Close-in Exoplanets as Candidates for Strange Quark Matter Objects

Since the true ground state of hadrons may be strange quark matter (SQM), pulsars may actually be strange stars rather than neutron stars. According to this SQM hypothesis, strange planets can also stably exist. The density of normal matter planets can hardly be higher than 30 g cm^-3. They will be tidally disrupted when the orbital radius is less than ∼5.6 × 10¹⁰ cm, or when the orbital period (P_orb) is less than ∼6100 s. However, an SQM planet can safely survive even when it is very close to the host, due to its high density. This feature can help us identify SQM objects. Here, we have tried to search for SQM objects among close-in exoplanets orbiting around pulsars. It is found that four pulsar planets (XTE J1807-294 b, XTE J1751-305 b, PSR 0636 b, PSR J1807-2459A b) completely meet the criterion of P_orb < 6100 s, and are thus good candidates for SQM planets. The periods of two other planets (PSR J1719-14 b and PSR J2051-0827 b) are only slightly larger than the criterion value. They could be regarded as potential candidates. Additionally, we find that the periods of five white dwarf planets (GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b) are less than 0.1 day; they might also be SQM planets. Gravitational wave emissions from these close-in planetary systems are calculated from the view of various gravitational wave detectors.