Free Astronomy Magazine January-February 2020

30 JANUARY-FEBRUARY 2020 EXOPLANETS nomenon of or- bital migration. According to the authors of the study: “If the origin of the co- pious amount of warm dust orbit- ing BD +20 307 is an extreme colli- sion between planetary-sized bodies, then this system may help unlock clues into planetary sys- tems around bi- nary stars, along with providing a glimpse into cat- astrophic colli- sions occurring late in a planetary system’s history. Understanding BD +20 307 and other systems like it with extremely dusty debris disks could advance our knowledge of catastrophic collisions, the effects of bi- nary stars on debris disks and the evolution of planetary systems.” Indeed, the knowledge of systems like BD +20 307 can help us to trace the history of other star systems that are more interesting to us, such as the triple one of Alpha Cen- tauri, where we know some planets exist. But the study of the collisional evolution of planetary systems can also be useful to bet- include leftover planetesimals and are thought to evolve through collisions or evaporation of solid bodies, ranging from small planetesimal to protoplanet/planet- sized. Just like the Solar System’s Kuiper Belt located beyond the orbit of Neptune, most debris disks contain low-temperature dust ( ≤ 100 K) orbiting far from the host star. However, there exists a small class of known stars with unusually warm, dusty debris disks that serve as a key sample to probe in order to understand cascade models and ex- treme collisions that likely lead to the final configurations of planetary systems. Collisional cascade, a process in which larger planetesimals in the disk collide and are continually broken up into smaller objects can explain most debris disks. In a collisional cascade, small debris disks with warm dust do not last for very long be- cause once the dust has reached a small enough size, removal mechanisms operate quickly, such as radiation pressure that blows the dust out of the system or Poynt- ing-Robertson drag that causes dust parti- cles to fall into the star.” BD +20 307, therefore, represents a rare op- portunity to study the catastrophic collisions that probably occur in many planetary sys- tems in the phases following their forma- tion, perhaps simultaneously with the phe- T he two main protagonists of the discovery of shattered plan- ets around the double star BD +20 307: Maggie Thompson (with the pretty Rocket) on the left and Alycia Wein- berger below.