Comets and the Oort Cloud

Comets are icy bodies from the outer solar system. When they approach the Sun, ice turns straight to gas, creating spectacular tails that can stretch millions of kilometers. Short-period comets come from the Kuiper Belt beyond Neptune; long-period comets originate in the Oort Cloud, a vast shell extending nearly a light-year from the Sun.

Comets carry water and organic molecules, so they were long suspected of seeding the early Earth. But the picture is debated. Asteroids are now the favored source of most of Earth's water, partly because of Rosetta itself: when it reached Comet 67P in 2014, it measured a deuterium-to-hydrogen ratio about three times higher than Earth's oceans — a mismatch that argues against comets like 67P being the main supplier. Rosetta did, however, detect glycine, an amino acid used to build proteins, keeping comets in the running as possible deliverers of life's ingredients.