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This is a list of astronomical objects formerly widely considered planets under any of the various definitions of this word in the history of astronomy. As the definition of planet has evolved, the de facto and de jure definitions of planet have changed over the millennia. As of 2024, there are eight official planets in the Solar System per the International Astronomical Union (IAU),[1] which has also established a definition for exoplanets. Several objects formerly considered exoplanets have been found actually to be stars or brown dwarfs.

Background

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Throughout antiquity, several astronomical objects were considered Classical Planets, meaning "wandering stars", not all of which are now considered planets. The moons discovered around Jupiter, Saturn and Uranus after the advent of the telescope were also initially considered planets by some. The development of more powerful telescopes resulted in the discovery of the asteroids, which were initially considered planets. Then Pluto, the first Trans-Neptunian Object, was discovered. More Trans-Neptunian Objects of the Kuiper Belt were found with the help of electronic imaging. One of these, Eris, was widely hailed as a "new planet", which prompted the 2006 recategorization of solar system bodies.

Some planetary scientists reject the 2006 definition of planet, and thus would still consider some of the objects on this list to be planets under a geophysical definition. See the list of gravitationally rounded objects of the Solar System for a list of geophysical planets.

List

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Former planets of the Solar System
Former planet Discovery Removal Current status Notes
The Morning Star[NB 1] Antiquity Antiquity Aspects of Venus "Phosphorus", the Morning Star of Greek antiquity (Eosphorus, the Dawn-Bringer; called "Lucifer" by the Romans), and "Hesperus", the Evening Star (called "Vesper" by the Romans), were later identified as a single planet, Venus (Aphrodite).

[2][3]

The Evening Star[NB 1] Antiquity Antiquity
Apollo[NB 2] Antiquity Antiquity Aspect of Mercury Like the Morning and Evening Stars, Mercury was deemed to be a distinct planet when it was visible during daytime, and dedicated to Apollo by the Greeks. Eventually, in the 4th century BC, Mercury and Apollo were found to be one and the same. [4]
☉ Sun Antiquity 1543 Star In antiquity, it was believed that the Sun and all the planets orbit the Earth. Thus the Sun was categorised as a planet. Following the acceptance of the Copernican model, it was recognized that the planets (including Earth) orbit the Sun, and it was no longer regarded as a planet. Subsequent discoveries show that the Sun is a star.

[5][6][7]

☾ Moon Antiquity 1543 Moon of Earth Following the acceptance of the Copernican model, planets were defined as objects which orbit the Sun. Since the Moon can be said to orbit the Earth, it was no longer regarded as a planet, but this is debated; see double planet.

[5][6][7]

Io 1610 1700s Moons of Jupiter Originally presented as satellite planets orbiting the planet Jupiter. Planetary status later rescinded, leaving them only as satellites. Ganymede is the largest satellite in the Solar System, and is slightly larger than Mercury, but is about half as massive.

[8][9][10] [11][6][7]

Europa 1610 1700s
Ganymede 1610 1700s
Callisto 1610 1700s
Titan 1656 1700s Moons of Saturn Originally presented as satellite planets orbiting the planet Saturn. Planetary status later rescinded, leaving them only as satellites. Titan is the second largest satellite in the Solar System, and is slightly larger than Mercury, but less massive. [12][6][7]
Iapetus 1671 1700s

[13][11] [6][7]

Rhea 1672 1700s
Tethys 1684 1700s

[14][11] [6][7]

Dione 1684 1700s
Titania 1787 1700s Moons of Uranus Originally presented as satellite planets orbiting the planet Uranus.[15] Planetary status later rescinded, leaving them only as satellites.
Oberon 1787 1700s
⚳ Ceres 1801 1867 Asteroid and dwarf planet

The first asteroids to be discovered were accepted as planets in the Copernican system, since they directly orbited the Sun. By 1855 the number of known bodies in the asteroid belt had grown to 15, at which point astronomers started distinguishing these from the eight known major planets. The 1867 edition of Berliner Astronomisches Jahrbuch placed all the new bodies in the asteroid belt into a separate category as 'minor planets' or 'asteroids', by which point almost 100 asteroids had been observed.[16]

