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 and Orbital Characteristics

The first table lists some of the physical and orbital characteristics of the planets. 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. Inclination measures the angle of inclination of the orbit to the ecliptic (which is defined by the orbit of the Earth around the Sun). 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 Equatorial
Radius
(Earth radius)
Mass

(Earth mass)
Rotational
Period
(Earth day)
Obliquity Semi-Major
Axis
(AU)
Orbital
Period
(Earth year)
Inclination Eccentricity
Mercury 0.3829 0.055 58.646 0.0° 0.387 0.24 7.01° 0.20564
Venus 0.9499 0.815 -243.018 177.3° 0.723 0.62 3.39° 0.00678
Earth 1.0000 1.000 0.997 23.4° 1.000 1.00 0.00° 0.01671
Mars 0.5320 0.107 1.026 25.2° 1.524 1.88 1.85° 0.09339
Jupiter 10.9733 317.828 0.414 3.1° 5.203 11.86 1.31° 0.04839
Saturn 9.1402 95.161 0.444 26.7° 9.537 29.46 2.49° 0.05386
Uranus 3.9809 14.536 -0.718 97.8° 19.189 84.01 0.77° 0.04726
Neptune 3.8647 17.148 0.671 28.3° 30.070 164.79 1.77° 0.00859

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 project was 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)
Earth I Moon    
Mars II Deimos 1877 A. Hall
I Phobos 1877 A. Hall
Jupiter XV Adrastea 1979 Voyager Science Team
XXXI Aitne 2001 S. Sheppard, D. Jewitt, J. Kleyna
V Amalthea 1892 E. Barnard
XII Ananke 1951 S. Nicholson
XLI Aoede 2003 S. Sheppard
XLIII Arche 2002 S. Sheppard
XXVIII Autonoe 2001 S. Sheppard, D. Jewitt, J. Kleyna
XVII Callirrhoe 1999 J. Scotti, T. Spahr, R. McMillan, J. Larson, J. Montani, A. Gleason, T. Gehrels
IV Callisto * 1610 Galileo
XI Carme 1938 S. Nicholson
XLVI Carpo 2003 S. Sheppard
XXI Chaldene 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XLVIII Cyllene 2003 S. Sheppard
LIII Dia 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
VII Elara 1905 C. Perrine
XXV Erinome 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XXXIII Euanthe 2001 S. Sheppard, D. Jewitt, J. Kleyna
XLVII Eukelade 2003 S. Sheppard
XXXIV Euporie 2001 S. Sheppard, D. Jewitt, J. Kleyna
II Europa * 1610 Galileo
XXXII Eurydome 2001 S. Sheppard, D. Jewitt, J. Kleyna
III Ganymede * 1610 Galileo
XXII Harpalyke 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XXXIX Hegemone 2003 S. Sheppard
XLV Helike 2003 S. Sheppard
XXX Hermippe 2001 S. Sheppard, D. Jewitt, J. Kleyna
L Herse 2003 B. Gladman, J. Kavelaars, J.-M. Petit, L. Allen
VI Himalia 1904 C. Perrine
I Io * 1610 Galileo
XXIV Iocaste 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XXVI Isonoe 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XXXVII Kale 2001 S. Sheppard, D. Jewitt, J. Kleyna
XLIV Kallichore 2003 S. Sheppard
XXIII Kalyke 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XLIX Kore 2003 S. Sheppard, D. Jewitt, J. Kleyna
XIII Leda 1974 C. Kowal
X Lysithea 1938 S. Nicholson
XIX Megaclite 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XVI Metis 1979 Voyager Science Team
XL Mneme 2003 B. Gladman, L. Allen
XXXV Orthosie 2001 S. Sheppard, D. Jewitt, J. Kleyna
VIII Pasiphaë 1908 P. Melotte
XXXVIII Pasithee 2001 S. Sheppard, D. Jewitt, J. Kleyna
XXVII Praxidike 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
IX Sinope 1914 S. Nicholson
XXXVI Sponde 2001 S. Sheppard, D. Jewitt, J. Kleyna
XX Taygete 2000 S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier
XIV Thebe 1979 Voyager Science Team
XLII Thelxinoe 2003 S. Sheppard
XVIII Themisto 1975 C. Kowal, E. Roemer
(Lost and then rediscovered in 2000 by S. Sheppard, D. Jewitt, Y. Fernandez, G. Magnier, M. Holman, B. Marsden, G. Williams)
XXIX Thyone 2001 S. Sheppard, D. 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 S. Sheppard, D. Jewitt, J. Kleyna
XXVI Albiorix 2000 M. Holman
XLIX Anthe 2007 Cassini Imaging Science Team
XV Atlas 1980 Voyager Science Team
XXXVII Bebhionn 2004 S. Sheppard, D. Jewitt, J. Kleyna
XXXVIII Bergelmir 2004 S. Sheppard, D. Jewitt, J. Kleyna
XXXIX Bestla 2004 S. Sheppard, D. Jewitt, J. Kleyna
XIV Calypso 1980 D. Pascu, P. Seidelmann, W. Baum, D. Currie
XXXV Daphnis 2005 Cassini Imaging Science Team
IV Dione 1684 G. Cassini
II Enceladus 1789 W. Herschel
