Free Astronomy Magazine January-February 2018

7 JANUARY-FEBRUARY 2018 SOLAR SYSTEM as the size decreases. In this case, we refer to asteroids and comets, whose presence in interstellar space is also indicated by the most reliable models of planetary system formation. These models predict that the stages of formation and migration of plan- ets within each system are responsible for the expulsion of countless billions of rocky and icy bodies, which in escaping the grav- itational attraction of their stars would end up wandering through the galaxy indefi- nitely. It is estimated that, on average, any existing planetary system may have ejected the equivalent of approximately 20 Earth masses in the form of asteroids and comets. Considering that, as far as we know today, the majority of stars possess planetary sys- tems, it goes without saying that there is a multitude of objects around the galaxy whose presence could only be revealed if they were close enough to be within reach of our most efficient photographic tele- scopes. Researchers have calculated how many asteroids or comets expelled from other planetary systems we should be able to discover every year, and the result is 0.2, or one every five years on average. This as- sessment is inevitably approximate since the variables in the equation are so numer- ous that this calculation seems a mere probabilistic exercise. Nevertheless, a sur- prising thing happened last October, which in all respects appears to confirm that hy- pothetical scenario. Here are the facts. O umuamua spotted whizzing through the Solar System in images taken with the WIYN telescope. The faint streaks are back- ground stars. The green circles highlight the position of the asteroid in each image. In these images Oumuamua is about 10 million times fainter than the faintest stars visible with the naked eye. [R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF]