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 Planetary status later rescinded, leaving them only as satellites.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

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. 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. https://books.google.com.my/books?id=AbZqDwAAQBAJ&pg=PA130&lpg=PA130&dq=mercury+venus+earth+mars+ceres+pallas+juno+vesta+astraea+hebe+iris&source=bl&ots=ZAdAaM2ueu&sig=ACfU3U1EGC0geGEFcL1KbtH8WeEaOTlC6Q&hl=en&sa=X&ved=2ahUKEwjr-qi-n77mAhWy9nMBHZzTA3EQ6AEwDXoECA8QAQ
  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. 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.
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