List of former planets

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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.

Contents

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 planetDiscoveryRemovalCurrent statusNotes
The Morning Star [NB 1] AntiquityAntiquityAspects of Venus Saturn, Moon, Venus and Mercury at dawn - 10 Dec. 2012.jpg "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] AntiquityAntiquity Winterabend im Dorf Cyriaxweimar in Elnhausen-Michelbacher Senke mit Venus am Nachthimmel, Radroute von Marburg-Stadtwald 2016-12-30.jpg
Apollo [NB 2] AntiquityAntiquityAspect of Mercury Merkur04122019.png 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 symbol (fixed width).svg Sun Antiquity1543 Star Sun white.jpg 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 decrescent symbol (fixed width).svg Moon Antiquity1543 Moon of Earth FullMoon2010.jpg 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 16101700s Moons of Jupiter Io highest resolution true color.jpg 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 16101700s Europa in natural color.png
Ganymede 16101700s Ganymede - Perijove 34 Composite.png
Callisto 16101700s Callisto - July 8 1979 (38926064465).jpg
Titan 16561700s Moons of Saturn Lakes Through the Haze.jpg 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 16711700s Japeto es una luna de saturno.jpg

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

Rhea 16721700s PIA07763 Rhea full globe5.jpg
Tethys 16841700s PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg

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

Dione 16841700s Dione in natural light.jpg
Titania 17871700s Moons of Uranus Titania (moon) color, edited.jpg Originally presented as satellite planets orbiting the planet Uranus. [15] Planetary status later rescinded, leaving them only as satellites.
Oberon 17871700s Voyager 2 picture of Oberon.jpg
Ceres symbol (fixed width).svg Ceres 18011867 Asteroid and dwarf planet Ceres - RC3 - Haulani Crater (22381131691) (cropped).jpg

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 symbol (fixed width).svg Pallas 18021867 Asteroid Potw1749a Pallas crop.png
Juno symbol (fixed width).svg Juno 18041867 Juno from Hooker telescope.jpg
Vesta symbol (original, fixed width).svg Vesta 18071867 Vesta in natural color (cropped).jpg
Astraea symbol (fixed width).svg Astraea 18451867
Hebe symbol (fixed width).svg Hebe 18471867 6hebe.png
Iris symbol (fixed width).svg Iris 18471867 Iris asteroid eso.jpg
Chiron symbol (fixed width).svg Chiron 19771980 Centaur 2060 Chiron Hubble.jpg 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 monogram (fixed width).svg
Pluto symbol (large orb, fixed width).svg 		Pluto 19302006 Dwarf planet Pluto in True Color - High-Res.jpg 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 19782006 Moon of Pluto Charon in True Color - High-Res.jpg 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 1992unknown Trans-Neptunian object 1992 QB1 crop.jpg 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 1992unknown [28]
Eris symbol (fixed width).svg Eris 20052006 Dwarf planet Eris and dysnomia-cut.jpg 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

Notes

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

Related Research Articles

<span class="mw-page-title-main">Kuiper belt</span> Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

<span class="mw-page-title-main">Planet</span> Large, round non-stellar astronomical object

A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.

<span class="mw-page-title-main">Planets beyond Neptune</span> Hypothetical planets further than Neptune

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

<span class="mw-page-title-main">2060 Chiron</span> Large, ringed 200km centaur/comet with 50-year orbit

2060 Chiron is a ringed small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orbiting between the asteroid belt and the Kuiper belt. Chiron is named after the centaur Chiron in Greek mythology.

5145 Pholus is an eccentric centaur in the outer Solar System, approximately 180 kilometers in diameter, that crosses the orbit of both Saturn and Neptune. It was discovered on 9 January 1992 by American astronomer David Rabinowitz (uncredited) of UA's Spacewatch survey at the Kitt Peak National Observatory in Arizona, United States. The very reddish object has an elongated shape and a rotation period of 9.98 hours. It was named after the centaur Pholus from Greek mythology.

<span class="mw-page-title-main">Minor-planet moon</span> Natural satellite of a minor planet

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of January 2022, there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights into their physical properties that are generally not otherwise accessible.

