One of the major goals of planetary science is to understand the formation of all the bodies within our solar system, including the nearly one million known asteroids. There are two main competing theories (see the figure). The first and classical theory suggests that these bodies formed incrementally, starting as dust grains and accumulating bit by bit until they reached their final size. The second and more recent theory (1, 2) suggests that these bodies formed almost instantly through the gravitational collapse of clusters of pebble-sized material in the protoplanetary disk into single bodies hundreds or thousands of kilometers in diameter. This method skips the meter-to-kilometer intermediate size range that has been problematic to quantify with the classical method.
At the south pole of the moon is a giant crater called the South Pole-Aitken Basin, about 2500 kilometres across. It is thought to have been created by a large asteroid striking the moon 4 billion years ago and is among the largest craters in the solar system. Now researchers say the remains of that asteroid may have been found under the lunar surface.
The birds that live on Earth today have all descended from dinosaurs, the creatures who dominated the planet for a couple of hundred million years, before an asteroid impact caused their extinction. It turns out birds at the time, better called avian dinosaurs, nearly got plucked too, with only those species that lived on the ground surviving the Cretaceous-Paleogene (K-Pg) mass extinction event. Some 66 million years ago, when an asteroid (now called the Chicxulub impactor, for the crater it left behind near the eponymous town in present-day Mexico) crashed into Earth, it led to sudden and severe changes in the climate worldwide, other than the immediate deathly impact on most life in a very large adjoining area, up to a few thousand miles away. Those changes together led to the death of about 75 percent of all animal and plant life on Earth. It was commonly thought that avian dinosaurs survived the mass extinction event, and later evolved into birds.