Minor Planets

Minor planets

A minor planet is not a planet, but may be a number of things, which are dealt with below.

Dwarf planets. This includes Pluto.
Asteroids. These are mostly found in the asteroid belt between Mars and Jupiter.
Trojans. These are asteroids that preceeed or follow Jupiter in its orbit.
Centaurs. These are icy bodies between Saturn and Neptune.
Trans-Neptunian objects. These are objects beyond Neptune's orbit, and include plutinos and other Kuiper belt objects.

Since Pluto is the first plutino found and a dwarf planet, it's clear that it's possible fo a body to be more than one type of minor planet.

Dwarf planets

A dwarf planet conforms to points 1 and 2 in the planet definition, but not 3. A dwarf planet is one type of minor planet, but most minor planets are not dwarf planets.

Point 3 of the planet definition excludes the asteroids from being planets. Point 2 excludes most of the asteroids from being dwarf planets, with the asteroid Ceres being a notable exception (it's big enough to be round).

A dwarf planet also needs to orbit the sun, not orbit a planet. This rules out various moons as being dwarf planets.

Asteroids

Asteroids are a type of minor planet. They orbit the Sun but have not cleared their orbit of other bodies. The main area they orbit in is called the asteroid belt, located between the orbits or Mars and Jupiter.

The total mass of the asteroid belt is not huge, only about 4% that of the Moon. About half the asteroid belt's mass is in the four largest asteroids, Ceres, Vesta, Pallas and Hygiea, together equivalent to Pluto's main moon Charon. The asteroid belt is so spread out that travelling through the asteroid belt has not been at all exciting for the many space probes which have headed out through it.

The biggest asteroid is Ceres. It is the only asteroid known to be big enough to be round, and hence it is the only asteroid to be a dwarf planet. It has a diameter of 950 km. It was discovered in the first day of 1801 by Giuseppe Piazzi. It was originally considered a planet until other objects with about the same orbit were discovered. Ceres is too dim to be seen with the naked eye.

Trojans

Trojans are asteroids that accompany some planets (or moons), particularly Jupiter, distributed around two points 60° ahead of and 60° behind the planet. Estimates are that there could be as many Jupiter trojans as there are asteroids in the main asteroid belt.

Mars and Neptune also have trojans, as do a couple of Saturn's moons. One Earth trojan was found in 2010.

The "Greeks" and "Trojans" labels in the diagram at right refer to the naming scheme that the trojans in those two groups have; individual objects are named after Greek heros and heros of Troy.

The Hilda asteroids have a 3:2 resonance with Jupiter; for each 3 orbits around the sun they make, Jupiter makes 2.

Centaurs

Centaurs (love that name) are icy planetoids between Saturn and Neptune. These are shown in yellow in the chart below and are all pretty small. They are named after mythical centaurs (and nymphs).

Distance from the Sun is shown along the top of the chart in astronomical units (AU). 1AU = the distance of Earth's orbit from the Sun. The figures up the left of the chart are the inclination (angle) from the ecliptic (the plane Earth orbits in or the path the Sun takes across the sky).

There are an estimated 44,000 centaurs bigger than 1 km diameter. The largest is 10199 Chariklo, at 232 km across. It lies close to being in 4:3 resonance with Uranus, and also has the distinction of being the smallest known body with rings (two of them). Saturn's moon Phoebe may be a captured centaur.

Centaurs and trans-Neptunian objects.

Semi-major axis on the horizontal axis.
Orbital inclination is on the vertical axis.

Trans-Neptunian objects (TNOs)

These are objects that orbit the Sun at a greater average distance than Neptune. Pluto was the first trans-Neptunan object discovered, in 1930. They orbit the Sun in elliptical orbits which are highly inclined to the ecliptic, unlike the planets which mostly have nearly circular orbits. (Mercury's orbit is a bit more elliptical than the orbits of the other planets.)

There are presently hundreds of known TNOs that may be dwarf planets. Astronomer Mike Brown maintains a list.

Sedna and Eris are two trans-Neptunian objects that along with Pluto definitely qualify for dwarf planet status. They are classed as scattered disk objects (SDO) because they are believed to have been scattered from the Kuiper Belt. Kuiper rhymes with (windscreen) wiper.

Sedna is about ⅔ the size of Pluto. At the time of discovery it was the furthest known object orbiting the Sun.

Eris – formerly called 2003 UB₃₁₃ – is about 27% more massive than Pluto (but is thought to be a little smaller). For this reason some people were very keen on it being designated a planet. However, others were equally keen on it not being a planet; its orbit is highly inclined to the ecliptic, 44°, and is highly elliptical with an eccentricity of 0.44. It has a very high albedo (reflectivity) of 0.96, which is higher than any other large body in the Solar System except Enceladus, a moon of Saturn.

Like Pluto and Charon, Eris also has a moon, Dysnomia.

Eris and Dysnomia.
Formerly 2003 UB ₃₁₃ and S/2005 (2003 UB₃₁₃)

Pluto/Charon

For several decades Pluto was thought of as the Solar System's ninth planet. With the discovery of several trans-Neptunian objects in the 1990s (resulting in much discussion about what a planet is) it became clear that these objects could not really be regarded as planets.

Thus Pluto lost its planet status, and is now officially regarded as a dwarf planet, as is the asteroid Ceres. This video nicely explains it. (Note that if it's orbiting the Sun rather than another planet it won't be a satellite.)

Another reason Pluto should not be regarded as a planet is that Pluto and its largest moon, Charon, orbit about a point in space between them (its barycentre), not a point within Pluto. This makes Pluto and Charon a binary system. In the Earth-Moon system the two bodies orbit each other about a point roughly 1,700 km under Earth's surface.

NASA's space probe New Horizons passed Pluto in July 2015, and was the first time a space probe has visited Pluto.

Interestingly, Pluto is in a stable 2:3 resonance with Neptune. That means for each 2 orbits around the Sun that Pluto makes, Neptune makes 3. However, even though Pluto's elliptical orbit sometimes takes it closer to the Sun than Neptune ever gets, Pluto approaches closer to Uranus than it ever does to Neptune.

There are other objects which also have a 2:3 resonance with Neptune, and these are called plutinos. Orcus and Ixion are plutinos.