Dark matter remains frustratingly elusive. We have not seen any direct detection in any of our laboratory experiments, and at cosmic scales we only have circumstantial evidence that it exists.
Now, a new hypothesis proposes that a large fraction of dark matter may be bound up inside tight balls the size of Neptune — so-called dark matter planets. But while those planets would remain invisible to us, they might collect atmospheres of helium and hydrogen that could undergo nuclear fusion, giving us a major clue to their presence.
Dark matter planets
We see evidence of dark matter all around us. Stars orbit the centers of their galaxies way too quickly. Galaxies move too fast within their clusters. The cosmic microwave background, the afterglow light pattern emitted when the universe was only 380,000 years old, could only have the properties it does thanks to dark matter. And there is so much more. No other alternative can explain the wealth of observational evidence.
And yet, we haven't been able to directly detect dark matter in experiments designed to do exactly that. Some of that may be just bad luck; whatever dark matter is, it interacts with normal matter extremely rarely (if at all). Perhaps our instruments may just not be sensitive enough, or we haven't been running our experiments long enough, to collect sufficient data to see a definite signal.
Or maybe there is a ton of dark matter in the universe — just not right here.
Holding on to a secret
The formation of dark matter planets would explain why no dark matter has turned up in our experiments. Instead of being smoothly distributed throughout the galaxy, most of the dark matter would be in these balls, with masses ranging from that of an asteroid to that of Neptune. Unless one of these balls just happened to pass through our detectors, we wouldn't see it.
It's an interesting idea, but as with any scientific hypothesis, we need to test it.
And the paper proposes how to do exactly that. The authors outlined how dark matter planets don't simply form in the early universe and just sit there for billions of years.
Instead, the dark matter planets would form earlier than pretty much anything. At early times, the universe was still a plasma, with the normal matter locked in constant struggle with radiation, which kept everything atomic and prevented the formation of larger objects. But dark matter doesn't interact with normal matter or with light and so was perfectly free to start collecting into planets.
Later, the universe cooled down enough to neutralize the plasma and allow normal matter to accumulate. Eventually, that matter would grow to become stars and galaxies, but in the meantime, some matter could find itself gravitationally attracted to the dark matter planets, and that's where things could get interesting.
The researchers found that these hypothetical dark matter planets would first accumulate a layer of helium, since that was the first element to disassociate from the plasma state of the early universe. Next came hydrogen, building up into a thick atmosphere around the helium.
To plunge into a dark planet would be very strange. The hydrogen layer would be warm, since it's gravitationally bound to a dense object, and the friction would cause it to glow. You could pass through it and reach the layer of helium beneath it. And once you passed through that, you'd see … nothing. The core of the dark matter planet itself would be completely invisible, and you would find yourself surrounded by a shell of glowing plasma.
The researchers found that if too much helium and hydrogen gathered onto the dark matter planets, they could reach critical temperatures and densities and undergo runaway nuclear fusion. Sometimes, this would take the form of a mere flash or ejection of material, and sometimes, it could completely detonate the entire mass of hydrogen and helium, rivaling a supernova in brightness from the resulting explosion.
All this activity wouldn't affect the dark matter planet, since dark matter doesn't care what normal matter does with itself. But we might be able to see these explosions, giving away the presence of the underlying hidden planet.
The researchers found that these explosions would have similar energies and frequencies as X-ray bursts, which are a common observation in astronomy. That's not a slam dunk, however, since the researchers still need to determine if and how these dark matter planet-driven explosions would be different from the more familiar astrophysical variety. But if there is such a difference, we may be able to use our existing extensive catalog of recorded X-ray bursts to determine if dark matter exists and if it forms planets.