Close-in Exoplanets as Candidates for Strange Quark Matter Objects

Abstract

Since the true ground state of the 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 . As a result, they will be tidally disrupted when its orbital radius is less than ∼ 5.6 × 10 10 cm, or when the orbital period (P orb ) is less than ∼ 6100 s. On the contrary, a strange planet can safely survive even when it is very close to the host, due to its high density. The feature can help us identify SQM objects. In this study, we have tried to search for SQM objects among close-in exoplanets orbiting around pulsars. Encouragingly, 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 criteria of P orb < 6100 s, and are thus good candidates for SQM planets. The orbital periods of two other planets (PSR J1719+14 b and PSR J2051-0827 b) are only slightly higher than the criteria. 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 days. We argue that they might also be SQM planets. It is further found that the persistent gravitational wave emissions from at least three of these closein planetary systems are detectable to LISA. More encouragingly, the advanced LIGO and Einstein Telescope are able to detect the gravitational wave bursts produced by the merger events of such SQM planetary systems, which will provide a unique test for the SQM hypothesis.

Gravitational-wave Emission from a Primordial Black Hole Inspiraling inside a Compact Star: a Novel Probe for Dense Matter Equation of State

“…Such GWs may be recognized as originating from a "planet"-compact star system. Geng et al (2015) and Kuerban et al (2020) have suggested that GWs from such "planet"-compact star systems could be efficiently used to identify strange quark planets, because a normal matter planet will be tidally disrupted by the compact star when it is still further away so that no GWs are available (Geng et al 2015;Kuerban et al 2020). Here, we would like to further remind that the possibility that the "planet" is actually a PBH should be further repelled before finally identifying it as a strange quark planet.…”

“…However, a 1.4 M SS has a radius very similar to that of a normal NS with comparable mass, thus it is hard to distinguish between these two types of compact stars via observations (Geng et al 2015(Geng et al , 2021. An interesting method to identify strange quark objects is to search for very close-in binary systems containing a strange planet and a compact star (Geng et al 2015;Kuerban et al 2019Kuerban et al , 2020. When the orbital radius of a planet is less than ∼ 5.6 × 10 10 cm or the orbital period is less than ∼ 6100 s, then it cannot be a normal matter planet, but should be a strange planet, because the tidal force is too strong to allow any kinds of normal matter planets to stably exist there (Huang & Yu 2017;Kuerban et al 2019Kuerban et al , 2020.…”

“…An interesting method to identify strange quark objects is to search for very close-in binary systems containing a strange planet and a compact star (Geng et al 2015;Kuerban et al 2019Kuerban et al , 2020. When the orbital radius of a planet is less than ∼ 5.6 × 10 10 cm or the orbital period is less than ∼ 6100 s, then it cannot be a normal matter planet, but should be a strange planet, because the tidal force is too strong to allow any kinds of normal matter planets to stably exist there (Huang & Yu 2017;Kuerban et al 2019Kuerban et al , 2020. However, in these close-in "planetary" systems, there is still a possibility that the planetary-mass object is actually a PBH.…”

Oscillating Strange Quark Matter Objects Excited in Stellar Systems

“…Recent papers [1,2] renew the interest in strange quark matter SQM objects of planetary masses [3] as candidates for high density planets orbiting some pulsars. Also, SQM planets around neutron stars are proposed to be new sources of gravitational radiation that could be detected by new generation detectors [4].…”

“…In [1,2] the pulsar PSR B1257+12 is proposed as one of the hosts of candidates for SQM planets. It is the first stellar object discovered to harbour three planets [9] of 4.3 M ⊕ , 3.9 M ⊕ and 0.025 M ⊕ .…”