From the December 2005 issue

Have we ever seen comets from beyond the solar system?

Nozomu Kaneda
By | Published: December 1, 2005 | Last updated on May 18, 2023
The notion that clouds of comets surround stars — including the Sun, of course — predates by some two decades Jan Oort’s concept of what we nowadays call the Oort comet cloud.
Astronomer Ernst Öpik believed many meteors were moving through the solar system on unbound, or hyperbolic, orbits with respect to the Sun. A particle following such a path would pass only once through the solar system, never to return. Öpik championed the idea that such clouds around nearby stars created a large population of interstellar comets and meteors.

An observing program initiated by Fred Whipple in the late 1930s quickly showed no convincing case of a hyperbolic meteor. And Oort’s calculations based on the orbits of comets before they entered the inner solar system showed little evidence for any hyperbolic comets.

Indeed, what few slightly hyperbolic comets there seemed to be could be eliminated by amending the calculations to take into account the comet’s reaction to solar heating. According to Whipple’s “dirty snowball” model, ice vaporizing off a comet nucleus near the Sun creates a small thrust, forcing a slight change in the comet’s speed.

What we have learned about comets and meteors does not completely exclude the possibility of an occasional visitor from some other star’s cometary cloud arriving in Earth’s vicinity. An interstellar comet would be moving slightly faster than those native to the solar system, but astronomers estimate such an event occurs only a couple of times a millennium. — BRIAN G. MARSDEN, HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS, CAMBRIDGE, MASSACHUSETTS