[17][18]

⚴ Pallas 1802 1867 Asteroid
⚵ Juno 1804 1867
Vesta 1807 1867
Astraea 1845 1867
Hebe 1847 1867
Iris 1847 1867
⚷ Chiron 1977 1980 Centaur The discovery of Chiron was hailed by the press and astrologers as that of a new planet. Astronomically, it was different from any other planets, asteroids and comets known at the time, and it was classified as unique at that time. Later it was called an asteroid, and then was found to exhibit characteristics of a comet, leading to multiple classifications. Later it was placed into its own category of centaurs, and many other centaurs have been discovered subsequently.

[19][20][21] [22][23]

♇
⯓ 		Pluto 1930 2006 Dwarf planet Following its discovery in 1930, Pluto was considered the ninth planet. Numerous scientific discoveries in the 1990s and early 2000s placed doubt on this classification, and after the discovery of Eris, which was thought to be larger than Pluto, the International Astronomical Union met to determine a definition of a planet. Like the asteroids before them, Pluto was grouped with Eris and similar bodies as members of a group of smaller objects designated as dwarf planets.

[24][25]

Charon 1978 2006 Moon of Pluto When discovered, Charon, the moon of Pluto, was found to be very large, leading to the declaration by many that the Pluto-Charon system was a double planet (binary planet). The 2006 IAU redefinition of planet excludes the possibility of double planets.

[24][25][26]

15760 Albion 1992 unknown Trans-Neptunian object When discovered, these bodies were briefly hailed as the tenth and eleventh planets by the press, but it was then decided that 15760 Albion was the prototype of trans-Neptunian objects or cubewanos. [27][28]
(181708) 1993 FW 1992 unknown [28]
⯰ Eris 2005 2006 Dwarf planet The discovery of Eris, hailed worldwide by the press as the tenth planet, prompted the International Astronomical Union to meet and establish a new definition of planet. It was recategorised as a dwarf planet, together with Pluto and numerous other objects.

[25][29]

See also

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Notes

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  1. ^ a b It is an aspect of the planet Venus.
  2. ^ It is an aspect of the planet Mercury.

References

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  1. ^ IAU (August 24, 2006). "Definition of a Planet in the Solar System: Resolutions 5 and 6" (PDF). IAU 2006 General Assembly. International Astronomical Union. Retrieved January 26, 2008.
  2. ^ Leonard Linsky (October 1959). "Hesperus and Phosphorus". The Philosophical Review. 68 (4): 515–518. doi:10.2307/2182495. JSTOR 2182495.
  3. ^ "The Great Cosmic Light Called Hesperus, Brother Of Lucifer, Reaches Greatest Brilliancy". Star Gazer. Episode 09-07 (1628). February 2009.
  4. ^ "Planet Mercury: Some Surprising Facts for Skywatchers". Space.com. 29 April 2015.
  5. ^ a b Elizabeth Howell (23 December 2015). "What Is The Difference Between the Geocentric and Heliocentric Models of the Solar System?". Universe Today.
  6. ^ a b c d e f Holli Riebeek (7 July 2009). "Planetary Motion: The History of an Idea That Launched the Scientific Revolution". The Science: Orbital Mechanics. NASA Earth Observatory.
  7. ^ a b c d e f Eric G. Blackman (2006). "The Copernican Model: A Sun-Centered Solar System". Astronomy 104 – The Solar System. University of Rochester, School of Arts and Sciences, Department of Physics and Astronomy.
  8. ^ Galileo Galilei (1610). "Sidereus Nuncius" (in Latin). {{cite journal}}: Cite journal requires |journal= (help)
  9. ^ Al Van Helden (1995). "Satellites of Jupiter". The Galileo Project. Rice University.
  10. ^ Calvin J. Hamilton (2009). "The Discovery of the Galilean Satellites". Views of the Solar System.
  11. ^ a b c Jean-Pierre Luminet (31 December 2016). "Montaigne, Peiresc, Gassendi, and Cassini - The Provençal Humanists and Copernicus". Inference: International Review of Science. 2 (4).
  12. ^ Cristiani Hugenii (Christiaan Huygens) (1659). "Systema Saturnium" (in Latin). {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ Jean-Dominique Cassini (Giovanni Domenico Cassini) (1673). "Découverte de deux nouvelles planètes autour de Saturne" (in French). Paris Observatory. {{cite journal}}: Cite journal requires |journal= (help)
  14. ^ Jean-Dominique Cassini (Giovanni Domenico Cassini) (1686–1692). "An Extract of the Journal Des Scavans. Of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris". Philosophical Transactions of the Royal Society of London. 16 (179–191): 79–85. Bibcode:1686RSPT...16...79C. doi:10.1098/rstl.1686.0013. JSTOR 101844.
  15. ^ Herschel, W. S. (1787). "An Account of the Discovery of Two Satellites Revolving Round the Georgian Planet". Philosophical Transactions of the Royal Society of London. 77: 125–129. doi:10.1098/rstl.1787.0016. JSTOR 106717. And the heavens now displayed the original of my drawing, by shewing, in the situation I had delineated them, The Georgian Planet attended by two satellites.

