This is a list of
astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
s formerly widely considered ''
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. 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 you ...
s'' under any of the various definitions of this word in the history of astronomy. As the
definition of ''planet'' has evolved, the
de facto
''De facto'' ( ; , "in fact") describes practices that exist in reality, whether or not they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with ''de jure'' ("by la ...
and
de jure
In law and government, ''de jure'' ( ; , "by law") describes practices that are legally recognized, regardless of whether the practice exists in reality. In contrast, ("in fact") describes situations that exist in reality, even if not legally ...
definitions of
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. 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 you ...
have changed over the millennia. As of 2022, there are
8 official planets in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
by the
IAU
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
,
and many more
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s. Several objects formerly considered exoplanets have been found actually to be
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s or
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s.
Background
Throughout antiquity, several astronomical objects were considered
Classical Planet
In classical antiquity, the seven classical planets or seven luminaries are the seven moving astronomical objects in the sky visible to the naked eye: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The word ''planet'' comes fro ...
s, meaning "wandering stars", not all of which are now considered planets. The
moons discovered around
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
,
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
and
Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
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
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s, which were initially considered planets. Then
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, the first
Trans-Neptunian Object, was discovered. More Trans-Neptunian Objects of the
Kuiper Belt
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 ...
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
See also
*
List of gravitationally rounded objects of the Solar System
*
List of possible dwarf planets
The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond.
However, consideration of the surprisingly low densities of many large trans-Neptunian object ...
*
List of hypothetical Solar System objects
A hypothetical Solar System object is a planet, natural satellite, subsatellite or similar body in the Solar System whose existence is not known, but has been inferred from observational scientific evidence. Over the years a number of hypothetic ...
Notes
References
{{Portal bar, Stars, Spaceflight, Outer space, Science
Former planets
Planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. 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 ...