From the August 2012 issue

Is there evidence of Oort-Cloud-like structures around other stars?

Laurence Rohr, Warrenville, Illinois
By | Published: August 28, 2012 | Last updated on May 18, 2023
Oort Cloud
Scientists think a structure called the Oort Cloud exists in the solar system’s outer reaches. Such regions likely lie around other stars, but researchers haven’t directly detected any yet. Credit: Astronomy: Roen Kelly
For more than 20 years, scientists have speculated whether structures of small orbiting bodies, like the Oort Cloud at the outskirts of the Sun’s gravitational hold, exist. In the late 1980s and early 1990s, researchers like Scott Tremaine (now at the Institute for Advanced Study in Princeton, New Jersey) and I, working independently, were the first to model and hunt for “extrasolar Oort Clouds” (ESOCs). We showed that if the solar system is typical, then Oort-Cloud-like structures should commonly exist around other stars. Later, Mike Shull (now at the University of Colorado Boulder) and I together calculated that if planetary systems with giant planets at large distances are common, as in the solar system, then interstellar comets should be common as well.

Since then, the field of extrasolar planet detection has, of course, exploded, and we now know that a wide variety of planetary system architectures exist. Many of these compositions likely support Oort-Cloud-like structures, but scientists haven’t directly detected any such regions yet. This is largely because comets are so small. However, scientists have found indirect evidence of ESOCs from observations of water-rich bodies (likely comets) falling onto both protostars and evaporating around red giants.

In the future, it may be possible to detect active comets by their water, hydroxide, and hydrogen emissions around stars and track their orbits, and from that infer whether the source regions of such extra-solar comets are in ESOCs, “Kuiper Belts,” or other structures. One of the most interesting questions we can then answer will be how the masses of these ESOCs vary, particularly as a function of the architecture type of planetary systems. — S. Alan Stern, Southwest Research Institute, Boulder, Colorado