800 km in diameter, due to their greater rigidity and strength, which resist deformation more effectively than ice.[6] Indicators of hydrostatic equilibrium include a smooth, rounded surface with minimal deviations from sphericity, the absence of disproportionately large craters that would persist on non-relaxed bodies, and rotational stability that aligns with fluid-like equilibrium figures.[7] Assessment methods often involve shape modeling from rotational light curves, which reveal the body's triaxial dimensions and deviation from ideal equilibrium ellipsoids, as well as direct measurements from spacecraft gravity data to confirm internal density distributions consistent with fluid balance.[8] Determining hydrostatic equilibrium poses challenges, particularly for smaller trans-Neptunian objects, where limited observational data leads to ambiguity between true relaxation and coincidental roundness from other formation processes.[9] Ongoing research employs numerical simulations of viscous flow and collisional evolution, alongside sparse spacecraft flybys, to refine these assessments, though comprehensive gravity mapping remains rare beyond a few targets.[10] In contrast to dwarf planets, smaller bodies like asteroids and comets lack the necessary mass—typically below $10^{19} kg—to achieve this equilibrium, retaining irregular, rubble-pile shapes dominated by rigid cohesion and impact history rather than self-gravitational rounding.[11]History of the Concept
Early Classifications
The discovery of Ceres on January 1, 1801, by Italian astronomer Giuseppe Piazzi marked the first identification of an object in the asteroid belt between Mars and Jupiter, initially classified as the eighth planet due to its planetary appearance and orbit.[12] However, as additional similar bodies were found—such as Pallas in 1802 and Juno in 1804—astronomers reclassified Ceres and these objects as asteroids by the mid-19th century, recognizing them as a distinct population rather than full planets.[12] This shift introduced the concept of "minor planets" around 1850, an intermediate category proposed to describe these smaller bodies orbiting between Mars and Jupiter, distinguishing them from the major planets while acknowledging their planetary-like orbits.[13] In 1930, Clyde Tombaugh discovered Pluto at Lowell Observatory in Arizona, identifying it as the ninth planet based on its trans-Neptunian orbit and perceived mass influence on outer planets.[14] Pluto's classification as a planet solidified its place in solar system models for decades, despite its small size compared to other planets. Mid-20th-century developments began challenging this view; in 1951, Gerard Kuiper proposed a hypothesis for a disk of icy planetesimals beyond Neptune, suggesting a reservoir of Pluto-like bodies that could explain comet origins and imply Pluto was not unique.[15] This idea, amid ongoing discoveries of thousands of asteroids, fueled debates about planetary boundaries, with some astronomers advocating for refined categories to accommodate small, icy outer solar system objects between traditional planets and mere asteroids.[15] The 1990s brought empirical evidence supporting Kuiper's hypothesis through the discovery of large trans-Neptunian objects (TNOs), starting with 1992 QB1 (later named Albion) on August 30, 1992, by David Jewitt and Jane Luu using the University of Hawaii's 2.2-meter telescope.[15] Measuring about 100-170 kilometers in diameter, Albion's orbit beyond Neptune demonstrated a population of ancient, icy bodies similar to Pluto, undermining Pluto's exceptional status as the sole trans-Neptunian planet and highlighting the need for intermediate classifications.[15] Subsequent TNO finds, such as Varuna in 2000, intensified these discussions among astronomers like Jewitt and Kuiper's intellectual successors. The concept of dwarf planets gained traction in 2005 when Mike Brown, Chad Trujillo, and David Rabinowitz discovered Eris, a TNO larger than Pluto, using Palomar Observatory data from 2003.[16] Eris's size—approximately 2,326 kilometers in diameter—prompted urgent International Astronomical Union (IAU) discussions that year on redefining planetary categories to address the growing roster of Pluto-like objects. These pre-2006 debates culminated in the IAU's formal resolution the following year.2006 IAU General Assembly
The 26th General Assembly of the International Astronomical Union (IAU) took place in Prague, Czech Republic, from August 14 to 25, 2006, attended by 2412 astronomers.[17] The meeting was prompted by the 2005 discovery of Eris (then designated 2003 UB313), a trans-Neptunian object larger than Pluto, which raised questions about Pluto's planetary status and the need for a formal definition of a planet.[2] Debates centered on competing proposals for defining planets, including a geophysical approach advocated by Alan Stern, principal investigator of the New Horizons mission, which emphasized an object's mass and ability to achieve hydrostatic equilibrium (roundness) without requiring orbital dominance.[17] This was rejected in favor of a dynamical criterion that included clearing the orbital neighborhood of other debris, leading to intense discussions among attendees on whether to prioritize physical properties or orbital behavior.[17] On August 24, 2006, only 424 members voted on the resolutions after preliminary drafts were revised multiple times.[17] IAU Resolution B5, passed that day, established the modern definition of a planet as a celestial body orbiting the Sun, nearly spherical due to hydrostatic equilibrium, and having cleared its orbital path of other objects.[2] It introduced the category of dwarf planet for objects that orbit the Sun, are nearly round, but have not cleared their orbits and are not satellites, immediately recognizing Ceres, Pluto, and Eris as the first members.[2] Pluto was reclassified as a dwarf planet and designated the prototype for trans-Neptunian objects of this type, reducing the number of planets in the Solar System to eight.[2] The decision sparked significant public backlash and criticism from the planetary science community, with figures like Stern arguing that the definition was flawed and excluded geophysically similar bodies.[17] In response to ongoing discoveries, the IAU added Haumea and Makemake to the dwarf planet list in 2008, expanding the category to five recognized members and introducing the term "plutoid" for those beyond Neptune's orbit.[18] The resolution has spurred increased research into the Kuiper Belt and trans-Neptunian objects, though it remains controversial with calls for revision from some scientists, and no major updates have occurred since.[19]Population
Recognized Dwarf Planets
The International Astronomical Union (IAU) recognizes five dwarf planets in the Solar System: Ceres, Pluto, Eris, Haumea, and Makemake.[4] These bodies meet the IAU's criteria, including hydrostatic equilibrium as confirmed through shape, density, and rotational observations.[20] Ceres is the only one located within the inner Solar System, orbiting in the main asteroid belt between Mars and Jupiter, while the others reside in the outer Solar System beyond Neptune.[21]| Dwarf Planet | Location | Approximate Diameter (km) | Key Characteristics |
|---|---|---|---|
| Ceres | Main asteroid belt | 946 | Rocky body with a significant ice mantle, comprising about one-third of the asteroid belt's total mass. |
| Pluto | Kuiper Belt | 2,376 | Icy surface with mountains, plains, and a thin nitrogen-methane atmosphere; explored by the New Horizons spacecraft. |
| Eris | Scattered disc | 2,326 | The most massive dwarf planet, with a highly eccentric orbit reaching up to 97 AU from the Sun; icy composition similar to Pluto. |
| Haumea | Kuiper Belt | ~1,600 (mean equivalent) | Elongated, rugby-ball shape due to rapid rotation (period of 3.9 hours); primarily rocky with water ice. |
| Makemake | Kuiper Belt | 1,430 | Bright, reddish surface rich in methane and ethane ices; no detected atmosphere.[22] |