“New Horizons shattered a major paradigm of planetary science,” says Alan Stern, the mission’s principal investigator. “Pluto turns out to have as much complexity as Mars or Earth, so much so that I ...

Instead of a barren ice ball, Pluto featured vast plains of nitrogen ice, jagged mountains made of water ice, and intricate networks of frozen canyons. Its heart-shaped region, informally called ...

(Correction: An earlier version of this story misstated how far Pluto is from the sun. The correct number is 3.7 billion miles.) What was discovered in Flagstaff, Arizona, and killed off in Prague?

But instead of surface water being evaporated by the Sun and falling as rain, Pluto experiences a daily nitrogen cycle. Nitrogen is heated and cooled as Pluto rotates. A New Horizons image of ...

The secret to all of that activity is Pluto’s probable underground ocean. The water ice on the surface of the world suggests that there should be more water hidden below ground. Over the course ...