Astronomy ABCs: D is for Debris Disk

Happy Honda Days, everyone! It’s December, which start with D, which is also the next letter in Astronomy ABCs! And this month D is for debris disk!

A debris disk in astronomy is pretty much what it sounds like: a disk made up of debris from some formation process that orbits a celestial body. Usually astronomers talk about debris disks when discussing the formation of planets around a star, so a debris disk in this case would be the gas and dust left over from the formation of a star that orbits that star in a disk.

A debris disk shouldn’t be confused with a protoplanetary disk. Protoplanetary disks have much more gas, but in debris disks this dust is nearly gone. Protoplanetary disks also tend to be found around younger stars than debris disks, leading to the hypothesis that protoplanetary disks evolve into debris disks over time. The link between protoplanetary disks and debris disks is an active area of research.

The term “debris disk” sounds like what is left after something catastrophic happens. And really, I guess that’s true! But not catastrophically bad. It’s the natural consequence of the formation of stars.

Stars form out of clouds of hydrogen gas and dust called nebulae. A star-forming nebula like the Orion Nebula will form many, many stars. In the very earliest stage of a star’s life, it is still surrounded by an envelope of gas and dust. This material will form planetesimals (basically a general term for small bodies like asteroids and comets), which will continue to collect material until they form planets. This might go on for a few tens of millions of years, but the pressure from the radiation given off by the star will eventually clear out all the teeny tiny bits. But this is just the beginning of the debris disk. Collisions between these planetesimals may cause a second generation of dust in the system.

OK, so we have these planetesimals flying around, potentially crashing into each other and creating dust that turns into a debris disk. Dust grains in a debris disk tend to be about 10 microns wide at their biggest. For reference, a human hair is something like 50 microns wide, so these dust grains are pretty tiny. These tiny grains will further collide to make even smaller, sub-micron sized pieces that won’t stick around the system for long. These dust grains could also spiral into their host star. These processes mean that, without any process to replenish it, the debris disk will only last about 10 million years. However, the cycle of collisions often keeps the debris disk around for longer.

Studying debris disks around other stars can shed light on the formation of our own solar system, but it’s also especially relevant for astronomers who want to find exoplanets, or planets around stars that are not our Sun. You see, for collisions to happen, planetesimals need to have their orbit altered in some way. Gravitational perturbations can draw two objects together and bam! Collision. The central star could cause this, as could a second star in a binary star system. But you know what else could cause these perturbations? Planets. So how can astronomers narrow down where to look for planets? Find systems with a debris disk. These systems are easier to spot and, while it doesn’t guarantee a planet exists there, it increases the odds.

Even though debris disks are easier to find than individual exoplanets, they are still difficult to find. Because of the thermal emission properties of dust grains, usually debris disks are detected with telescopes designed to detect relatively cool things out in space. It’s hard to believe that we can actually use telescopes on Earth to see other stars in enough detail to make out their disks and, possibly, detect planets. I’m never not astounded by what we’re able to learn!

Featured image credit: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center), M. Fitzgerald