One of the things that deeply amuses me about NASA is they have a handful of science divisions: there is Planetary Science, Earth Science, Helioscience, Biological and Physical Sciences, and everything else gets lumped in under Astrophysics. Put another way, the vast majority of the universe is outside our solar system, and it is all lumped together in one of five science divisions. 

This tendency to spend a lot of our energy observing what is near and bright is a pretty common habit for us humans. As a whole, the universe has three major components: there is the stuff that makes up us and every single thing we feel and touch inside the universe; this is called baryonic matter. There is also dark matter that we trace through its gravitational pulls on light and matter. And in 1998, we learned there is a third something out there: a mysterious thing that is accelerating the expansion of the universe. We don’t know what it is, but we named it dark energy. Mind you, it might not actually be a form of energy; that’s just the placeholder name. We could have just as easily named it Fred, but that would have been less interesting. 

We can’t see dark matter or dark energy except through how they pull and push on everything around them. Nevertheless, 68% of the universe is dark energy, while 27% is dark matter, and less than a measly 5% is regular stuff.

It is super annoying to not be able to identify 95% of the stuff in the universe, and annoyance can lead to creativity.

Filed under things I never expected to read, super creative cosmologists have figured out a possible way to use exoplanets to find and map the distribution of dark matter in our galaxy. As weird as that sounds, their logic is pretty straightforward, and while the required observations are super hard, hard doesn’t mean impossible.

Here is how it works: the more matter you cram into a space, the higher the temperature will be. Planets have mass, and their gravity is capable of drawing in more matter, including dark matter. Since dark matter particles are super small and generally don’t interact with anything, it is possible for dark matter to sink into the centers of worlds and just hang out there, not doing much beyond driving up temperatures.

In this work, which is published in Physical Review Letters by Rebecca Leane and Juri Smirnov, it’s predicted that all other things being equal, planets in regions of the galaxy with more dark matter will pull more dark matter into their core and be warmer, and planets in volumes of the galaxy with less dark matter will be cooler. The catch is the “all things being equal” part. 

The temperatures of planets vary with distance from their star, their interactions with other objects, their age, their composition, and a whole lot more. To keep things simple, they propose looking at gas giants, rogue planets, and even those borderline objects – brown dwarfs. If these objects are harboring dark matter, there should be an observable change in temperature from place to place. In the core of the galaxy, where we expect more dark matter, things should be hotter, while things further outwards should be cooler.

Unfortunately, this only works for the smallest theorized forms of dark matter, but it is better than nothing.

Also, unfortunately, this particular theory can’t be tested until the JWST is launched and working, and they can get a significant amount of observing time on it, so we can’t know this answer for a while. This is a reminder that studying astronomy is a long game with years and years sometimes passing from idea to observation to a new understanding of our universe.

More Information

The Ohio State University press release

“Exoplanets as Sub-GeV Dark Matter Detectors,” Rebecca K. Leane and Juri Smirnov, 2021 April 22, Physical Review Letters