Free Astronomy Magazine January-February 2018

6 JANUARY-FEBRUARY 2018 SOLAR SYSTEM microlensing (an increase in the light from a background star following the transit of a dark body in front of it) has allowed us to close that gap, revealing the existence of sub-stellar and planet-sized objects wan- dering the galaxy. About fifteen of these bodies are known today, and the presence of many others is suspected. Conserva- tive assessments indicate that their total number is twice the num- ber of stars in the Milky Way, and thus some hun- dreds of billions. De- spite this enormous amount, the galaxy is so gigantic that the population of large objects is hugely dispersed. While it is true that, when it comes to sub-stellar and planetary sized objects, interstellar space is essentially empty, the same conclusion may not be valid for smaller bodies, the num- ber of which increases ex- ponentially T he Pan-STARRS1 Observatory on Halealakala, Maui, opens at sunset to begin a night of map- ping the sky. [Rob Ratkowski]