Strange Quark Matter (SQM) Planets
At extreme densities, normal matter may exist in the form of strange quark matter (SQM). A consequence of this hypothesis is the existence of SQM stars which are hard to distinguish from neutron stars. SQM stars and neutron stars are both ultra-compact stars that measure only several kilometres across but can contain 1 to 2 times the Sun’s mass. Unlike neutron stars which have a minimum mass limit, SQM stars can have arbitrarily small masses. Since SQM is stable in bulk, planetary-mass clumps of SQM can exist (i.e. SQM planets). The detection of SQM planets would be very useful for testing the SQM hypothesis.
Gravitational waves are ripples in the curvature of spacetime that propagate outward from their source. Sources of detectable gravitational waves include binary systems composed of compact objects such as white dwarfs, neutron stars or black holes. A more tightly bound binary system emits stronger gravitational waves. For a normal matter planet orbiting an ultra-compact star, the emitted gravitational wave power is negligible (i.e. non-detectable) since the planet cannot come close enough to the star without being tidally disrupted.
However, things become very different for a SQM planet orbiting a SQM star. Due to its extreme compactness, a SQM planet can spiral very close to its host SQM star without being tidally disrupted. Such a compact system then becomes very efficient in producing strong gravitational waves. Upcoming gravitational wave detectors such as Advanced LIGO and the Einstein Telescope can detect gravitational waves arising from the in-spiral of a SQM planet into its host SQM star.
There are a number of possible mechanisms that can result in the formation of a SQM planet. One mechanism involves newly-born SQM stars that are very hot and exceedingly turbulent. The strong turbulence can eject planetary-mass clumps of SQM. If these clumps remain gravitationally bound, SQM planets are produced. A SQM planet with 1/10th the mass of Jupiter (i.e. 32 Earth masses) would measure only ~1000m in diameter.
J. J. Geng, Y. F. Huang, T. Lu (2015), “Coalescence of Strange-Quark Planets with Strange Stars: a New Kind of Sources for Gravitational Wave Bursts”, arXiv:1501.02122 [astro-ph.HE]