Dense strange quark matter(SQM)composed of up,down,and strange quarks may be the absolute ground state of strong-interaction matter.Based on this SQM hypothesis,pulsars may actually be strange stars.Strange stars are different from conventional neutron stars in features such as mass-radius relationship and cooling rate,but current astronomical observations cannot discriminate between them unambiguously yet.Strange stars can power fast radio bursts and gravitational-wave bursts.The ultimate stability of SQM enables self-bound planetary-mass SQM clumps,i.e.strange planets,to exist stably.A strange planet,being very dense,can revolve around a central object in a very close orbit with a period shorter than 6 100 s.By con-trast,a plant made up of normal matter shall be tidally disrupted at such a short distance.Therefore,these close-in planetary systems would strongly evidence the existence of strange planets once they were discovered.Moreover,strange dwarfs can stably exist under the SQM hypothesis.Future multiwavelength and multimessenger astronomical observations may help clari-fv the nature of dense matter.
dense nuclear matterstrange starfast radio burstgravitational wavenuclear astrophysics
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