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Science Astronomy Planets (1) Planets Dwarf Planets Satellites (Moons)

Astronomy: Planets (1)

This page lists the basic characteristics of the planets and other major bodies of the Solar System.  For details of their atmosphere, surface and interior structure, see Planets (2).  For anything else, see Solar System.


... in order of the radii of their orbits around the Sun – in other words, their distances from the Sun:

Name Discovered By Relative Mass Diameter (miles) Mean radius of orbit (miles) Rotational period ('day') Orbital period ('year') Satellites (known in November 2018)
Mercury 0.0563,030 36,000,00059 88 daysNone
Venus 0.827,500 67,200,000243 225 daysNone
Earth 17,923 92,860,0001 365.24 daysThe Moon (see below)
Mars 0.114,210 141,600,0001.02 687 daysPhobos, Deimos (see below)
Jupiter 31888,700 484,000,0009hr 51min 11.86 years79 (Ganymede is the largest)
Saturn 9575,000 866,000,00010hr 14m 29.46 years62 (Titan is the largest)
Uranus1781William Herschel 14.531,600 1,800,000,00017hrs 24min 84 years27 (named after Shakespearean characters)
Neptune1846J. G. Galle 17.230,200 2,794,000,00016hr 7min 164.8 years14 (2 are visible from Earth)

Twelve new moons of Jupiter were discovered in July 2018.

The four largest moons of Jupiter (Io, Europa, Ganymede and Callisto) were discovered in 1610 by Galileo Galilei and are known as the Galilean satellites. They were the first celestial objects to be discovered by telescope.

Dwarf planets

... in order of discovery:

Name Discovered By Relative Mass Diameter (miles) Mean radius of orbit (miles) Rotational period ('day') Orbital period ('year') Satellites
Ceres1801Giuseppe Piazzi 0.00016590 256,000,0009 hours 1,680 days
Pluto1930Clyde Tombaugh 0.0021,438 3,600,000,0006.39 days 248.5 yearsCharon (see below), Nix, Hydra
Eris2003Mike Brown Unknown1,490 6,288,000,000> 8 hours? 556 yearsDysnomia
Sedna2003Mike Brown 5.6 * 10–41,100 17,000,000,00010 hours 11,487 years 
Makemake2004Mike Brown 0.0005450 4,278,000,000Unknown 310 yearsNone so far
Haumea2005Mike Brown 0.00066400 3,900,000,0003.9 hours 283 yearsHi'iaka, Namaka

Note: the IAU does not currently classify Sedna as a dwarf planet, although many astronomers do. Officially it's a minor planet, or a Trans–Neptunian Object (TNO).

Satellites (Moons)

This section lists the ten most significant satellites or moons in the Solar System (the ones you're most likely to get asked about in a quiz.) For which satellite belongs to which planet, see the Planets table above. But for the record: Titan belongs to Saturn, and the other four biggest ones (other than The Moon) belong to Jupiter; Triton belongs to Neptune, Charon belongs to Pluto, and Phobos and Deimos both belong to Mars.

Name Discovered By Relative Mass Diameter (miles) Mean radius of orbit (miles) Rotational period ('day') Orbital period ('year') Satellites
Ganymede1610Galileo Galilei 0.0253,270 700,000Synchronous 7.2 daysThe ninth largest object in the solar system
Titan1655Christiaan Huygens 0.0233,200 759,000Synchronous 15.95 daysThe only 'moon' with an atmosphere (nitrogen)
Callisto1610Galileo 0.0183,000 1,170,100Synchronous 16.7 days
Io1610Galileo 0.0152,250 262,100Synchronous 1.8 days
The Moon 0.0122,160 238,855Synchronous 27.32 days *
Europa1610Galileo 0.0081,950 417,000Synchronous 3.6 days
Triton1846William Lasell 0.00361,680 220,400Synchronous –5.877 daysThe only large moon with a retrograde orbit
Charon1978James W. Christy 3 * 10–51,200 12,500Synchronous 6.39 days
Phobos1877Asaph Hall (US) 5.6 * 10–917 5,840Synchronous 0.32 daysIrregularly shaped – diameter given is largest
Deimos1877Asaph Hall (US) 3.8 * 10–109 15,000Synchronous 1.26 daysIrregularly shaped – diameter given is largest

* 27 days, 7 hours, 41 minutes. The 'lunation period' – the time taken to complete the lunar cycle of phases – is 29.53 days

© Haydn Thompson 2017–20