Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. This field developed after the discovery of sub-surface oceans in Saturn's moon Titan and Jupiter's moon Europa during the Voyager missions. The Cassini mission observed surface lakes of liquid methane on Titan, and directly sampled a plume of sub-surface ocean water from Enceladus.

Early in their geologic histories, Mars and Venus are theorized to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, and a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia, when dissolved in water, will lower water's freezing point, so that water might exist in large quantities in extraterrestrial environments as brine, or convecting ice. Oceans are thought to exist beneath the surfaces of many dwarf planets and natural satellites; notably, the ocean of the moon Europa is estimated to have over twice the water volume of Earth's.

The Solar System's giant planets are thought to have liquid or supercritical atmospheric layers of yet-to-be-confirmed compositions. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.

Extraterrestrial oceans may be composed of water, or other elements and compounds. The only confirmed large, stable bodies of extraterrestrial surface liquids are the lakes of Titan, which are made of hydrocarbons instead of water. However, there is strong evidence for the existence of subsurface water oceans elsewhere in the Solar System. The best-established candidates for subsurface water oceans in the Solar System are Jupiter's moons Europa, Ganymede, and Callisto, and Saturn's moons Enceladus and Titan.

Although Earth is the only known planet with large stable bodies of liquid water on its surface, and the only such planet in the Solar System, other celestial bodies are thought to have large oceans. In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.

The inner structure of gas giants remain poorly understood. Scientists suspect that, under extreme pressure, hydrogen would act as a supercritical fluid, hence the likelihood of oceans of liquid hydrogen deep in the interior of gas giants like Jupiter. Oceans of liquid carbon have been hypothesized to exist on ice giants, notably Neptune and Uranus. Magma oceans exist during periods of accretion on any planet and some natural satellites when the planet or natural satellite is completely or partly molten.

The gas giants, Jupiter and Saturn, are thought to lack surfaces and instead have a stratum of liquid hydrogen; however their planetary geology is not well understood. The possibility of the ice giants Uranus and Neptune having hot, highly compressed, supercritical water under their thick atmospheres has been hypothesised. Although their composition is still not fully understood, a 2006 study by Wiktorowicz and Ingersall ruled out the possibility of such a water "ocean" existing on Neptune, though oceans of metallic liquid carbon are possible.

The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, though the water on Mars is no longer oceanic (much of it residing in the ice caps). The possibility continues to be studied along with reasons for their apparent disappearance. Some astronomers now propose that Venus may have had liquid water and perhaps oceans for over 2 billion years.