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.
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.
|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).|
|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.|||
|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.|
|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.|
|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 Planetary status later rescinded, leaving them only as satellites.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.|
|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.|||
|Titania||1787||1700s||Moons of Uranus||Originally presented as satellite planets orbiting the planet Uranus. Planetary status later rescinded, leaving them only as satellites.|
|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.
|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.|
|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.|
|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.|
|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.|||
|(181708) 1993 FW||1992||unknown|||
|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.|
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.