List of former planets

Short description: none

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 2022, there are 8 official planets in the Solar System by the IAU,[1] and many more exoplanets. Several objects formerly considered exoplanets have been found actually to be stars or brown dwarfs.

## Contents

* 1 Background
* 2 List
* 3 See also
* 4 Notes
* 5 References

## Background

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

Former planets of the Solar System  | Former planet  | Discovery | Removal | Current status  |  | Notes  |

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| Morning Star The Morning Star

[NB 1]

| -9999 Antiquity

| -400 Antiquity

| Aspect 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]

| Evening Star The Evening Star

[NB 1]

| -9999 Antiquity

| -400 Antiquity

| Apollo[NB 2] | -9999 Antiquity

| -300 Antiquity

| Aspect 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 | -9999 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 | -9999 Antiquity

| 1543  | Moon 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  | Moon 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  | Moon 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  | Moon Moons of Uranus

|  |
| 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.[15]

[16][17]

| Pallas | 1802  | 1867  | Asteroid |
| Juno | 1804  | 1867  |
| Vesta | 1807  | 1867  |
| Astraea | 1845  | 1867  |
| Hebe | 1847  | 1867  |
| Iris | 1847  | 1867  |
| Chiron | 1977  | 1980  | Centaur 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.  |

[18][19][20] [21][22]

| Pluto | 1930  | 2006  | Dwarf planet |  | Following its discovery in 1930, Pluto was widely regarded as the ninth planet. Numerous scientific discoveries cast doubt on this classification, and after the discovery of Eris, which was larger than Pluto, the International Astronomical Union met to hammer out a definition of planet. Like the asteroids before them, Pluto was grouped together with Eris and similar bodies as members of a group of smaller objects designated as dwarf planets.  |

[23][24]

| Charon | 1978  | 2006  | Moon 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.  |

[23][24][25]

| 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.  | [26][27]
| (181708) 1993 FW | 1992  | unknown  |  | [27]
| 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 hammer out a new definition of planet. It was recategorised as a dwarf planet, together with Pluto and numerous other objects.  |

[24][28]

## See also

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

## Notes

1. ↑ 1.0 1.1 It is an aspect of the planet Venus
2. ↑ It is an aspect of the planet Mercury

