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Dark matter planets might exist, and we have ways to find them
Maybe we should seek out the dark planets.
It’s important to be reminded, every now and again, that we know next to nothing. Like the fabled hydra, every question we successfully answer only reveals several new questions, each of them more mind-boggling than the last. When we think of the universe, we usually imagine planets orbiting stars, and those stars orbiting the core of their galaxy. We imagine nebulae, asteroids, and rogue black holes with incredible expanses of nearly empty space between.
As diverse as the cosmos can be, all of these things have one thing in common: they are made of ordinary matter. When you get right down to it, all of those things, from the biggest stars to you, your house, and the stack of empty diet coke cans piling up on your desk (you should probably take care of those, by the way) are all made of the same ingredients. And that’s what we think of when we imagine the universe, but it’s actually only a very small part of what we know is out there.
According to our best knowledge, which is admittedly lacking in this area, about 68% of all the stuff in the universe is dark energy, a further 27% is invisible dark matter. Only the leftovers, about 5%, is made up of ordinary matter.
Point a telescope at any random point in the sky and you're practically guaranteed to get 19 parts of mystery stuff for every star, planet, or galaxy you see. That we aren’t able to figure out what dark matter is and how it works remains one of the biggest challenges of modern astronomy.
Not only is dark matter apparently invisible, but it also doesn’t behave like other matter. As far as we can tell, of the four fundamental forces, dark matter only really interacts through gravity. Even still, the conventional wisdom has been that it doesn’t clump together into stars or planets like ordinary matter does. In recent years, however, some scientists have suggested that may not be the case and we might actually have large planet-like objects made of dark matter orbiting distant stars.
We feel like we’ve seen this movie before. A diverse group of explorers, some of them with questionable relationships to the law, land themselves on a dark planet filled with ravenous creatures. Vin Diesel was there. It didn’t end well, but Pitch Black sure is fun to watch. Still, scientists have never been bothered by the warnings of our fiction.
Perhaps that why Yang Bai, a researcher from the Department of Physics at the University of Wisconsin-Madison, and colleagues, have proposed that dark matter planets could exist under the right conditions and there may even be ways for us to find them with existing astronomical instruments. Their study was posted to the pre-print server ArXiv.
Their proposed methods are similar to those already in use for finding ordinary exoplanets. Usually, we find distant planets by measuring the way they influence their parent star. A planet which orbits directly in front of its star (an event called a transit) will block some of its light and the dimming tells us a planet is there. We can also measure some of the light absorbed and re-emitted by the planet’s atmosphere to tell what the planet is made of. Even planets which don’t go in front of their star can have a measurable influence. Any orbiting planet pulls on the star gravitationally, causing it to wobble. If the wobble is large enough, we can pick it up on our telescopes.
Those methods can reveal a surprising amount of information about a planet. Scientists have been able to tell everything from the size of a planet, to the length of its day and year, its density, and atmospheric composition, even when those planets are hundreds or thousands of light years away. According to Bai and colleagues, the same methods could be used in the hunt for dark matter planets, if they do indeed exist.
Researchers focused primarily on the transit light curve detection method. That’s the curve measured in a star’s apparent brightness when a planet passes in front of it. The measurement begins steady, then dips down and back up again as the planet passes in front and then away again. In simulated experiments, they found that a planet-sized collection of dark matter orbiting a star might create a distinguishable light curve. According to the paper, if a dark exoplanet has a radius larger than 10% of its host star and a low optical depth, we should be able to see them with existing instruments. The underlying thinking is that dark matter, and therefore dark exoplanets, may react weakly with electromagnetic fields. If that’s the case, dark matter may not be totally transparent.
A transiting dark exoplanet might therefore look, in reality and in our measurements, like a sort of semi-transparent ghost planet, blocking out only a small portion of the star’s light but letting the rest of it pass through. If we do succeed at finding a dark exoplanet, the light measurements we get could help unravel some of dark matter’s mysteries.
And it probably won’t unleash an army of nocturnal alien predators, but you never know. Catch Pitch Black, available now from Universal Entertainment.