Everyone knows that the Earth has huge oceans. But are there oceans on other planets? As far as we currently know, there aren't -- except for hydrocarbon oceans on one planetary moon, Titan, the largest moon of Saturn. Scientists also suspect there may be subsurface salt-water oceans on some of the largest moons of Jupiter, like Europa, Callisto, and Ganymede, but they haven't confirmed these yet. Some scientists even optimistically believe there may be life in these subsurface oceans, but this seems extremely unlikely.
The most impressive, and the only confirmed extraterrestrial oceans are on the surface of the Saturnian moon Titan. Titan is a huge moon, about 50% larger and 80% times more massive than our Moon. It is the second largest moon in the solar system, after Jupiter's moon Ganymede. Frequently described as a "planet-like moon," Titan has a thick nitrogen atmosphere which appears as a deep orange haze. This thick atmosphere has blocked our view of the moon for hundreds of years, and it wasn't until 2007 that the Cassini-Huygens space probe mission visited the moon and dropped a probe through its atmosphere and onto its surface.
Scientists had long suspected the existence of hydrocarbon lakes on Titan, but it wasn't confirmed until Cassini-Huygens made a close flyby and imaged the surface with its cloud-penetrating radar. On July 22nd, 2006, a radar-imaging flyby confirmed the existence of smooth, dark patches near the north pole of the moon, which were the oceans scientists were looking for. The largest body of hydrocarbons was named Ontario Lacus, after Lake Ontario on Earth. The hydrocarbons were primarily methane and ethane. The lakes discovered ranged from about a kilometer to 100 kilometers in size. They are the only large stable bodies of liquid known to exist anywhere but Earth. The hydrocarbons found on Titan far exceed the quantity of hydrocarbons anywhere on Earth.
Aside from Titan, there are probably liquid oceans deep beneath the crust of ice moons like Europa. These moons consist of balls of rock surrounded by a thick layer of ice. As depth increases, the temperature of the ice is thought to increase (due to tidal heating) until it melts and turns to liquid. On Europa, this ocean is thought to be about 100 km (60 mi) deep, and its existence has been hinted at by the presence of an induced magnetic field, only possible if it has a surface conducting layer. Many planetary scientists are interested in exploring this ocean by launching a probe to Europa that melts through the outer layer until it penetrates through the surface ice layer.