Could Pluto's icy surface hide an ocean underneath? This question and other mysteries surrounding the distant object, once considered the farthest planet from the sun, may be answered when NASA's New Horizons spacecraft finally reaches Pluto in 2015.

Little is known about the dwarf planet. Pluto has never been visited by a spacecraft and even the Hubble Space Telescope can only resolve the largest features on its surface. Pluto's composition is unknown and scientists can only estimate that it's made of mixture of 80 percent rock and 10 percent water ice.

Planetary scientists Guillame Robuchon and Francis Nimmo from the University of California at Santa Cruz have worked out a checklist of features to look for when New Horizons passes by Pluto. They wanted to find out if an ocean could exist underneath Pluto's outer shell of nitrogen ice.

The pair believe the first sign to look for is whether Pluto has a bulge or not. As spherical bodies spin, their momentum pushes material to the equator, creating a bulge. If there is liquid beneath the surface, there would be less of a bulge because the ice would flow. If there isn't an ocean there would be frozen bulge on the equator.

"If the bulge is present, it will be about six miles (10 km) high, so it should be readily detectable," Nimmo told Space.com.

It might seem Pluto's distance from the Sun would make impossible for liquid water to exist but there could be enough heat from the core to melt the ice. The main source of energy could come from the rocky interior, more specifically from the potassium in the core. Enough potassium could melt the ice above it.

"I think there is a good chance that Pluto has enough potassium to maintain an ocean," Nimmo said.

It wouldn't be the first time water was discovered so far out of the sun's reach. Jupiter's moons Europa, Ganymede and Callisto may each contain a sea beneath their frozen surfaces, and Saturn's moon Titan is also believed to harbor an underground water ocean.

NASA's New Horizons spacecraft will begin imaging the dwarf planet three months before its closest approach. The spacecraft will not be able to map the entire surface of Pluto, but its images of the dwarf planet will be 10 times more detailed than any images captured by the Hubble. Launched in 2006, the New Horizons spacecraft is already halfway between Earth and Pluto.