There could be several stars made of antimatter in our solar system’s neighbourhood. There have been small hints that these strange and unlikely objects, called antistars, could exist, and a search for the gamma rays that they are expected to produce has turned up 14 candidates.
When matter and antimatter meet, they annihilate in a shower of radiation, including high-energy gamma rays. This is expected to happen fairly often at the surfaces of antistars – if they exist – as regular matter falls onto them.
Simon Dupourqué at the University of Toulouse in France and his colleagues examined data from the Fermi Gamma-ray Space Telescope for objects emitting the sort of radiation expected from these annihilations that weren’t already explained by some other astronomical phenomenon.
From the 14 candidates that they found, they calculated that there could be as many as one antistar per 400,000 regular stars in our galaxy. “That may seem high, but it’s an upper limit,” says Dupourqué. “That’s assuming that all 14 candidates are antistars, but they’re probably not.”
There is no formation mechanism for antistars that fits into our standard model of cosmology, so it is fairly unlikely that they exist – but there are models in which they are possible, and there is one small piece of observational evidence that suggests they might be real. Since it was bolted to the outside of the International Space Station in 2011, the Alpha Magnetic Spectrometer (AMS) experiment has detected tentative signals of eight antihelium atoms.
There is very little naturally occurring antimatter in the universe, and most of it is in the form of smaller antimatter particles produced by cosmic rays. “They found 100 million helium [atoms] for every one antihelium,” says Vivian Poulin at the University of Montpellier, France. “That rate, as small as it is, is actually around 500 times greater than you expect from regular astrophysical processes.”
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The simplest way to produce antihelium is in an antistar, which would fuse antihydrogen into antihelium in the same way that a star does with regular hydrogen and helium. Even though antimatter annihilates immediately when it meets regular matter, space is so empty that Dupourqué and his colleagues calculated that an antistar could survive well beyond the current age of the universe without disappearing completely.
However, if antistars exist, they are incredibly tough to distinguish from afar. “It’s not like, ‘oh my god they’re green!’ The anti-sun would probably look similar to the sun,” says Poulin.
Even up close, an antistar would behave just like a regular star, except when matter fell onto its surface and annihilated. “If you jumped into an antimatter star – but don’t do that – it would not be so different from jumping into a regular star, except that maybe there would be more gamma rays,” says Dupourqué.
That means that proving that these 14 candidates are truly antistars is next to impossible, he says. It would be far easier to prove that they aren’t antistars, either by searching for less exotic explanations for the gamma rays they give off or by observing them in different wavelengths of light looking for signatures of other types of object. If even one of them is an antistar, though, we will have to reconsider our entire understanding of the early universe to figure out how it could have formed.
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Journal reference: Physical Review D, DOI: 10.1103/PhysRevD.103.083016