## References

1. ↑ IAU (August 24, 2006). "Definition of a Planet in the Solar System: Resolutions 5 and 6". IAU 2006 General Assembly (International Astronomical Union). http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf.
2. ↑ Leonard Linsky (October 1959). "Hesperus and Phosphorus". The Philosophical Review 68 (4): 515–518. doi:10.2307/2182495.
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". 29 April 2015. https://www.space.com/29265-mercury-planet-facts-for-skywatchers.html.
5. ↑ 5.0 5.1 Elizabeth Howell (23 December 2015). "What Is The Difference Between the Geocentric and Heliocentric Models of the Solar System?". Universe Today. http://www.universetoday.com/36487/difference-between-geocentric-and-heliocentric/.
6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Holli Riebeek (7 July 2009). "Planetary Motion: The History of an Idea That Launched the Scientific Revolution". The Science: Orbital Mechanics. NASA Earth Observatory. http://earthobservatory.nasa.gov/Features/OrbitsHistory/page1.php.
7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 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. http://www.pas.rochester.edu/~blackman/ast104/copernican9.html.
8. ↑ Galileo Galilei (1610) (in la). Sidereus Nuncius.
9. ↑ Al Van Helden (1995). "Satellites of Jupiter". The Galileo Project. Rice University. http://galileo.rice.edu/sci/observations/jupiter_satellites.html.
10. ↑ Calvin J. Hamilton (2009). "The Discovery of the Galilean Satellites". Views of the Solar System. http://solarviews.com/eng/galdisc.htm.
11. ↑ 11.0 11.1 11.2 Jean-Pierre Luminet (31 December 2016). "Montaigne, Peiresc, Gassendi, and Cassini - The Provençal Humanists and Copernicus". Inference: International Review of Science 2 (4). http://inference-review.com/article/the-provencal-humanists-and-copernicus.
12. ↑ Cristiani Hugenii (Christiaan Huygens) (1659) (in la). Systema Saturnium.
13. ↑ Jean-Dominique Cassini (Giovanni Domenico Cassini) (1673) (in fr). Découverte de deux nouvelles planètes autour de Saturne. Paris Observatory.
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. doi:10.1098/rstl.1686.0013. Bibcode: 1686RSPT...16...79C.
15. ↑ "When did the asteroids become minor planets? — Naval Oceanography Portal". http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/minor-planets/.
16. ↑ James L. Hilton (18 April 2016). "When did the asteroids become minor planets?". Astronomical Applications Department. U.S. Naval Observatory. http://aa.usno.navy.mil/faq/docs/minorplanets.php.
17. ↑ Benamran, Bruce (4 September 2018). How to Speak Science: Gravity, Relativity, and Other Ideas That Were Crazy Until Proven Brilliant. The Experiment. ISBN 9781615194032. https://books.google.com/books?id=AbZqDwAAQBAJ&q=mercury+venus+earth+mars+ceres+pallas+juno+vesta+astraea+hebe+iris&pg=PA130.
18. ↑ 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.
19. ↑ "Chiron and the Centaurs". Emerald Visions. Mystic Visions Spiritual Astrology. 2004. https://www.spiritualastrology.com/article15.html.
20. ↑ Barbara Hand Clow (1987). Chiron: Rainbow Bridge Between the Inner and Outer Planets. Llewellyn. ISBN 087542094X.
21. ↑ Richard Nolle (1983). Chiron: The New Planet in Your Horoscope, The Key to Your Quest. American Federation of Astrologers. ISBN 0866902368.
22. ↑ "In Greenwich". Popular Mechanics 158 (1): 28. January 1982.
23. ↑ 23.0 23.1 Robert Roy Britt (24 August 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". SPACE.com. http://www.space.com/2791-pluto-demoted-longer-planet-highly-controversial-definition.html.
24. ↑ 24.0 24.1 24.2 Mike Wall (19 November 2010). "The Man Who Killed Pluto: Q & A with Astronomer Mike Brown". SPACE.com. http://www.space.com/9563-man-killed-pluto-astronomer-mike-brown.html.
25. ↑ E. Mostra (1998). "Pluto and Charon". Voyage in the Universe. Astronomical Observatory of Padua. http://archive.oapd.inaf.it/E-MOSTRA/NEW/A2024PLU.HTM.
26. ↑ Collander-Brown, S.; Maran, M.; Williams, I. P. (2000-10-11). "The effect on the Edgeworth-Kuiper Belt of a large distant tenth planet" (in en). Monthly Notices of the Royal Astronomical Society 318 (1): 101–108. doi:10.1046/j.1365-8711.2000.03640.x. ISSN 0035-8711. Bibcode: 2000MNRAS.318..101C.
27. ↑ 27.0 27.1 Coote, Roger. / (August 2008). The earth. London. ISBN 9781842399491. OCLC 671197414. https://archive.org/details/earth0000coot.
28. ↑ David Whitehouse (30 July 2005). "Astronomers detect '10th planet'". BBC News. http://news.bbc.co.uk/2/hi/science/nature/4730061.stm.

