SkyEye

Planets and Their Satellites

What is a Planet?

The solar system boasts a population of eight planets, five dwarf planets, dozens of satellites and countless small solar-system bodies. The charts below list a few properties of the planets and their named satellites.

Although the innermost six planets of the solar system were known from antiquity, Uranus was discovered in 1781 by W. Herschel. Irregularities in the orbit of Uranus led J. Adams and U. Le Verrier to predict the existance of an eighth planet and in 1846, J. Galle discovered Neptune. Interestingly, Galileo was the first astronomer to observe Neptune in 1612 and 1613 but he recorded it as a fixed star!

The official definition (adopted by the International Astronomical Union on 24 August 2006) of a planet is a celestial body that

Thus Pluto was demoted from planet to dwarf planet status and the solar system shrank from nine to eight planets.

Physical Characteristics

The rotational period is the length of a day on a planet. Obliquity measures the angle of inclination of the equator to the orbital plane. Notice that Venus is nearly "upside-down" as a result of a large axial tilt. Thus, it appears to rotate retrograde or backwards from our point of view. Uranus is practically laying on its side, 'rolling' in its orbit around the Sun. It is the axial tilt on the Earth that is largely responsible for the seasons.

Planet Equatorial
Radius
(Earth radius)
Mass

(Earth mass)
Rotational
Period
(Earth day)
Obliquity
Mercury 0.383 0.055 58.646 0.0°
Venus 0.949 0.815 −243.018 177.3°
Earth 1.0000 1.000 0.997 23.4°
Mars 0.532 0.107 1.026 25.2°
Jupiter 11.201 317.828 0.414 3.1°
Saturn 9.449 95.161 0.444 26.7°
Uranus 4.007 14.536 −0.718 97.8°
Neptune 3.883 17.148 0.671 28.3°

Orbital Characteristics

The semi-major axis defines the size of the orbit. The sidereal period is the length of time it takes for a planet to complete one orbit around the Sun. The synodic period, however, is the time it takes a planet to return to the same configuration with respect to the Earth. For instance, it takes Jupiter nearly 12 years to orbit the Sun but the time between successive oppositions, say, is around 400 days. Inclination measures the angle of inclination of the orbit to the ecliptic (which is defined by the orbit of the Earth around the Sun). Most of the planets are found close to the ecliptic but minor planets and comets often have much higher inclinations. Finally, eccentricity indicates the shape of the orbit: An eccentricity of zero denotes a circular orbit but if the eccentricity is between zero and one, the orbit is elliptical. The closer the eccentricity is to one, the more stretched-out the orbit becomes. Many comets have highly elliptical orbits. An eccentricity of exactly one means that the orbit is parabolic and an eccentricity higher than one gives a hyperbolic orbit.

Planet Semi-Major
Axis
(au)
Sidereal
Period
(Earth year)
Synodic
Period
(Earth day)
Inclination Eccentricity
Mercury 0.387 0.241 115.9 7.01° 0.206
Venus 0.723 0.615 583.9 3.39° 0.007
Earth 1.000 1.000 0.00° 0.017
Mars 1.524 1.881 779.9 1.85° 0.093
Jupiter 5.203 11.863 398.9 1.31° 0.048
Saturn 9.537 29.447 378.1 2.49° 0.054
Uranus 19.189 84.017 369.7 0.77° 0.047
Neptune 30.070 164.791 367.5 1.77° 0.009

Satellites

The next table lists alphabetically the named satellites of the major planets. Many tiny new satellites have been discovered since space probes have been sent to explore the solar system. The Voyager and Cassini projects were particularly successful. The Galilean satellites of Jupiter are marked with an asterisk * and are easily seen through a small telescope.

