Planet Nine’s existence is not the first time astronomers have predicted a planet they couldn’t see. Clyde Tombaugh was looking for Planet X when he accidentally found Pluto. After Voyager 2 flew by Neptune in 1989, new calculations determined there was no Planet X. But three years ago, a disturbance was detected at the far reaches of the Solar System.

Follow two astronomers, Mike Brown and Konstantin Batygin, as they’re on the hunt for Planet Nine — a hypothesized ninth planet at the edge of our solar system.

Two astronomers argue that the farthest objects in the Kuiper Belt are being pushed and pulled by the gravity of Planet Nine, a giant planet orbiting over 10 times farther from the Sun than Neptune. If they find it, it’ll be the first real expansion of the Sun’s planetary album in 170 years.

This robot might look like a Furby. But, using artificial intelligence, it can learn to understand a child's emotional state to better engage them during a lesson.

Bioarchaeologists investigate a ninth-century mass grave in a rural English village. Will the remains unlock the mystery of the “Great Heathen Army,” a legendary Viking fighting force that once invaded England?

Archaeologists discovered a mass grave of what they thought to be ninth-century Vikings. But for some reason, the radiocarbon dates of their bones, found by measuring a radioactive carbon isotope, appeared much older.

This Viking’s skeleton, which was buried alongside weapons and horses, was found in 1889. Over a century later, archaeologists still assumed the skeleton was that of a high-status male warrior. But DNA analysis revealed the surprising truth.

Horse riding played a key role in human expansion and civilization. But when and how did people first master these animals? Scientists use archeology and genetics to uncover clues about the first horse riders and how they shaped the world.

Anthropologist and filmmaker Niobe Thompson attempts to milk a horse, and discovers that the process is a lot more difficult than it sounds. Milking a mother horse, he explains, is all about tricking her. The process begins by having her foal suckle. Then, the foal is taken away and a human rushes in to take its place. But if the mother horse suspects anything is different, things can go awry.

Paradise, California's Camp Fire accelerated as it moved uphill. U.S. Department of Agriculture scientists conduct an experiment to discover why.

The California wildfires of 2018 took a worrisome trend to a new extreme, claiming scores of lives and over a million acres. Scientists investigate how forestry practices, climate change, and drought may contribute to the rise of deadly megafires.

As the Camp Fire scorches a community in Northern California, residents leave by car and on foot. Their journey is captured on harrowing cell phone videos.

The Camp Fire, which began on Nov. 8, 2018, was the deadliest and most destructive wildfire in California history to date.

After the largest and deadliest wildfire in California's history to date scorches Paradise, California, a resident returns home and witnesses the Camp Fire's destruction firsthand.

Many Muslim women are not permitted to be home alone with a male who is not a family member, making it tough to schedule home repairs. Now, in Jordan, billions of gallons of already-scarce water is being lost to leaky pipes. In an effort to take matters into their own hands, many Muslim Jordanian women are learning to do their own plumbing.

As the Dead Sea shrinks, engineers prepare a daring solution: connect it with the Red Sea by way of a massive desalination plant. If it works, it could stabilize the legendary lake and ease regional tensions. But will it put the environment at risk?

A mass of fossilized fish, uncovered in North Dakota, appear to preserve the catastrophic fallout of the asteroid impact that wiped out the dinosaurs.