* v
* t
* e

Solar System

* Sun
* Mercury
* Venus
* Earth
* Mars
* Ceres
* Jupiter
* Saturn
* Uranus
* Neptune
* Pluto
* Haumea
* Makemake
* Eris

| Planets|

* Terrestrial planets
* Mercury
* Venus
* Earth
* Mars

* Giant planets
* Jupiter
* Saturn
* Uranus
* Neptune

* Dwarf planets
* Ceres
* Pluto
* Haumea
* Makemake
* Eris

|

Rings|

* Jovian
* Saturnian (Rhean)
* Charikloan
* Chironean
* Uranian
* Neptunian
* Haumean

Moons|

* Terrestrial
* Moon
* other near-Earth objects
* Martian
* Phobos
* Deimos
* Jovian
* Ganymede
* Callisto
* Io
* Europa
* all 79
* Saturnian
* Titan
* Rhea
* Iapetus
* Dione
* Tethys
* Enceladus
* Mimas
* Hyperion
* Phoebe
* all 82
* Uranian
* Titania
* Oberon
* Umbriel
* Ariel
* Miranda
* all 27
* Neptunian
* Triton
* Proteus
* Nereid
* all 14
* Plutonian
* Charon
* Nix
* Hydra
* Kerberos
* Styx
* Eridian
* Dysnomia
* Haumean
* Hiʻiaka
* Namaka
* Makemakean
* S/2015 (136472) 1

Exploration
(outline)|

* Colonization
* Discovery
* astronomy
* historical models
* timeline
* Human spaceflight
* space stations
* list
* Space probes
* timeline
* list

* Mercury
* Venus
* Moon
* mining
* Mars
* Ceres
* Asteroids
* mining
* Comets
* Jupiter
* Saturn
* Uranus
* Neptune
* Pluto
* Deep space

Hypothetical
objects|

* Fifth giant
* Nemesis
* Phaeton
* Planet Nine
* Planet V
* Planet X
* Theia
* Tyche
* Vulcan
* Vulcanoids
* Subsatellites

Lists|

* Comets
* Dwarf planets (possible)
* Gravitationally rounded objects
* Minor planets
* names
* Natural satellites
* Solar System models
* Solar System objects
* by size
* by discovery date

| Small Solar
System bodies|

* Comets
* Damocloids
* Meteoroids
* Minor planets
* moons
* Planetesimal

* Mercury-crossers
* Venus-crossers
* Venus trojans
* Near-Earth objects
* Earth-crossers
* Earth trojans
* Mars-crossers
* Mars trojans

* Asteroid belt
* Asteroids
* Ceres
* Pallas
* Juno
* Vesta
* first 1000
* families
* exceptional
* Kirkwood gap
* Main-belt comets

* Jupiter-crossers
* Jupiter trojans
* Centaurs
* Saturn-crossers
* Uranus-crossers
* Uranus trojans
* Neptune-crossers
* Neptune trojans
* Cis-Neptunian objects
* Trans-Neptunian objects

* Plutoids
* Kuiper belt
* Cubewanos
* Plutinos
* Detached objects
* Hills cloud
* Oort cloud
* Scattered disc
* Sednoids

|

Formation
and evolution|

* Accretion
* Accretion disk
* Asteroid belt
* Circumplanetary disk
* Circumstellar disc
* Circumstellar envelope
* Cosmic dust
* Debris disk
* Detached object
* Disrupted planet
* Excretion disk
* Exoplanetary Circumstellar Environments and Disk Explorer
* Exozodiacal dust
* Extraterrestrial materials
* Extraterrestrial sample curation
* Giant-impact hypothesis
* Gravitational collapse
* Hills cloud
* Interplanetary dust cloud
* Interplanetary medium
* Interplanetary space
* Interstellar cloud
* Interstellar dust
* Interstellar medium
* Interstellar space
* Kuiper belt
* List of interstellar and circumstellar molecules
* Merging stars
* Molecular cloud
* Nebular hypothesis
* Oort cloud
* Outer space
* Planetary migration
* Planetary system
* Planetesimal
* Planet formation
* Protoplanetary disk
* Ring system
* Rubble pile
* Sample-return mission
* Scattered disc
* Star formation

* Outline of the Solar System
*   *   *

Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe
Each arrow (→) may be read as "within" or "part of".

