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Pluto, originally considered the ninth planet, was recently classified as a dwarf planet and is the least massive planet in the solar system.

Scientists speculate that ours might not be the only universe out there — we might instead live in a "multiverse".

UFOs, or Unidentified Flying Objects, and extraterrestrials are perennial favorites in science fiction, but to date there have been no confirmed encounters on Earth.

NASA is the federal space agency run by the United States to oversee American space exploration, research and technology.

Meteors, also known as shooting stars, are pieces of dust and debris from space that burn up in Earth's atmosphere, where they can create a bright streak across the night sky.

Albert Einstein's theory of relativity explains how space and time are linked.

Big Bang theory is the leading explanation for how our universe began.

Interstellar travel is the ability to travel between stars in the exploration of space.

There are eight distinct phases every month, four of them occurring roughly a week apart.

Gravitational waves are ripples in space-time created by the interaction of massive objects in space, such as black holes and neutron stars.

See news, images and videos about comets, icy wanderers in the solar system.

Uranus is the seventh planet from the sun and the third-largest planet in the solar system.

The International Space Station is a $100 billion research outpost in low-Earth orbit. See the latest news, photos and videos from ISS missions here.

Rocket launches used to be the realm of national governments, but with the rise of private spaceflight companies more rockets are launching into space than ever before.

NASA's Jupiter-bound Juno probe will study the gas giant's atmosphere, magnetosphere and gravitational field. Juno will orbit Jupiter for about a year.

The aurora borealis, also known as the northern lights, is the rippling night-sky light display found at the northernmost reaches of the Earth.

Roughly 80 percent of the mass of the universe appears to be dark matter: a material that seems to interact with ordinary matter only through gravity, without emitting light or energy.