The NASA New Horizons probe wowed the world with its amazing photos of Pluto in recent weeks — and with that flyby accomplished, it’s taking on a new mystery: a possible “Planet X” deep in the Kuiper Belt.

NASA flew by Pluto on July 14 and sent back spectacular images, and will continue to send back data over the next few months. But now that New Horizons has gone past Pluto’s orbit and heads for the Kuiper Belt, which it will reach in 2019, it will take a close glimpse at an interesting object that is essentially a big chunk of icy rock, according to a Washington Post report.

A total of 1,500 icy bodies have been detected in the Kuiper Belt, with a few of them large enough to qualify as a “dwarf planet.” But there might be a much bigger object out there, one that is even bigger than Earth and could rival the size of Neptune, according to the report.

A 2014 report in the journal Nature from the Gemini Observatory in Hawaii indicated that a massive Solar System “perturber” may lurk somewhere out there. They called it a dwarf planet and named it 2012 VP113, and it is way out there — about three times farther from the sun than distant Pluto is, and even out past the Kuiper Belt.

It’s tough to see Planet X because it’s way out there and quite faint. Because it’s about 200 Astronomical Units from the sun — the Earth is one Astronomical Unit — it doesn’t get much sunlight, and therefore it’s tough for astronomers to pick it up. Stars are much easier to study even at great distances because they emit their own light.