1932

The Öpik-Oort Cloud

Ernst Öpik (1893–1985), Jan Oort (1900–1992)

By the early twentieth century, hundreds of years of careful observations had enabled accurate orbits to be calculated for dozens of bright comets. They appeared to come in two varieties: periodic short-period comets, with orbits that bring them back into the inner solar system roughly every 20 to 200 years; and periodic long-period comets, with long, highly eccentric orbits that could take hundreds to thousands of years or more to complete (or not to repeat at all, for the related nonperiodic single-apparition comets).

The trajectories of the longest-period comets take them extremely far from the Sun, with some traveling as far out as 50,000 to 100,000 astronomical units (almost a third of the way to the nearest star) when near their aphelion, or farthest point in their orbit. Several researchers independently noted that the cluster of aphelion distances at those extreme ranges probably meant that there was a supply or reservoir of comets that originated at those extreme distances. The fact that long-period comets come from all directions in the sky (not just along the plane of the ecliptic, like the planets and most short-period comets) also meant that this reservoir was likely spherical, like a huge cloud surrounding the solar system.

In 1932 the Estonian astrophysicist Ernst Öpik was the first to postulate the existence of this vast reservoir of comets in a paper describing the role that passing stars might have in gently nudging comets from this distant cloud into new orbits that would take them in toward the Sun. Independently, in 1950, the Dutch astronomer Jan Oort also came up with a similar idea, but expanded to include the role of Jupiter and the other giant planets in flinging inner solar system comets out into this enormous distant cloud.

Subsequent studies of new long-period comets (about one new one is discovered every year) confirm the idea: even though it’s never been directly seen, a vast cloud of distant comets appears to surround the Sun. Astronomers now call this the Oort cloud (or Öpik-Oort cloud). By some estimates there could be a few Earth masses in a trillion or more kilometer-size comet nuclei out there, some formed closer in to the Sun but ejected into a perennial deep freeze, others formed at the edge of the Sun’s gravity and waiting for their first gentle stellar nudge into the warmth.

The inner solar system (top) extends out to the main asteroid belt, and the outer solar system (middle) out to the Kuiper Belt. The Oort cloud (bottom) is hypothesized to extend out much farther, perhaps a third of the way or more to the nearest star.