Schematic showing how liquid water from Enceladus is released into space through plumes. NASA/JPL-Caltech Most astrobiologists will agree that one of the key ingredients for life is liquid water.

That's why, whenever planetary scientists discover evidence of liquid water on other celestial bodies in space, like Jupiter's moon Europa or Saturn's moon Enceladus, it's an exciting time to speculate whether these watery worlds could harbor alien life.

Right now, Enceladus seems like the most likely place for alien life in our solar system because of evidence pointing to active vents heating the waters along the ocean floor.

But there's a surprising number of other moons and dwarf planets that, scientists suspect, contain vast, deep oceans beneath an icy outer shell. Since we can't actually see underneath the surface, scientists usually detect evidence for the presence of liquid water by using satellites that take pictures during flybys. If there's liquid water underneath, it tends to escape through cracks in the surface and create distinctive plumes, which spacecraft then study for evidence of water vapor or molecules.

Here's a graphic showing the 12 possible places in the solar system where scientists suspect an underground ocean of liquid water:

Most of these objects are moons, like Titan and Callisto, or dwarf planets, like Eris and Pluto, all of which are much smaller than Earth. They are almost all completely unexplored except by brief satellite flybys. The one exception is Saturn's largest moon, Titan.

In 2005, a space probe named Huygens landed on Titan, becoming the first spacecraft to successfully touch down on any celestial body in the outer solar system, beyond Mars. Huygens studied the moon's atmosphere and relayed the data to Earth for about 90 minutes after touchdown.

At nearly 900 million miles from Earth, Huygens is still to this day the most distant successful landing by any man-made spacecraft.