    I confess that this scene appeared to me with additional beauty, as the little secondary planets seemed to give a dignity to the primary one, which raises it into a more conspicuous situation among the great bodies of our solar system.
  16. ^ "When did the asteroids become minor planets? – Naval Oceanography Portal". Archived from the original on 2012-04-06.
  17. ^ James L. Hilton (18 April 2016). "When did the asteroids become minor planets?". Astronomical Applications Department. U.S. Naval Observatory. Archived from the original on 24 March 2008.
  18. ^ Benamran, Bruce (2018). How to Speak Science: Gravity, Relativity, and Other Ideas That Were Crazy Until Proven Brilliant. The Experiment. ISBN 9781615194032.
  19. ^ Hodgson, Richard G. (March 1978). "The Discovery of Chiron: Some Reflections". The Minor Planet Bulletin. 5 (3). IAU MPC: 21–22. Bibcode:1978MPBu....5...21H.
  20. ^ "Chiron and the Centaurs". Emerald Visions. Mystic Visions Spiritual Astrology. 2004.
  21. ^ Barbara Hand Clow (1987). Chiron: Rainbow Bridge Between the Inner and Outer Planets. Llewellyn. ISBN 087542094X.
  22. ^ Richard Nolle (1983). Chiron: The New Planet in Your Horoscope, The Key to Your Quest. American Federation of Astrologers. ISBN 0866902368.
  23. ^ "In Greenwich". Popular Mechanics. Vol. 158, no. 1. January 1982. p. 28.
  24. ^ a b Robert Roy Britt (24 August 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". SPACE.com.
  25. ^ a b c Mike Wall (19 November 2010). "The Man Who Killed Pluto: Q & A with Astronomer Mike Brown". Space.com.
  26. ^ E. Mostra (1998). "Pluto and Charon". Voyage in the Universe. Astronomical Observatory of Padua.
  27. ^ Collander-Brown, S.; Maran, M.; Williams, I. P. (2000-10-11). "The effect on the Edgeworth-Kuiper Belt of a large distant tenth planet". Monthly Notices of the Royal Astronomical Society. 318 (1): 101–108. Bibcode:2000MNRAS.318..101C. doi:10.1046/j.1365-8711.2000.03640.x. ISSN 0035-8711.
  28. ^ a b Coote, Roger. / (2008). The earth. London. ISBN 9781842399491. OCLC 671197414.{{cite book}}: CS1 maint: location missing publisher (link)
  29. ^ David Whitehouse (30 July 2005). "Astronomers detect '10th planet'". BBC News.
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