You might have a pretty clear mental picture of what a planet is, but the official definition is surprisingly contentious. The shock "demotion" of Pluto from planet to dwarf planet in 2006 shows just ...

Add Yahoo as a preferred source to see more of our stories on Google. The word “planet” came from the ancient Greek words that mean “wandering star.” That makes sense, because for thousands of years, ...

This illustration shows the rapidly rotating, highly magnetized white dwarf with the smallest radius ever found, with Earth's moon shown nearby for a size comparison. With a radius of ~2,140 km, just ...

Learning the parts of the solar system used to be a simple thing — basically, all that really mattered was the sun and nine planets. The mnemonic My Very Educated Mother Just Served Us Nine Pizzas ...