XI Epimetheus 1977 J. Fountain, S. Larson
(Confirmed in 1980 by D. Cruikshank)
XXVIII Erriapus 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XL Farbauti 2004 S. Sheppard, D. Jewitt, J. Kleyna
XLI Fenrir 2004 S. Sheppard, D. Jewitt, J. Kleyna
XLII Fornjot 2004 S. Sheppard, D. Jewitt, J. Kleyna
LI Greip 2006 S. Sheppard, D. Jewitt, J. Kleyna
XLIII Hati 2004 S. Sheppard, D. Jewitt, J. Kleyna
XII Helene 1980 P. Laques, J. Lecacheux
VII Hyperion 1848 W. Bond, G. Bond, W. Lassell
XLIV Hyrrokkin 2004 S. Sheppard, D. Jewitt, J. Kleyna
VIII Iapetus 1671 G. Cassini
XXII Ijiraq 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
X Janus 1966 A. Dollfus
(Confirmed in 1980 by D. Pascu)
L Jarnsaxa 2006 S. Sheppard, D. Jewitt, J. Kleyna
XLV Kari 2006 S. Sheppard, D. Jewitt, J. Kleyna
XXIV Kiviuq 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XLVI Loge 2006 S. Sheppard, D. Jewitt, J. Kleyna
XXXII Methone 2004 Cassini Imaging Science Team
I Mimas 1789 W. Herschel
XXV Mundilfari 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XXXI Narvi 2003 S. Sheppard, D. Jewitt, J. Kleyna
XX Paaliaq 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XXXIII Pallene 2004 Cassini Imaging Science Team
XVIII Pan 1990 M. Showalter
XVII Pandora 1980 Voyager Science Team
IX Phoebe 1898 W. Pickering
XXXIV Polydeuces 2004 Cassini Imaging Science Team
XVI Prometheus 1980 Voyager Science Team
V Rhea 1672 G. Cassini
XXIX Siarnaq 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XXVII Skathi 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XLVII Skoll 2006 S. Sheppard, D. Jewitt, J. Kleyna
XLVIII Surtur 2006 S. Sheppard, D. Jewitt, J. Kleyna
XXIII Suttungr 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
LII Tarqeq 2007 S. Sheppard, D. Jewitt, J. Kleyna
XXI Tarvos 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
XIII Telesto 1980 B. Smith, H. Reitsema, S. Larson, J. Fountain
III Tethys 1684 G. Cassini
XXX Thrymr 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
VI Titan 1655 C. Huygens
XIX Ymir 2000 B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, M. Holman, B. Marsden, P. Nicholson, J. Burns
Uranus I Ariel 1851 W. Lassell
XIV Belinda 1986 Voyager Science Team
VIII Bianca 1986 Voyager Science Team
XVI Caliban 1997 B. Gladman, P. Nicholson, J. Burns, J. Kavelaars
VI Cordelia 1986 Voyager Science Team
IX Cressida 1986 Voyager Science Team
XXVII Cupid 2003 M. Showalter, J. Lissauer
X Desdemona 1986 Voyager Science Team
XXIV Ferdinand 2001 D. Milisavljevic, M. Holman, J. Kavelaars, T. Grav
XXII Francisco 2001 J. Kavelaars, M. Holman, D. Milisavljevic, T. Grav
XI Juliet 1986 Voyager Science Team
XXVI Mab 2003 M. Showalter, J. Lissauer
XXIII Margaret 2003 S. Sheppard, D. Jewitt
V Miranda 1948 G. Kuiper
IV Oberon 1787 W. Herschel
VII Ophelia 1986 Voyager Science Team
XXV Perdita 1986 E. Karkoschka
XII Portia 1986 Voyager Science Team
XVIII Prospero 1999 M. Holman, 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. Kavelaars, B. Gladman, M. Holman, J.-M. Petit, H. Scholl
XX Stephano 1999 B. Gladman, M. Holman, J. Kavelaars, J.-M. Petit, H. Scholl
XVII Sycorax 1997 P. Nicholson, B. Gladman, J. Burns, J. Kavelaars
III Titania 1787 W. Herschel
XXI Trinculo 2001 M. Holman, J. Kavelaars, D. Milisavljevic
II Umbriel 1851 W. Lassell
Neptune V Despina 1989 Voyager Science Team
VI Galatea 1989 Voyager Science Team
IX Halimede 2002 M. Holman, J. Kavelaars, T. Grav, W. Fraser, D. Milisavljevic
VII Larissa 1989 Voyager Science Team
(After the Voyager discovery, it was found that H. Reitsema, W. Hubbard, L. Lebofsky and D. Tholen had observed this satellite during a 1981 occultation event)
XII Laomedeia 2002 J. Kavelaars, M. Holman, T. Grav, W. Fraser, D. Milisavljevic
III Naiad 1989 Voyager Science Team
II Nereid 1949 G. Kuiper
XIII Neso 2002 M. Holman, J. Kavelaars, T. Grav, W. Fraser, D. Milisavljevic
VIII Proteus 1989 Voyager Science Team
X Psamathe 2003 S. Sheppard, D. Jewitt, J. Kleyna
XI Sao 2002 T. Grav, M. Holman, J. Kavelaars, W. Fraser, D. Milisavljevic
IV Thalassa 1989 Voyager Science Team
I 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 Gazetteer of Planetary Nomenclature database.