Dark matter could be detected on alien worlds orbiting distant suns, a new study suggests.
This elusive form of matter is one of the most frustrating and mysterious aspects of modern astronomy. Thought to account for 80% of all matter in the universe, it is completely invisible, detectable only through its slight gravitational pull on its surroundings.
But in some situations, it can settle into the core of a massive object, releasing energy in the form of heat. Now, a pair of astronomers is advocating a daring new research program: to turn our widening search for life beyond Earth into a hunt for dark matter.
The frustrating darkness
We know very little about dark matter, other than that it exists. In the 1970's, astronomer Vera Rubin noticed something funny about the way that galaxies were rotating. Rubin found that stars were orbiting around their galaxies far too quickly, given how much visible matter there was if you add up the gravitational attraction of everything we can see in a galaxy, then at the observed rotational speeds the galaxies she trained her telescope on should have torn themselves apart billions prior to her observation.
In the decades since Rubin's revelation, more mysteries have piled up. The gas inside galaxy clusters is too hot. Galaxies move around too quickly. The universe has too many large-scale structures, given the age of the universe. The remnant radiation from the early universe is too bumpy to be explained by normal matter alone. Light from distant background galaxies curves too strongly when passing near massive galaxy clusters.
The list goes on, but one answer has risen to the top: In order to explain all these observations, the universe must have some hidden ingredient. It's a form of matter (because it can obviously clump together and has gravity), but it doesn't interact with light or normal matter in any significant way. It's dark matter.
Based on computer simulations of giant clusters of galaxies, whatever the dark matter is, we expect it to be more heavily clumped toward the centers of galaxies and generally thin out the farther you get from those centers. And it's those differences in dark matter density throughout a galaxy that may help astronomers identify this mysterious substance.
If only we had large dark matter detectors scattered through the galaxy.
According to a pair of researchers in a paper published in October to the preprint journal arXiv, dark matter detectors are indeed scattered through the Milky Way galaxy. And we're already finding thousands of them orbiting distant suns every year. They're exoplanets, or the alien worlds beyond our solar system, that we're spotting with the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS).
Indeed, the thousands of confirmed exoplanets known so far represent only a tiny percentage of all possible worlds. For the Milky Way alone, estimates of the true number of exoplanets range from the extreme (300 billion) to the ludicrous (1 trillion).
Signs of light
Here's what that has to do with dark matter. Dark matter — as far as we can tell — hardly ever interacts with normal matter, or even with itself. When it does interact, it does so through an interaction involving the weak nuclear force, which is incredibly wimpy. Almost every time there's an encounter, a dark matter particle and a normal matter particle simply slide on by each other without comment … or even a quick glance.
But rarely, occasionally, dark matter and ordinary matter interact, allowing the dark matter particle to pass on some of its energy to the normal matter particle, slowing down the dark matter particle in the process. These interactions are especially common when two things happen: there's a large, dense concentration of normal matter that acts as a gravitational trap for dark matter, and there's lots of dark matter just floating around.
These two criteria could be met for exoplanets near the center of the Milky Way. The dark matter density in those neighborhoods is much higher than it is around the solar system, and large planets (say, Jupiter-size and up) could collect dark matter particles in their cores. They would do this through their gravity: In high-density environments, the normal matter can attract the dark matter to them, pulling it to their centers.
These interactions wouldn't just slow down dark matter, they would also heat up the planet. And sometimes dark matter particles might occasionally interact with themselves, annihilating each other in a brief flash of energy. This energy would be too feeble to see directly, but over the course of billions of years the sustained flashes from countless interactions could contribute an extra source of heat to the planet.
The end result, according to the research: Planets closer to the center of the galaxy might experience a significant amount of heating from dark matter, causing their temperatures to rise by thousands of degrees.
In order to test this, we need to take the temperatures of a lot of exoplanets. Thankfully, this is exactly what missions like the James Webb Space Telescope (JWST), which is set to reach space in October 2021, are explicitly designed to do.
The researchers noted that the JWST has just enough sensitivity (in both recording the temperatures of exoplanets and in searching close enough to their galactic center) that if this effect of dark matter is real, we should be able to see a distinct and noticeable warming of planets the closer they are to the galactic center. If the surveys pan out, it would be the first non-gravitational detection of dark matter ever seen.
And in the process of searching through all those exoplanets, we might just discover life on another world, which would be a nice bonus.
Originally published on Live Science.
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Paul M. Sutter is a research professor in astrophysics at SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.