Hypothetical astronomical object

{{Short description|Astronomical bodies believed or speculated to exist}}

File:Eclipsing binary star animation 2.gif, a possible eclipsing binary initially mistaken for a moon of Mercury]]

File:Carbon Planet.JPG. The surface is dark and reddish from hydrocarbon deposits.]]

File:Transiting planet HD 209458b.png, a possible Chthonian planet, transiting its star]]

Various unknown astronomical objects have been hypothesized throughout recorded history. For example, in the 5th century BCE, the philosopher Philolaus defined a hypothetical astronomical object which he called the "Central Fire", 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, 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
For hypothetical stars, see: Hypothetical star
For hypothetical brown dwarfs, see: List of brown dwarfs
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 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:

class="wikitable" ! Type ! Description
Ammonia planet {{citation needed\|date=May 2023}}	A planet with significant amounts of ammonia. May have lakes or oceans of ammonia.
Blanet	A planet that directly orbits a black hole.
Carbon planet	A terrestrial planet composed primarily of carbon, rather than silicon.
Chthonian planet	A hot Jupiter whose outer layers have been completely stripped off by its parent star.
{{Anchor\|Chlorine planet}}Chlorine planet	A planet with significant amounts of free chlorine or hydrochloric acid.{{Cite journal \|last=Haas \|first=Johnson R. \|date=November 2010 \|title=The potential feasibility of chlorinic photosynthesis on exoplanets \|url=https://pubmed.ncbi.nlm.nih.gov/21118026/ \|journal=Astrobiology \|volume=10 \|issue=9 \|pages=953–963 \|doi=10.1089/ast.2009.0364 \|issn=1557-8070 \|pmid=21118026\|bibcode=2010AsBio..10..953H }}
Coreless planet	A terrestrial planet that has no metallic core.
Desert planet	A terrestrial planet with little to no water.
Extragalactic planet \|A planet that is located outside the Milky Way galaxy
Eyeball planet	A tidally locked planet where uneven heating of the surface induces spatial features resembling a human eye.
Helium planet	A gas giant composed mainly of helium instead of hydrogen.
Hycean planet {{IPAc-en\|ˈ\|h\|aɪ\|ʃ\|ən}}	A hot, water-covered planet with a hydrogen-rich atmosphere that is possibly capable of harboring extremophilic life.{{cite journal \|last1=Madhusudhan \|first1=Nikku \|last2=Piette \|first2=Anjali a. A. \|last3=Constantinou \|first3=Savvas \|title=Habitability and Biosignatures of Hycean Worlds \|date=26 August 2021 \|journal=The Astrophysical Journal \|volume=918 \|number=1 \|page=1 \|doi=10.3847/1538-4357/abfd9c \|arxiv=2108.10888 \|bibcode=2021ApJ...918....1M \|s2cid=237290118 \|doi-access=free }}{{Cite web \|last=Anderson \|first=Paul Scott \|title=Hycean planets might be habitable ocean worlds \|url=https://earthsky.org/space/hycean-planets-exoplanets-habitability/ \|date=29 August 2021 \|work=Earth & Sky \|language=en-US} \|access-date=12 December 2021 }}{{cite news \|last=Davis \|first=Nicola \|title='Mini-Neptunes' beyond solar system may soon yield signs of life – Cambridge astronomers identify new hycean class of habitable exoplanets, which could accelerate search for life \|url=https://www.theguardian.com/science/2021/aug/26/mini-neptune-beyond-solar-system-may-soon-yield-sign-life-hycean-exoplanet-cambridge-astronomer \|date=25 August 2021 \|work=The Guardian \|accessdate=12 December 2021 }}
Ocean planet	A planet whose surface is covered entirely by deep oceans.
Superhabitable planet	A terrestrial planet that is more habitable than Earth.
Tidally detached exomoon	A planet that was originally a moon but has become gravitationally detached.
Toroidal planet	A planet whose shape resembles a torus or doughnut.
Trojan planet	A planet that orbits near the {{L4}} or {{L5}} Lagrange points of a more massive object.

Hypothetical astronomical object

Types of hypothetical astronomical objects

Hypothetical planet types

References