Various unknown astronomical objects have been hypothesized throughout

recorded history Recorded history or written history describes the historical events that have been recorded in a written form or other documented communication which are subsequently evaluated by historians using the historical method. For broader world his ...

. For example, in the 5th century BCE, the philosopher

Philolaus Philolaus (; grc, Φιλόλαος, ''Philólaos''; ) was a Greek Pythagorean and pre-Socratic philosopher. He was born in a Greek colony in Italy and migrated to Greece. Philolaus has been called one of three most prominent figures in the Pyt ...

defined a hypothetical astronomical object which he called the "

Central Fire An astronomical system positing that the Earth, Moon, Sun, and planets revolve around an unseen "Central Fire" was developed in the fifth century BC and has been attributed to the Pythagorean philosopher Philolaus. The system has been called "the ...

", around which he proposed other celestial bodies (including the Sun) moved.Marco Ceccarelli, ''Distinguished Figures in Mechanism and Machine Science'' (2007), p. 124.

Types of hypothetical astronomical objects

Hypothetical astronomical objects have been speculated to exist both inside and outside of the

Solar System The Solar System Capitalization 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 ...

, and speculation has included different kinds of stars, planets, and other astronomical objects. * For hypothetical astronomical objects in the Solar System, see:

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 ...

* For hypothetical stars, see:

Hypothetical star A hypothetical star is a star, or type of star, that is speculated to exist but has yet to be definitively observed. Hypothetical types of stars have been conjectured to exist, have existed or will exist in the future universe. Types Scientific ...

* For hypothetical

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, see:

List of brown dwarfs This is a list of brown dwarfs. These are objects that have masses between heavy gas giants and low-mass stars. The first isolated brown dwarf discovered was Teide 1 in 1995. The first brown dwarf discovered orbiting a star was Gliese 229 B, al ...

* For hypothetical black holes, see: Hypothetical black hole * For extrasolar moons, all of which are currently hypothetical, see: Extrasolar moon * For stars, planets or moons whose existence is not accepted by science, see:

Planetary objects proposed in religion, astrology, ufology and pseudoscience There are a number of planetary objects proposed in religion, astrology, ufology and pseudoscience whose existence is not supported by scientific evidence. Lilith Lilith is a fictitious second moon of Earth, supposedly about the same mass as ...

and Stars proposed in religion * For hypothetical planets in fiction, see: Fictional planets of the Solar System

Hypothetical planet types

Hypothetical types of extrasolar planets include: , - , Iron planet , , A planet similar to Mercury that consists mainly of an iron core with little mantle. , - ,

Ocean planet An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surf ...

, , A planet whose surface is covered entirely by deep oceans. , - ,

Superhabitable planet A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong, who have criticized the l ...

, , A terrestrial planet more habitable than Earth. , - ,

Tidally detached exomoon Tidally detached exomoons, also known as orphaned exomoons or ploonets, are hypothetical exoplanets that were formerly exomoons of another planet, before being ejected from their orbits around their parent planets by tidal forces during planetary m ...

, , A planet that was originally a moon but has become gravitationally detached. , - ,

Toroidal planet A toroidal planet is a hypothetical type of terrestrial planet, telluric exoplanet with a Torus, toroidal or doughnut shape. While no firm theoretical understanding as to how toroidal planets could Planetary formation, form naturally is necessari ...

, , A planet that is shaped like a

torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanar with the circle. If the axis of revolution does not tou ...

or doughnut. , - ,

Trojan planet In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonan ...

, , A planet that orbits near the or

Lagrange points In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of t ...

of a more massive object. , - , Vitriolic planet , , A planet with significant amounts of acid.

References

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