* v
* t
* e

Exoplanetology

* Planet
* Definition
* IAU
* Planetary science

Main topics|

* Exoplanet
* Methods of detecting exoplanets
* Planetary system

Sizes
and types|

| Terrestrial|

* Carbon planet
* Coreless planet
* Desert planet
* Dwarf planet
* Ice planet
* Iron planet
* Lava planet
* Mega-Earth
* Ocean planet
* Sub-Earth
* Super-Earth

|

Gaseous|

* Eccentric Jupiter
* Gas dwarf
* Helium planet
* Hot Jupiter
* Hot Neptune
* Ice giant
* Mini-Neptune
* Super-Neptune
* Super-Jupiter

Other types|

* Brown dwarf
* Chthonian planet
* Circumbinary planet
* Disrupted planet
* Double planet
* Eyeball planet
* Mesoplanet
* Planemo
* Planet/Brown dwarf boundary
* Planetesimal
* Protoplanet
* Pulsar planet
* Sub-brown dwarf
* Ultra-cool dwarf
* Ultra-short period planets (USP)

Formation
and evolution|

* Accretion
* Accretion disk
* Asteroid belt
* Circumplanetary disk
* Circumstellar disc
* Circumstellar envelope
* Cosmic dust
* Debris disk
* Detached object
* Disrupted planet
* Excretion disk
* Exoplanetary Circumstellar Environments and Disk Explorer
* Exozodiacal dust
* Extraterrestrial materials
* Extraterrestrial sample curation
* Giant-impact hypothesis
* Gravitational collapse
* Hills cloud
* Interplanetary dust cloud
* Interplanetary medium
* Interplanetary space
* Interstellar cloud
* Interstellar dust
* Interstellar medium
* Interstellar space
* Kuiper belt
* List of interstellar and circumstellar molecules
* Merging stars
* Molecular cloud
* Nebular hypothesis
* Oort cloud
* Outer space
* Planetary migration
* Planetary system
* Planetesimal
* Planet formation
* Protoplanetary disk
* Ring system
* Rubble pile
* Sample-return mission
* Scattered disc
* Star formation

Systems|

* Exocomet
* Exomoon
* Interstellar comet
* Mean-motion resonances
* Retrograde planet
* Rogue planet
* Tidally detached exomoon
* Titius–Bode laws
* Trojan planet

Host stars|

* A
* B
* Binary star
* Brown dwarfs
* Extragalactic planet
* F/Yellow-white dwarfs
* G/Yellow dwarfs
* Herbig Ae/Be
* K/Orange dwarfs
* M/Red dwarfs
* Planets in globular clusters
* Pulsar
* Red giant
* Subdwarf B
* Subgiant
* T Tauri
* White dwarfs
* Yellow giants

Detection|

* Astrometry
* Direct imaging
* list
* Microlensing
* list
* Polarimetry
* Pulsar timing
* list
* Radial velocity
* list
* Transit method
* list
* Transit-timing variation

Habitability|

* Astrobiology
* Circumstellar habitable zone
* Earth analog
* Extraterrestrial liquid water
* Habitability of natural satellites
* Superhabitable planet

Catalogues|

* Catalog of Nearby Habitable Systems
* Exoplanet Data Explorer
* Extrasolar Planets Encyclopaedia
* NASA Exoplanet Archive
* NASA Star and Exoplanet Database

Lists|

* Exoplanetary systems
* Host stars
* Multiplanetary systems
* Stars with proplyds
* Exoplanets
* Lists of exoplanets
* Discoveries
* Extremes
* Firsts
* Nearest
* Largest
* Most massive
* Terrestrial candidates
* Kepler
* Potentially habitable
* Discovered exoplanets by year
* before 2000
* 2000–2009
* 2010
* 2011
* 2012
* 2013
* 2014
* 2015
* 2016
* 2017
* 2018
* 2019

Other|

* Carl Sagan Institute
* Exoplanet phase curves
* Nexus for Exoplanet System Science
* Planets in science fiction
* Sudarsky's gas giant classification

* Discoveries of exoplanets
* Search projects

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