Pluto’s recategorization as a dwarf planet may have caused some past anger, but there has never been a better time to be a Pluto fan. Since the New Horizons mission gave us our first real look at Pluto in 2015, researchers have been digging into the rich reality of this icy world. The latest question under the magnifying glass: what was Pluto like at its birth?

Beneath its surface—composed of frozen water, nitrogen, carbon monoxide, and more—Pluto is today thought to have a liquid water ocean surrounding a rocky core. But scientists are trying to figure out how long it has been that way. In the “cold start” model, Pluto’s interior ocean is thought to have been frozen but then gradually thawed due to heat from radioactive decay in the core. But it remains possible that a “hot start” model is more accurate, one where the planet started out warm enough to form some liquid water.

Cold start vs. hot start

Although the dwarf planet’s beginnings are buried deep in the past, its two possible origins should have left marks on the surface.