Abstract: 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 contrast, 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 clarify the nature of dense matter.