WATCH: Stephen Colbert and Neil deGrasse Tyson engage in hilarious feud over Pluto

NASA's probe New Horizons reached Pluto, the most far-flung dwarf planet in the solar system on Tuesday night and images of the silvery orb reached Earth for the first time.

Stephen Colbert and astrophysicist Neil deGrasse Tyson commemorated the historic occasion by trading jokes. Broadcasting from "Late Show Mission Control," Colbert held up new pictures of Pluto.

It looks like "a malted milk ball left in the rain 4.7 billion miles from the sun," according to Colbert. He noted what looks like a heart-shaped patch of white.

"It hearts us!" Colbert exclaimed. "But you know who doesn't 'heart' it? Director of the Hayden Planetarium Neil deGrasse Tyson."

Tyson was sitting just out of frame. Colbert turned to him and they had a faux-confrontation over how cool Pluto is, with Tyson pointing out Pluto is smaller than scientists believed. In the 1930s, he said, scientists thought Pluto was the size of Earth, but it's in fact far smaller. Colbert then picked up his smart phone and "fact checked" him.

"Already scientists have learned that Pluto is 20 to 30 kilometers larger than previously thought," Colbert read, prompting laughter from Tyson.

In a brief moment of seriousness, Tyson said, "It's not every day where you get to be the first eyes that set on a completely undiscovered land." He also pointed out that Pluto and its main moon, Charon, orbit a center of gravity that lies outside both objects because Charon is so large in comparison to Pluto.

Watch their exchange here: