Dutch astronomer Jan Oort was born on April 28, 1900. He visualized a vast reservoir of icy comets on the fringes of our solar system, which now bears his name.
An oddly textured region of southern Mars is not ice-covered today. Yet landforms here - and now certain minerals - are associated with under-ice volcanoes.
Earlier this year, scientists presented evidence for a Planet Nine in the far outer solar system. Scientists think it exists, but - if so - how did it get there?