Planet Named Satellite Year of
Discovery
Discoverer(s)
Mercury [none]
Venus [none]
Earth Moon    
Mars Deimos 1877 A. Hall
Phobos 1877 A. Hall
Jupiter ⅩⅤ Adrastea 1979 Voyager Science Team
ⅩⅩⅩⅠ Aitne 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
Amalthea 1892 E.E. Barnard
ⅩⅡ Ananke 1951 S.B. Nicholson
ⅩⅬⅠ Aoede 2003 S.S. Sheppard
ⅩⅬⅢ Arche 2002 S.S. Sheppard
ⅩⅩⅧ Autonoe 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅩⅦ Callirrhoe 1999 J.V. Scotti, T.B. Spahr, R.S. McMillan, J.A. Larson, J. Montani, A.E. Gleason, T. Gehrels
Callisto * 1610 Galileo
ⅩⅠ Carme 1938 S.B. Nicholson
ⅩⅬⅥ Carpo 2003 S.S. Sheppard
ⅩⅩⅠ Chaldene 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅬⅧ Cyllene 2003 S.S. Sheppard
ⅬⅢ Dia 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
Elara 1905 C. Perrine
ⅩⅩⅤ Erinome 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅩⅩⅢ Euanthe 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅩⅬⅦ Eukelade 2003 S.S. Sheppard
ⅩⅩⅩⅣ Euporie 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
Europa * 1610 Galileo
ⅩⅩⅩⅡ Eurydome 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
Ganymede * 1610 Galileo
ⅩⅩⅡ Harpalyke 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅩⅩⅨ Hegemone 2003 S.S. Sheppard
ⅩⅬⅤ Helike 2003 S.S. Sheppard
ⅩⅩⅩ Hermippe 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
Herse 2003 B. Gladman
Himalia 1904 C. Perrine
Io * 1610 Galileo
ⅩⅩⅣ Iocaste 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅩⅥ Isonoe 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅩⅩⅦ Kale 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅩⅬⅣ Kallichore 2003 S.S. Sheppard
ⅩⅩⅢ Kalyke 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅬⅨ Kore 2003 S.S. Sheppard
ⅩⅢ Leda 1974 C. Kowal
Lysithea 1938 S.B. Nicholson
ⅩⅨ Megaclite 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅥ Metis 1979 Voyager Science Team
ⅩⅬ Mneme 2003 S.S. Sheppard, B. Gladman
ⅩⅩⅩⅤ Orthosie 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
Pasiphaë 1908 P. Melotte
ⅩⅩⅩⅧ Pasithee 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅩⅩⅦ Praxidike 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
Sinope 1914 S.B. Nicholson
ⅩⅩⅩⅥ Sponde 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅩⅩ Taygete 2000 S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier
ⅩⅣ Thebe 1979 Voyager Science Team
ⅩⅬⅡ Thelxinoe 2003 S.S. Sheppard, B. Gladman
ⅩⅧ Themisto 1975 C. Kowal, E. Roemer
(Lost and then rediscovered in 2000 by S.S. Sheppard, D.C. Jewitt, Y. Fernandez, G. Magnier)
ⅩⅩⅨ Thyone 2001 S.S. Sheppard, D.C. Jewitt, J. Kleyna
ⅬⅠ [unnamed] 2010 R. Jacobson, M. Brozovic, B. Gladman, M. Alexandersen
ⅬⅡ [unnamed] 2010 C. Veillet
Saturn ⅬⅢ Aegaeon 2008 Cassini Imaging Science Team
ⅩⅩⅩⅥ Aegir 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅩⅥ Albiorix 2000 M. Holman, T.B. Spahr
ⅩⅬⅨ Anthe 2007 Cassini Imaging Science Team
ⅩⅤ Atlas 1980 Voyager Science Team
ⅩⅩⅩⅦ Bebhionn 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅩⅩⅧ Bergelmir 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅩⅩⅨ Bestla 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅣ Calypso 1980 D. Pascu, K. Seidelmann, W. Baum, D. Currie
ⅩⅩⅩⅤ Daphnis 2005 Cassini Imaging Science Team
Dione 1684 G.D. Cassini
Enceladus 1789 W. Herschel
ⅩⅠ Epimetheus 1977 J. Fountain, S. Larson, H. Reitsema, B. Smith/Voyager 1
ⅩⅩⅧ Erriapus 2000 J.J. Kavelaars, B. Gladman
ⅩⅬ Farbauti 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅬⅠ Fenrir 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅬⅡ Fornjot 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅬⅠ Greip 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅬⅢ Hati 2004 D. Jewitt, S. Sheppard, J. Kleyna
