comets

In August 2019, an amateur astronomer in Crimea spotted a faint smudge moving at impossible speed. It turned out to be a comet from another star system — only the second interstellar object ever confirmed in our solar system, and the first one to look genuinely familiar.

Comet Hale-Bopp, which dominated the 1997 night sky, had not been seen by any human being before. Its previous perihelion passage was in approximately 2215 BCE, before writing systems existed in most of the world. It had spent the intervening 4,200 years falling inward from the outer solar system at speeds so slow that a bicycle could have overtaken it. When it finally arrived, its pristine composition — unchanged since the solar system formed 4.5 billion years ago — gave scientists a direct sample of the raw material from which planets were built. Long-period comets are the oldest, least-altered objects available for study. They are time capsules from the beginning.

Before dawn in a dark-sky location, a ghostly pyramid of light rises from the eastern horizon along the path of the zodiac. Ancient observers thought it was a meteorological phenomenon — moisture in the upper atmosphere catching the sun. It is not. It is sunlight scattered off trillions of interplanetary dust particles spread across the entire inner solar system, the ground-down debris of comets and asteroids, and it is far stranger than it looks. In 2021, a camera on NASA's Parker Solar Probe detected that this dust does not extend all the way to the Sun.

On October 19, 2017, astronomers found the first object confirmed to have come from outside our solar system. It tumbled, looked extremely elongated or flat, showed no gas or dust, and still sped up slightly as it left the Sun. Scientists have proposed a hydrogen iceberg, a nitrogen shard, and an alien lightsail. Here is what the evidence supports.

Oort Cloud vs Kuiper Belt: two icy reservoirs at the edge of the solar system. Sources of comets, Planet Nine, interstellar visitors, and Voyager.

Asteroids, comets, meteoroids, meteors, and meteorites — what each one is, where they come from, and how scientists study them.

Astrochemistry studies how molecules form and survive in space, from H₂ in dark clouds to amino acids and nucleobases in meteorites and returned asteroid samples.