List of Solar System objects by size - Alchetron, the free social encyclopedia

This is a partial list of Solar System objects by size, arranged in descending order of mean volumetric radius, and subdivided into several size classes. These lists can also be sorted according to an object's mass and, for the largest objects, volume, density and surface gravity, insofar as these values are available. This list contains 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.

The ordering may be different depending on whether one chooses radius or mass, because some objects are denser than others. For instance, Uranus is larger than Neptune but less massive, and although Ganymede and Titan are larger than Mercury, they have less than half Mercury's mass. This means some objects in the lower tables, despite their smaller radii, may be more massive than objects in the upper tables because they have a higher density.

Many trans-Neptunian objects (TNOs) have been discovered, and their approximate locations in this list are shown, even though there can be a large uncertainty in their measurement.

Solar System objects more massive than 10²¹ kilograms (one yottagram [Yg]) are known or expected to be approximately spherical. Astronomical bodies relax into rounded shapes (ellipsoids), achieving hydrostatic equilibrium, when the gravity of their mass is sufficient to overcome the structural strength of their material. Objects made of ice become round more easily than those made of rock, and many icy objects are spheroidal at far lower sizes. The cutoff boundary for roundness is somewhere between 100 km and 200 km in radius.

The larger objects in the mass range between 10¹⁸ kg to 10²¹ kg (1 to 1000 zettagrams [Zg]), such as Tethys, Ceres, and Mimas, have relaxed to an oblate-spheroid equilibrium due to their gravity, whereas the less massive rubble piles (e.g. Amalthea and Janus) are roughly rounded, but not spherical, dubbed "irregular".

Spheroidal bodies typically have some polar flattening due to the centrifugal force from their rotation, and can sometimes even have quite different equatorial diameters (scalene ellipsoids such as Haumea). Unlike bodies such as Haumea, the irregular bodies deviate significantly from the shape of an ellipsoid.

There can be difficulty in determining the diameter (within a factor of about 2) for typical objects beyond Saturn. (See 2060 Chiron as an example.) For TNOs there is some confidence in the diameters, but for non-binary TNOs there is no real confidence in the masses/densities. Many TNOs are often just assumed to have Pluto's density of 2.0 g/cm³, but it is just as likely that they have a comet-like density of only 0.5 g/cm³. For example, if a TNO is poorly assumed to have a mass of 3.59×10²⁰ kg based on a radius of 350 km with a density of 2 g/cm³ and is later discovered to only have a radius of 175 km with a density of 1 g/cm³, the mass estimate would be only 2.24×10¹⁹ kg.

The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous observations and interactions of the Galileo and Cassini orbiters. But many of the moons with a radius less than ~100 km, such as Jupiter's Himalia, still have unknown masses. Again, as we get further from the Sun than Saturn, things get less clear. There has not yet been an orbiter around Uranus or Neptune for long-term study of their moons. For the small outer irregular moons of Uranus, such as Sycorax, which were not discovered by the Voyager 2 flyby, even different NASA web pages, such as the National Space Science Data Center and JPL Solar System Dynamics, have somewhat contradictory size and albedo estimates depending on which research paper is being cited.

Data for objects has varying reliability including uncertainties in the figures for mass and radius, and irregularities in the shape and density, with accuracy often depending on how close it is to Earth or if it has been visited by a probe.

Larger than 400 km

It was once expected that any icy body larger than approximately 200 km in radius was likely to be in hydrostatic equilibrium (HE). However, Rhea is the smallest body where detailed measurements have been made and are consistent with hydrostatic equilibrium, whereas Iapetus is the largest determined not to be in hydrostatic equilibrium, bracketing a radius of 750 km.

For simplicity and comparative purposes, the values are manually calculated assuming a sphericity of 1. The size of solid bodies does not include an object's atmosphere. For example, Titan looks bigger than Ganymede, but its solid body is smaller. For the giant planets, the "radius" is the point at which the atmosphere reaches 1 bar of atmospheric pressure. The radius of Saturn's main rings is 136,775 km.

From 200 to 400 km

All imaged icy moons except Proteus with radii greater than 200 km are round, although those under 400 km that have had their shapes carefully measured are not in hydrostatic equilibrium. Most asteroids are rockier and less likely to be round; for example, 10 Hygiea is not, while 2 Pallas and 4 Vesta are borderline.

From 100 to 200 km

This list contains a selection of objects estimated to be between 100 and 200 km in radius (200 and 400 km in diameter). The largest of these may lie above the boundary for hydrostatic equilibrium, but most are irregular. Most of the trans-Neptunian objects listed with a radius smaller than 200 km have "assumed sizes based on a generic albedo of 0.09" since they are too far away to directly measure their sizes with existing instruments. Mass switches from 10²¹ kg to 10¹⁸ kg (Zg). Main-belt asteroids have orbital elements constrained by (2.0 AU < a < 3.2 AU; q > 1.666 AU) according to JPL Solar System Dynamics (JPLSSD). This list is not complete, missing many poorly known TNOs.

From 50 to 100 km

This list contains a selection of objects 50 and 100 km in radius (100 km to 200 km in average diameter). The listed objects currently include most objects in the asteroid belt and moons of the giant planets in this size range, but many newly discovered objects in the outer Solar System are missing, such as those included in the following reference. Asteroid spectral types are mostly Tholen, but some might be SMASS.

From 20 to 50 km

This list contains a few examples because there are about 589 asteroids in the asteroid belt with a measured radius between 20 and 50 km. Many thousands of objects of this size range have yet to be discovered in the Trans-Neptunian region. The number of digits is not an endorsement of significant figures. The table switches from ×10¹⁸ kg to ×10¹⁵ kg (Eg), and many of these mass values are assumed. (See list of minor planets.)

From 1 to 20 km

This list contains only a few examples of objects between 1 and 20 km in radius.

Below 1 km

This list contains only a few examples of objects below 1 km in radius.

In the asteroid belt alone there are estimated to be between 1.1 and 1.9 million objects with a radius above 0.5 km, many of which are in the range 0.5–1.0 km. Countless more have a radius below 0.5 km.

Very few objects in this size range have been explored or even imaged. The exceptions are objects that have been visited by a probe, or have passed close enough to Earth to be imaged. Radius is by mean geometric radius. Number of digits not an endorsement of significant figures. Mass scale shifts from × 10¹⁵ to 10¹² kg, which is 10¹⁵ grams (Petagram – Pg).

Currently most of the objects of mass between 10⁹ kg to 10¹² kg (less than 1000 teragrams (Tg)) listed here are near-Earth asteroids (NEAs). (See also List of NEAs by distance from Sun.) 1994 WR₁₂ has less mass than the Great Pyramid of Giza, 5.9 × 10⁹ kg.

For more about very small objects in the Solar System, see meteoroid, micrometeoroid, and interplanetary dust cloud. (See also Visited/imaged bodies.)

Surface gravity

The surface gravity at the equator of a body can in most cases be accurately calculated using Newton's law of universal gravitation and centrifugal force.

The gravitational acceleration at the equator is given by Newton's law of universal gravitation. The formula that follows from this law is:

a g = G m r 2

where

a_g is the magnitude of the gravitational acceleration G is the gravitational constant m is the mass of the celestial body r is the equatorial radius of the celestial body (if this varies significantly, the mean equatorial radius is used)

The magnitude of the outward acceleration due to centrifugal force is given by

a c = 4 π 2 r T 2

where

T is the rotation period of the celestial body

The surface gravity at the equator is then given by

g = a g − a c = G m r 2 − 4 π 2 r T 2

References

List of Solar System objects by size Wikipedia

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