Scientists were floored recently after getting back data from NASA’s MESSENGER space probe that showed that Mercury has a core filled with sloshing liquid metal, much like Earth — and they’re scratching their heads as to how that’s possible.

Mercury had been thought to be too small to have a liquid core, as scientists believed the metal would have cooled relatively quickly in its history, but the MESSENGER space probe sent back data before slamming into the planet on April 30 that indicated that yes, its core still contains molten metal, and that metal is creating a magnetic field similar to that of Earth, according to a Space.com report.

It’s nowhere near as powerful — scientists estimate that it is 100 times weaker than Earth’s. But the findings still surprised scientists who thought they would find a solid rock core much like the other rocky planets in our solar system. The findings shed new light on the evolution of Mercury, the closest planet to the sun and the smallest planet in the solar system, and will force scientists to rethink how it developed.

MESSENGER, which launched in 2004, had been monitoring Mercury since it entered orbit in 2011. It is the first probe to orbit the planet — NASA’s Mariner 10 probe paid a visit four decades ago, but only conduct a fly-by. MESSENGER was only supposed to last a year, but only just a couple weeks ago crashed into the surface of the planet after running out of fuel, outliving scientists’ expectations.

Mercury is the most volatile of all the planets in our solar system when it comes to temperature since it doesn’t have an atmosphere to retain a consistent level of heat. As a result, temperatures can dive to a low of -280 degrees Fahrenheit at night to 800 degrees during the day in some regions.