Pluto might have stolen the dwarf planet spotlight, but 2015 was also the year we started exploring Ceres, the only dwarf planet in the inner Solar System and the largest object in the asteroid belt.

For just over a year, NASA’s Dawn spacecraft has been studying Ceres in detail and has discovered incredible features. Among these, it found Ahuna Mons – a large, dome-shaped mountain.

Dawn’s latest images of Ahuna Mons shows an incredible geological formation. The mountain has an average height of 4 kilometers (2.5 miles), although its steepest side is about 5 kilometers (3 miles). This is extraordinary considering the dwarf planet is only about 900 kilometers (560 miles) in diameter.

"No one expected a mountain on Ceres, especially one like Ahuna Mons," said Chris Russell, Dawn's principal investigator, in a statement. "We still do not have a satisfactory model to explain how it formed."

"Ceres has defied our expectations and surprised us in many ways, thanks to a year's worth of data from Dawn," added Carol Raymond, deputy principal investigator for the mission, based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "We are hard at work on the mysteries the spacecraft has presented to us."

Ahuna Mons has another curious feature: Its slopes are not the same color. Some of them are covered in a bright material similar in appearance to the mysterious bright spots found in the famous Occator Crater. New findings of the spots and more insight into Ceres will be presented at the 47th Lunar and Planetary Science Conference.

"Dawn began mapping Ceres at its lowest altitude in December, but it wasn't until very recently that its orbital path allowed it to view Occator's brightest area," said Marc Rayman, Dawn's chief engineer and mission director at JPL. "This dwarf planet is very large and it takes a great many orbital revolutions before all of it comes into view of Dawn's camera and other sensors."

Ceres’ primary mission will finish on June 30, but further activities will likely be planned for after that date.