Free Astronomy Magazine January-February 2020

31 JANUARY-FEBRUARY 2020 EXOPLANETS ter understand some catastrophic events that certainly occurred in our own solar sys- tem, which led to the formation of the Moon (as a consequence of an impact be- tween the proto-Earth and a Mars-sized planet), the overturning of Uranus’ rotation axis, and the probable expulsion of a gas giant. It is therefore clear that prolonged observations over time of hot dusty debris around solar-type mature stars is not only fundamental for understanding the evolu- tion of those extrasolar sys- tems, but it is also useful for describing with greater preci- sion the evolutionary frame- work of our own solar system. If future observations of BD +20 307 reinforce the hypoth- esis of a clash between plan- ets, once again science will have surpassed science fiction, which before could only imagine such a kind of event as in the novel When Worlds Collide , written in 1932 by Edwin Balmer and Philip Wylie, who tell about the clash between the Earth and a planet arrived from who knows where. Al- though decidedly imagina- tive in the plot, and even a bit naive in detail, that novel must have left its mark, as it was adapted for the big screen in 1951. Curiously, in 1999, the alter- native metal band Power- man 5000 re-used the title When Worlds Collide for its “masterpiece,” thus demon- strating that there may be more disturbing things than the collision of planets... ! C ollisions be- tween planets like the one that occurred in the BD +20 307 sys- tem also occurred in our own solar system, but not at such a late age. The most proba- ble of these colli- sions (depicted above) gave rise to the Moon. [Ge- mini Observatory/ Lynette Cook] On the right, the cover of the first science fiction novel that de- scribed an impact between planets.