List of former planets

This is a list of astronomical objects formerly widely considered planets under any of the various definitions of this word in astronomy. As of 2016, there are 8 official planets of the Solar System, and many more exoplanets. Several objects formerly considered exoplanets have been found to actually be stars or brown dwarfs. As the definition of planet has evolved, the de facto and de jure definitions of planet have changed over the millennia.

Background

Throughout antiquity, there have been many Classical Planets, once "wandering stars", not all of which are now considered planets. With the advent of the telescope, the moons initially discovered around Jupiter and Saturn, were also considered planets by some. The development of more powerful telescopes resulted in the discovery of the asteroids, the first many of which were initially considered planets. Then Pluto was discovered, the first Trans-Neptunian Object. When electronic imaging came about, Trans-Neptunian Objects of the Kuiper Belt were found, and then Eris, widely hailed as the "new planet", was discovered, which prompted the 2006 round of recategorization of what is a planet.

List

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).	[1][2]
	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 dedicated to Apollo by the Greeks when it was visible during daytime. Eventually in the 4th century BC, Mercury and Apollo were found to be one and the same.	[3]
	Sun	Antiquity	1700s	Star		Following the acceptance of the Copernican model, the Sun was recognized as not being a planet, as it was the center, and did not orbit the center.	[4][5][6]
	Moon	Antiquity	1700s	Moon of Earth		Following the acceptance of the Copernican model, the Moon was recognized as not being a planet, as it orbited the Earth, and did not orbit the center, the Sun.	[4][5][6]
	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 Planetary status later rescinded, leaving them only as satellites.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 more voluminous than Mercury, but is about half as massive.	[7][8][9] [10][5][6]
	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 more voluminous than Mercury, but less massive.	[11][5][6]
	Iapetus	1671	1700s				[12][10] [5][6]
	Rhea	1672	1700s
	Tethys	1684	1700s				[13][10] [5][6]
	Dione	1684	1700s
	Titania	1787	1700s	Moons of Uranus		Originally presented as satellite planets orbiting the planet Uranus. 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, as they directly orbited the Sun. By 1855 the number of known bodies in the asteroid belt grew to 15, at which point astronomers started distinguishing between these from the known seven major planets. This went on until the 1867 edition of Berliner Astronomisches Jahrbuch which listed all the new bodies in the asteroid belt, including the first four ones, into a separate category as 'minor planets' or 'asteroids'—by which point almost 100 asteroids had been observed.[14]	[15][16]
	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 other objects, asteroids and comets, known at the time, and was classified independently uniquely at that time. Later, it was called an asteroid and then exhibited characteristics of a comet, leading to multiple classifications. Later it was placed into its own category of centaurs.	[17][18][19] [20][21]
	Pluto	1930	2006	Dwarf planet		Following the discovery of Eris, the International Astronomical Union met to hammer out a definition of planet, as Eris being larger than Pluto, pressured the IAU into making a formal statement on the matter. Either there were to be many planets, or the decision made at the time the asteroids were removed would be repeated. Pluto and Eris, and similar bodies were lumped together like the asteroids before them, and removed, being as representatives of a large group of smaller objects. Pluto would be listed as a "dwarf planet" from the reshuffle.	[22][23]
	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 removed the possibility of double planets being within the definition.	[22][23][24]
	15760 Albion	1992	unknown	Trans-Neptunian object		When discovered, these bodies were briefly hailed as the tenth and eleventh planets by the press, before being decided that 15760 Albion was the prototype of trans-Neptunian objects or cubewanos.	[25][26]
	(181708) 1993 FW	1992	unknown				[26]
	Eris	2005	2006	Dwarf planet		The discovery of Eris, hailed worldwide by the press as the 10th planet, prompted the International Astronomical Union to meet to hammer out a definition of planet. Its discovery pointed to a future of many more similar discoveries, forming an analogue to the time when asteroids were first discovered. In a similar fashion, all such objects were again lumped together, out of the planet category. Eris would be listed as a "dwarf planet" from the reshuffle.	[23][27]

See also

• List of gravitationally rounded objects of the Solar System
• List of possible dwarf planets
• List of hypothetical Solar System objects

Notes

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

References

1. Leonard Linsky (October 1959). "Hesperus and Phosphorus". The Philosophical Review. 68 (4): 515–518. doi:10.2307/2182495. JSTOR 2182495.
2. "The Great Cosmic Light Called Hesperus, Brother Of Lucifer, Reaches Greatest Brilliancy". Star Gazer. Episode 09-07 (1628). February 2009.
3. https://www.space.com/29265-mercury-planet-facts-for-skywatchers.html
4. Elizabeth Howell (23 December 2015). "What Is The Difference Between the Geocentric and Heliocentric Models of the Solar System?". Universe Today.
5. Holli Riebeek (7 July 2009). "Planetary Motion: The History of an Idea That Launched the Scientific Revolution". The Science: Orbital Mechanics. NASA Earth Observatory.
6. 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.
7. Galileo Galilei (1610). "Sidereus Nuncius" (in Latin). Cite journal requires |journal= (help)
8. Al Van Helden (1995). "Satellites of Jupiter". The Galileo Project. Rice University.
9. Calvin J. Hamilton (2009). "The Discovery of the Galilean Satellites". Views of the Solar System.
10. Jean-Pierre Luminet (31 December 2016). "Montaigne, Peiresc, Gassendi, and Cassini - The Provençal Humanists and Copernicus". Inference: International Review of Science. 2 (4).
11. Cristiani Hugenii (Christiaan Huygens) (1659). "Systema Saturnium" (in Latin). Cite journal requires |journal= (help)
12. Jean-Dominique Cassini (Giovanni Domenico Cassini) (1673). "Découverte de deux nouvelles planètes autour de Saturne" (in French). Paris Observatory. Cite journal requires |journal= (help)
13. 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.
14. https://web.archive.org/web/20120406222551/http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/minor-planets/
15. 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.
16. Benamran, Bruce (4 September 2018). How to Speak Science: Gravity, Relativity, and Other Ideas That Were Crazy Until Proven Brilliant. ISBN 9781615194032.
17. Hodgson, Richard G. (March 1978). "The Discovery of Chiron: Some Reflections". The Minor Planet Bulletin. IAU MPC. 5 (3): 21–22. Bibcode:1978MPBu....5...21H.
18. "Chiron and the Centaurs". Emerald Visions. Mystic Visions Spiritual Astrology. 2004.
19. Barbara Hand Clow (1987). Chiron: Rainbow Bridge Between the Inner and Outer Planets. Llewellyn. ISBN 087542094X.
20. Richard Nolle (1983). Chiron: The New Planet in Your Horoscope, The Key to Your Quest. American Federation of Astrologers. ISBN 0866902368.
21. "In Greenwich". Popular Mechanics. Vol. 158 no. 1. January 1982. p. 28.
22. Robert Roy Britt (24 August 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". SPACE.com.
23. Mike Wall (19 November 2010). "The Man Who Killed Pluto: Q & A with Astronomer Mike Brown". SPACE.com.
24. E. Mostra (1998). "Pluto and Charon". Voyage in the Universe. Astronomical Observatory of Padua.
25. 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.
26. Coote, Roger. / (August 2008). The earth. London. ISBN 9781842399491. OCLC 671197414.
27. David Whitehouse (30 July 2005). "Astronomers detect '10th planet'". BBC News.