The Solar System formed from a cloud of gas, which gave rise to a proto-star, a proto-planetary disk, and eventually the seeds of what would become planets. This story of planetary formation occurred not only hundreds of billions of times in our own galaxy, once for each of the stars that exists, but all throughout the Universe over our cosmic history as well as in systems that never grew massive enough to form a successful star. As a result, most estimates of the number of planets in the Universe grossly underestimate the total number that are out there. (Credit: NASA/Dana Berry)

How many planets are there in the Universe?

In 1990, we only knew of the ones in our Solar System. Today, we know of thousands, and even that’s only the tip of the iceberg.

For most of history, our Solar System contained the only known planets.