<span class="mw-page-title-main">Orcus (dwarf planet)</span> Trans-Neptunian dwarf planet

Orcus is a dwarf planet located in the Kuiper belt, with one large moon, Vanth. It has an estimated diameter of 870 to 960 km, comparable to the Inner Solar System dwarf planet Ceres. The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color, and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.

<span class="mw-page-title-main">15760 Albion</span> Trans-Neptunian object, prototype of cubewanos

15760 Albion (provisional designation 1992 QB1) was the first trans-Neptunian object to be discovered after Pluto and Charon. Measuring about 108–167 kilometres in diameter, it was discovered in 1992 by David C. Jewitt and Jane X. Luu at the Mauna Kea Observatory, Hawaii. After the discovery, they dubbed the object "Smiley" and it was shortly hailed as the tenth planet by the press. It is a "cold" classical Kuiper belt object and gave rise to the name cubewano for this kind of object, after the QB1 portion of its designation. Decoding its provisional designation, "QB1" reveals that it was the 27th object found in the second half of August of that year. As of January 2018, around 2,400 further objects have been found beyond Neptune, a majority of which are classical Kuiper belt objects. It was named after Albion from William Blake's mythology.

In ancient times, only the Sun and Moon, a few stars, and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects, and where relevant, features of those objects.

The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται, 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.

<span class="mw-page-title-main">Makemake</span> Dwarf planet in the Outer Solar System

Makemake is a dwarf planet and the largest of what is known as the classical population of Kuiper belt objects, with a diameter approximately that of Saturn's moon Iapetus, or 60% that of Pluto. It has one known satellite. Its extremely low average temperature, about 40 K (−230 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices. Makemake shows signs of geothermal activity and thus may be capable of supporting active geology and harboring an active subsurface ocean.

<span class="mw-page-title-main">38628 Huya</span> Trans-Neptunian object

38628 Huya ( hoo-YAH; provisional designation 2000 EB173) is a binary trans-Neptunian object located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Huya is classified as a plutino, a dynamical class of trans-Neptunian objects with orbits in a 3:2 orbital resonance with Neptune. It was discovered by the Quasar Equatorial Survey Team and was identified by Venezuelan astronomer Ignacio Ferrín in March 2000. It is named after Juyá, the mythological rain god of the Wayuu people native to South America.

<span class="mw-page-title-main">Dwarf planet</span> Small planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.

IAU definition of <i>planet</i> 2006 International Astronomical Union definition

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium, and
  3. has "cleared the neighbourhood" around its orbit.
<span class="mw-page-title-main">Eris (dwarf planet)</span> Dwarf planet beyond Pluto in the Solar System

Eris is the most massive and second-largest known dwarf planet in the Solar System. It is a trans-Neptunian object (TNO) in the scattered disk and has a high-eccentricity orbit. Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown and verified later that year. It was named in September 2006 after the Greco–Roman goddess of strife and discord. Eris is the ninth-most massive known object orbiting the Sun and the sixteenth-most massive overall in the Solar System. It is also the largest known object in the solar system that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometres (1,445 ± 7 mi) in diameter; its mass is 0.28% that of the Earth and 27% greater than that of Pluto, although Pluto is slightly larger by volume. Both Eris and Pluto have a surface area that is comparable to that of Russia or South America.

<span class="mw-page-title-main">Small Solar System body</span> Object in the Solar System

A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies'".

<span class="mw-page-title-main">Planetary-mass moon</span> Planetary-mass bodies that are also natural satellites

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton – are larger and more massive than the dwarf planets Pluto and Eris.

References

  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. 1 2 Elizabeth Howell (23 December 2015). "What Is The Difference Between the Geocentric and Heliocentric Models of the Solar System?". Universe Today.
  6. 1 2 3 4 5 6 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. 1 2 3 4 5 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.
  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. 1 2 3 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. 1 2 Robert Roy Britt (24 August 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". SPACE.com.
  25. 1 2 3 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. 1 2 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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