Despite the cold and the fact that it’s a moon, Ganymede may be the most ‘Earth-like’ body in our solar system. The revelation of an incredibly vast and deep ocean beneath Ganymede’s ice is the latest in a long string of positive signs for life on the moons of the outer planets.

Back in March of 2014, Emily Lakdawalla of the Planetary Society proclaimed that 2015 would be the “Year of the Dwarf Planet”. With the Dawn Spacecraft arriving at Ceres and New Horizons set to arrive at Pluto, we’re bound to learn a great deal more about dwarf planets than we knew before.

Lakdawalla was right, but 2015 is actually shaping up to be a pretty good year for anything in the solar system that is smaller than a planet. There is, of course, the ESA’s ongoing Rosetta mission to study a comet up close. There also seems to be a regular stream of news about moons.

There hasn’t really been much news about our moon. We already know quite a bit about that one but there are plenty of other moons. In February, NASA announced that its Europa Clipper mission is plunging ahead. A little later that month, NAS unveiled plans for a theoretical drone submarine that could explore the methane seas of Saturn’s moon Titan.

Early this month a chemical formula for life that could exist on Titan was laid out, yesterday evidence of warm water activity on Saturn’s moon Enceladus was revealed and today evidence has been produced for an enormous ocean on Ganymede.

Ganymede is the largest moon in the solar system and, according to the new data, could harbor an ocean with more water than all of the surface water on Earth.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish. In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth,” said John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate at NASA Headquarters, Washington, D.C. in a statement.

In addition to being the largest moon, Ganymede is the only one with its own magnetic field. In addition to providing great protection against radiation, magnetic fields causes aurorae, just like they do on Earth.

By watching the rocking motion of the aurorae at Ganymede’s northern and southern poles, researchers determined that a large amount of saltwater exists beneath the moon’s crust altering it’s magnetic field.

The close up views of the aurorae were provided by the Hubble Space Telescope.

“I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior,” said Joachim Saur of the University of Cologne in Germany.

The saltwater ocean underneath the moon’s crust counters Jupiter’s magnetic field which causes magnetic friction and surpasses the rocking of the aurorae. In this case the field generated by the ocean is strong enough that it reduces the rocking of the aurora by two degrees.

The researchers, whose work is published on March 12 in the Journal of Geophysical Research: Space Physics estimate that the ocean is 60 miles deep. The Mariana Trench, the deepest known part of the Earth’s surface oceans, is less than seven miles deep. In short, the ocean’s of Ganymede are deep enough to make a sailor afraid of heights.

It may be true that 2015 is the year of the dwarf planet but in just about six weeks the chances of finding life on Titan, Enceladus and Ganymede and plans to look for life on Europa and Titan have moved ahead. So, the year of the exomoon can’t be far away.