Did Pluto form hot or cold?
That may seem like a weird question. After all, at 6 billion kilometers from the Sun, things are chilly.
However, creating a world involves a great deal of heat. The question is, where does it come from? And how did that affect the Pluto that we see today?
A new paper just published has an idea about that. And it turns Pluto inside-out. Kinda.
The way people have traditionally thought Pluto formed is what's called a "cold start." 4.6 billion years ago it grew as objects made of rock and ice in the outer solar system collided together relatively slowly. These would have had small amounts of radioactive materials in them, blown into our solar system by nearby stars even as the Sun was forming. As this material (mostly aluminum-26) collected, it warmed the growing proto-Pluto's interior, causing it to melt from the inside out.
That would create a subsurface ocean over a rocky core and below a thick shell of ice. At first the increased heat caused the ocean to grow deeper, but that radioactive heat source didn't last long, decaying in only a few million years. At that point Pluto would begin ...