Until the recent discovery of gravitational waves, the only ‘sense’ that astronomers had was vision.  Granted our ‘vision’ with telescopes is far broader than human eyes, we still need to find ingenious ways to use the precious photons that rain down on Earth. One of the new ways astronomers are using light is to look at what we call a ‘light echo.’  In reality it’s a reflection of starlight.  When a new star is forming, it is accompanied by a protoplanetary disk, which will eventually form all the planets of the system.  Our own solar system went through this stage 4.5...