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 billion years ago. As the new star continues to suck up material from the surrounding disk, it pulls it in along magnetic field lines. When the material arrives at the star along the lines it produces a bright flash.
Now here’s the ingenious part: The bright flash not only reaches Earth directly, it also reflects off the protoplanetary disk and reaches Earth as a second flash. Using this fact, astronomers have used the time difference between the flashes to figure out the distance between the star and the protoplanetary disk. Since the speed of light is constant, a greater time delay means a greater distance to the disk.
This is an important piece of information because it helps us understand how planets form. It gives us an idea of how close to a star a planet can form, and allows us to constrain models of stellar planet formation. It may give insights into how such large planets (hot Jupiters) form so close to their parent stars.
Astronomers have used light reflections before, but only in the past few years have observational techniques been developed. Another key area is the study of exoplanet atmospheres. Light from a star reflects off a planet and reaches Earth, revealing information about it’s atmosphere or surface, if you can separate the signal from that of the star.
It’s no easy task, but it gives valuable information.