The world we see around us is built around quarks. They form the nuclei of the atoms and molecules that make up us and our world. While there are six types of quarks, regular matter contains only two: up quarks and down quarks. Protons contain two highs and lows, while neutrons contain two lows and lows. On earth, the other four types are only seen when they are generated in particle accelerators. However, some of them could also occur naturally in dense objects such as neutron stars.
Neutron star against a quark star. Photo credit: CXC / M. Weiss
The standard model for neutron stars states that neutrons remain largely intact inside. So a neutron star is like a giant atomic nucleus that is held together by gravity rather than strong nuclear force. However, we do not fully understand how neutrons interact at extreme temperatures and densities. It is possible that within a neutron star the neutrons decay into a quark soup and form a so-called quark star. Quark stars would look like neutron stars, but would be a bit smaller.
When quark stars exist, it is possible for high-energy up and down quarks to collide, creating strange quarks. And this is where things could get a little strange. Strange quarks are much heavier than up and down quarks, so strange quarks tend to form a new type of nucleon known as strangelets. A simple strangelet would consist of an up, down and a strange quark. Since strangelets are much denser than protons and neutrons, contact between the two would break the protons and neutrons to create more strangelets. Essentially, when strange matter comes into contact with normal matter, it does not take long to transform into strange matter. You could have anything from strange stars to strange planets.
Strange quarks can appear in regular nucleons. Photo credit: APS / Alan Stonebraker
Strange matter is an interesting idea, but not a popular one. If strange quark matter forms in some neutron stars, it should form in all of them first and lead to collapse. But we see a lot of neutron stars that are too big to be strange quarks. There's also the fact that strange quarks can appear in regular protons and neutrons. For example, although a proton consists of two up quarks and one down quark, this is really just an average. Quantum fluctuations mean that strange quarks can appear for a short time. But they are not stable and do not convert nucleons into strange matter. So if strange matter exists, it probably only exists in large and dense objects.
Still, it's worth looking for objects with strange matter in the universe, and a recent study found some candidates. The study was looking for some type of object known as the strange dwarfs. These hypothetical objects have a mass that resembles a white dwarf, but instead of being made of regular matter in a degenerate state, they are made of strange quark matter. As a result, they would be much smaller than white dwarfs.
The mass-radius relationship for white dwarfs. Photo credit: Brian Koberlein
To find these objects, the team examined data from the Montreal White Dwarf Database (MWDD), which contains data on more than 50,000 white dwarfs. For about 40,000 of them, the database lists both the mass and surface gravity of the white dwarfs. A white dwarf's mass can be determined by the Doppler shift of its light as it orbits a companion star or through gravitational lenses, while surface gravity can be measured by the gravitational redshift of its light.
If you know a star's mass and surface gravity, you can easily calculate its radius. The team did this and then compared it to the mass and radius relationship for white dwarfs. Most of them followed the relationship, but 8 of the stars did not. They were much smaller and agreed with the predictions for a cottage cheese dwarf.
The data from this work is not strong enough to prove that these objects are strange dwarfs, but they are worth investigating further. There is something strange about them and it would be good to see if this is due to strange quarks or something else.
Reference: Abudushataer Kuerban et al. "Search for strange objects of quark matter among white dwarfs." arXiv preprint arXiv: 2012.05748 (2020).