Small Solar System body in the context of Near-Earth asteroid

Ceto (/ˈsiːtoʊ/; Ancient Greek: Κητώ, romanized: Kētṓ, lit. 'sea monster') is a primordial sea goddess in Greek mythology, the daughter of Pontus and his mother, Gaia. As a mythological figure, she is considered to be one of the most ancient deities, and bore a host of monstrous children fathered by Phorcys, another child of Gaia and Pontus. The small Solar System body 65489 Ceto was named after her, and its satellite after Phorcys.

Ceto was also variously called Crataeis (Κράταιις, Krataiis, from κραταιίς "mighty") and Trienus (Τρίενος, Trienos, from τρίενος "within three years"), and was occasionally conflated by scholars with the goddess Hecate (for whom Crataeis and Trienus are also epithets).

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

In the Solar System, there are six planetary satellite systems, altogether comprising 419 natural satellites with confirmed orbits. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, and Eris. As of January 2022, there are 447 other minor planets known to have natural satellites.

A comet is an icy, small Solar System body or interstellar object that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids or minor planets. The identified objects are of many sizes, but much smaller than planets, and, on average, are about one million kilometers (or six hundred thousand miles) apart. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System.

The asteroid belt is the smallest and innermost circumstellar disc in the Solar System. Classes of small Solar System bodies in other regions are the near-Earth objects, the centaurs, the Kuiper belt objects, the scattered disc objects, the sednoids, and the Oort cloud objects. About 60% of the main belt mass is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is estimated to be 3% that of the Moon.

The International Astronomical Union (IAU) adopted in August 2006 the definition made by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi that stated, that in the Solar System, a planet is a celestial body that:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria (such as Pluto, which had hitherto been considered a planet) is classified as a dwarf planet. According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a small Solar System body (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The decision was a controversial one, and has drawn both support and criticism from astronomers.

A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets (including Earth), dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land (or ground) is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. Most bodies more massive than super-Earths, including stars and giant planets, as well as smaller gas dwarfs, transition contiguously between phases, including gas, liquid, and solid. As such, they are generally regarded as lacking surfaces.

In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary physics of Earth, the pressure-gradient force prevents gravity from collapsing the atmosphere of Earth into a thin, dense shell, whereas gravity prevents the pressure-gradient force from diffusing the atmosphere into outer space. In general, it is what causes objects in space to be spherical.

Hydrostatic equilibrium is the distinguishing criterion between dwarf planets and small solar system bodies, and features in astrophysics and planetary geology. Said qualification of equilibrium indicates that the shape of the object is symmetrically rounded, mostly due to rotation, into an ellipsoid, where any irregular surface features are consequent to a relatively thin solid crust. In addition to the Sun, there are a dozen or so equilibrium objects confirmed to exist in the Solar System.

According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term minor planet, but that year's meeting reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs). In contrast to the eight official planets of the Solar System, all minor planets fail to clear their orbital neighborhood.

Minor planets include asteroids (near-Earth objects, Earth trojans, Mars trojans, Mars-crossers, main-belt asteroids and Jupiter trojans), as well as distant minor planets (Uranus trojans, Neptune trojans, centaurs and trans-Neptunian objects), most of which reside in the Kuiper belt and the scattered disc. As of October 2025, there are 1,472,966 known objects, divided into 875,150 numbered, with only one of them recognized as a dwarf planet (secured discoveries) and 597,816 unnumbered minor planets, with only five of those officially recognized as a dwarf planet.

The Kuiper belt (/ˈkaɪpər/ ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

The Kuiper belt is named in honor of the Dutch astronomer Gerard Kuiper, who conjectured the existence of a version of the belt in 1951. There were researchers before and after him who proposed similar hypoetheses, such as Kenneth Edgeworth in the 1930s. The most direct prediction of the belt was by astronomer Julio Ángel Fernández, who published a paper in 1980 suggesting the existence of a comet belt beyond Neptune which could serve as a source for short-period comets.

Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.

In the Solar System, the orbits around the Sun of all planets and dwarf planets and most small Solar System bodies, except many comets and few distant objects, are prograde. They orbit around the Sun in the same direction as the sun rotates about its axis, which is counterclockwise when observed from above the Sun's north pole. Except for Venus and Uranus, planetary rotations around their axis are also prograde. Most natural satellites have prograde orbits around their planets. Prograde satellites of Uranus orbit in the direction Uranus rotates, which is retrograde to the Sun. Nearly all regular satellites are tidally locked and thus have prograde rotation. Retrograde satellites are generally small and distant from their planets, except Neptune's satellite Triton, which is large and close. All retrograde satellites are thought to have formed separately before being captured by their planets.

Panspermia (from Ancient Greek πᾶν (pan) 'all' and σπέρμα (sperma) 'seed') is the hypothesis that life exists throughout the universe, distributed by cosmic dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms, known as directed panspermia. The theory argues that life did not originate on Earth, but instead evolved somewhere else and seeded life as we know it.

Panspermia comes in many forms, such as radiopanspermia, lithopanspermia, and directed panspermia. Regardless of its form, the theories generally propose that microbes able to survive in outer space (such as certain types of bacteria or plant spores) can become trapped in debris ejected into space after collisions between planets and small Solar System bodies that harbor life. This debris containing the lifeforms is then transported by meteors between bodies in a planetary system, or even across planetary systems within a galaxy. In this way, panspermia studies concentrate not on how life began but on methods that may distribute it within the Universe. This point is often used as a criticism of the theory.

This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius. These lists can be sorted according to an object's radius and mass and, for the most massive objects, volume, density, and surface gravity, if these values are available.

These lists contain the Sun, the planets, dwarf planets, many of the larger small Solar System bodies (which includes the asteroids), all named natural satellites, and a number of smaller objects of historical or scientific interest, such as comets and near-Earth objects.

A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth. If an NEO's orbit crosses the Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but about a third of a percent are comets.

There are over 37,000 known near-Earth asteroids (NEAs) and over 120 known short-period near-Earth comets (NECs). A number of solar-orbiting meteoroids were large enough to be tracked in space before striking Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth. Asteroids as small as 20 metres (66 ft) in diameter can cause significant damage to the local environment and human populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact the sea. Interest in NEOs has increased since the 1980s because of greater awareness of this risk. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched.

In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because of this; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻāwela, which may be in a stable (though retrograde) orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.

The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo, which at 250 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings. It was discovered in 1997.

In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the March equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.