Dark matter remains frustratingly elusive. We haven’t seen any direct detection in any of our lab experiments, and on a cosmic scale we only have circumstantial evidence of its existence.
Now, a new hypothesis proposes that a large part of black matter can be bound inside tight balls the size of Neptune – so-called dark matter planets. But while these planets would remain invisible to us, they could collect helium and hydrogen atmospheres 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 too quickly around the center of their galaxies. Galaxies move too fast within their clusters. The cosmic microwave background, the afterglow pattern emitted when the universe was only 380,000 years old, could not have had the properties it has 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 just that. Some of this may just be bad luck; whatever dark matter is, it interacts with normal matter extremely rarely (if at all). Maybe our instruments aren’t sensitive enough, or we haven’t been running our experiments long enough, to collect enough data to see an accurate signal.
Or maybe there’s a ton of dark matter in the universe – just not here.
Related: Could the Large Hadron Collider discover dark matter?
hold on to a secret
The formation of dark matter planets would explain why no dark matter appeared in our experiments. Instead of being smoothly distributed throughout the galaxy, most dark matter would be found in these balls, with masses ranging from that of a asteroid for which Neptune. Unless one of those bullets passed through our detectors, we wouldn’t see it.
It’s an interesting idea, but like any scientific hypothesis, we need to test it.
And the document offers how to do just that. The authors explained how dark matter planets don’t just form in the early universe and stay there for billions of years.
Instead, dark matter planets would form earlier than virtually anything. In the beginning, the universe was still a plasma, with normal matter locked in a constant fight against radiation, which kept everything atomic and prevented the formation of larger objects. But dark matter does not interact with normal matter or with light and was therefore perfectly free to start accumulating in planets.
Later, the universe cooled enough to neutralize the plasma and allow normal matter to accumulate. Eventually, this question would become stars and galaxiesbut in the meantime, some of the matter could find itself gravitationally pulled towards 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 it was the first element to dissociate from the plasma state of the early universe. Next comes hydrogen, accumulating in a thick atmosphere around helium.
Diving into a dark planet would be very strange. The hydrogen layer would be hot, because it is gravitationally bound to a dense object, and the friction would cause it to glow. You could cross it and reach the helium layer below. And once you got there, you wouldn’t see…nothing. The planet’s core of dark matter 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 on dark matter planets, they could reach critical temperatures and densities and undergo rampant nuclear fusion. Sometimes it would take the form of a simple flash or ejection of matter, and sometimes it could completely explode the entire mass of hydrogen and helium, rivaling a supernova in the brightness of the resulting explosion.
All of this activity would not 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, revealing the presence of the underlying hidden planet.
The researchers found that these explosions would have energies and frequencies similar to those of X-ray bursts, which is a common observation in astronomy. It’s not a slam dunk, however, since researchers have yet to determine if and how these planetary explosions of dark matter would differ from the more familiar astrophysical variety. But if there is such a difference, we may be able to use our existing large catalog of recorded X-ray bursts to determine whether dark matter exists and whether it forms planets.
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