ⅩⅡ Helene 1980 P. Laques, J. Lecacheux
Hyperion 1848 W. Bond, G. Bond, W. Lassell
ⅩⅬⅣ Hyrrokkin 2004 S. Sheppard, D. Jewitt, J. Kleyna
Iapetus 1671 G.D. Cassini
ⅩⅩⅡ Ijiraq 2000 J.J. Kavelaars, B. Gladman
Janus 1966 A. Dollfus
Jarnsaxa 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅬⅤ Kari 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅩⅣ Kiviuq 2000 B. Gladman
ⅩⅬⅥ Loge 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅩⅩⅡ Methone 2004 Cassini Imaging Science Team
Mimas 1789 W. Herschel
ⅩⅩⅤ Mundilfari 2000 B. Gladman, J.J. Kavelaars
ⅩⅩⅩⅠ Narvi 2003 S.S. Sheppard
ⅩⅩ Paaliaq 2000 B. Gladman
ⅩⅩⅩⅢ Pallene 2004 Cassini Imaging Science Team
ⅩⅧ Pan 1990 M. Showalter/Voyager 2
ⅩⅦ Pandora 1980 Voyager Science Team
Phoebe 1898 W. Pickering
ⅩⅩⅩⅣ Polydeuces 2004 Cassini Imaging Science Team
ⅩⅥ Prometheus 1980 Voyager Science Team
Rhea 1672 G.D. Cassini
ⅩⅩⅨ Siarnaq 2000 B. Gladman, J.J. Kavelaars
ⅩⅩⅦ Skathi 2000 J.J. Kavelaars, B. Gladman
ⅩⅬⅦ Skoll 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅬⅧ Surtur 2006 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅩⅢ Suttungr 2000 B. Gladman, J.J. Kavelaars
ⅬⅡ Tarqeq 2007 S. Sheppard, D. Jewitt, J. Kleyna
ⅩⅩⅠ Tarvos 2000 J.J. Kavelaars, B. Gladman
ⅩⅢ Telesto 1980 B. Smith, H. Reitsema, S. Larson, J. Fountain/Voyager 1
Tethys 1684 G.D. Cassini
ⅩⅩⅩ Thrymr 2000 B. Gladman, J.J. Kavelaars
Titan 1655 C. Huygens
ⅩⅨ Ymir 2000 B. Gladman
Uranus Ariel 1851 W. Lassell
ⅩⅣ Belinda 1986 Voyager Science Team
Bianca 1986 Voyager Science Team
ⅩⅥ Caliban 1997 B.J. Gladman, P.D. Nicholson, J.A. Burns, J.J. Kavelaars
Cordelia 1986 Voyager Science Team
Cressida 1986 Voyager Science Team
ⅩⅩⅦ Cupid 2003 M.R. Showalter, J.J. Lissauer
Desdemona 1986 Voyager Science Team
ⅩⅩⅣ Ferdinand 2001 D. Milisavljevic, M. Holman, J.J. Kavelaars, T. Grav
ⅩⅩⅡ Francisco 2001 J.J. Kavelaars, M. Holman, D. Milisavljevic, T. Grav
ⅩⅠ Juliet 1986 Voyager Science Team
ⅩⅩⅥ Mab 2003 M.R. Showalter, J.J. Lissauer
ⅩⅩⅢ Margaret 2003 D.C. Jewitt, S.S. Sheppard
Miranda 1948 G. Kuiper
Oberon 1787 W. Herschel
Ophelia 1986 Voyager Science Team
ⅩⅩⅤ Perdita 1986 E. Karkoschka/Voyager 2
ⅩⅡ Portia 1986 Voyager Science Team
ⅩⅧ Prospero 1999 M. Holman, J.J. Kavelaars, B. Gladman, J.-M. Petit, H. Scholl
ⅩⅤ Puck 1985 Voyager Science Team
ⅩⅢ Rosalind 1986 Voyager Science Team
ⅩⅨ Setebos 1999 J.J. Kavelaars, B. Gladman, M. Holman, J.-M. Petit, H. Scholl
ⅩⅩ Stephano 1999 B. Gladman, M. Holman, J.J. Kavelaars, J.-M. Petit, H. Scholl
ⅩⅦ Sycorax 1997 P.D. Nicholson, B.J. Gladman, J.A. Burns, J.J. Kavelaars
Titania 1787 W. Herschel
ⅩⅩⅠ Trinculo 2001 M. Holman, J.J. Kavelaars, D. Milisavljevic
Umbriel 1851 W. Lassell
Neptune Despina 1989 Voyager Science Team
Galatea 1989 Voyager Science Team
Halimede 2002 M. Holman, J.J. Kavelaars, T. Grav, W. Fraser, D. Milisavljevic
ⅩⅣ Hippocamp 2013 M. Showalter
Larissa 1989 Voyager Science Team
ⅩⅡ Laomedeia 2002 M. Holman, J.J. Kavelaars, T. Grav, W. Fraser, D. Milisavljevic
Naiad 1989 Voyager Science Team
Nereid 1949 G. Kuiper
ⅩⅢ Neso 2002 M. Holman et al, B. Gladman et al
Proteus 1989 Voyager Science Team
Psamathe 2003 D.C. Jewitt, J. Kleyna, S.S. Sheppard
ⅩⅠ Sao 2002 M. Holman, J.J. Kavelaars, T. Grav, W. Fraser, D. Milisavljevic
Thalassa 1989 Voyager Science Team
Triton 1846 W. Lassell

Sources

Planetary orbital elements are provided by the Solar System Exploration database. Satellite names and discovery data are provided by the Jet Propulsion Laboratory.