Ah, dark matter, you keep playing to us. That stuff is incredibly difficult to study. It does not interact with light, so our evidence for this is based on its gravitational effects on light and visible matter. And the main difficulty is that we still don’t know what it is. Efforts to detect dark matter directly are as empty as indirect methods such as looking for evidence of dark matter through things like excess gamma rays in the Milky Way. But astronomers keep coming up with new ways to discover the stuff, like a study recently published in Physical Review Letters.
This new study takes an interesting approach. One of the popular ideas for dark matter is that it is made up of weakly interacting massive particles (WIMPs). These particles would be more massive than known particles such as electrons or protons. Due to the weak nuclear force, particles of dark matter should disintegrate into lighter particles. While it might be difficult to observe the decaying particles directly, the study suggests looking for their effect on exoplanets.
Since dark matter and regular matter interact gravitationally, exoplanets could trap a little dark matter that would tend to cluster around their nuclei. When dark matter decays, thermal energy is given off to the exoplanet, making it warmer than usual. So if we find that exoplanets are usually slightly warmer than expected, it would be evidence of dark matter.
The predicted warming of various dark matter models. Photo credit: Leane and Smirnov
A good candidate for this type of study would be large exoplanets like Super Jupiter. Their high mass would tend to trap a lot of dark matter, and their expected temperatures are well understood. Since Super Jupiter is made up primarily of hydrogen and helium, most of its core warming comes from gravitational contraction. Just as squeezing a rubber ball makes it warmer, so squeezing gravity warms Super Jupiters. These planets are not big enough to cause nuclear fusion like stars. So if they are warmer than we expect, it must be through radioactive decay.
Using their dark matter heating model, the team demonstrated that the effect should be visible with next-generation telescopes such as the James Webb Space Telescope. Since the amount of excess warming would depend on the amount of dark matter in the Milky Way, the technique could also be used to map the distribution of dark matter in our galaxy. We could then start narrowing down our models of dark matter.
It is possible that an online survey of the JWST found no warming in a survey of exoplanets. But that would also be a good result. It would confirm that dark matter doesn’t heat exoplanets, so either it doesn’t exist or it’s something even stranger than we thought. The only way to solve the dark matter puzzle is to keep collecting data.
Reference: Leane, Rebecca K. and Juri Smirnov. “Exoplanets as Sub-GeV Detectors for Dark Matter.” Physical Review Letters 126.16 (2021): 161101.