Figure 1: Artist’s impression of what could be a brown dwarf in orbit around a main-sequence star.
Bayliss et al. (2016) present the discovery of an ultra-dense brown dwarf in a 40.737 day orbit around its host star. This brown dwarf is identified as EPIC201702477b and it was first reported as a planet candidate based on two transit events observed by K2. Follow up observations, together with high precision radial velocity measurements, confirmed its nature as a brown dwarf. EPIC201702477b has 66.9 ± 1.7 times the mass and 0.757 ± 0.065 times the radius of Jupiter, giving it a remarkably high density of 191 ± 51 g/cm³, around ~25 times the density of iron.
EPIC201702477b has the smallest known radius for any brown dwarf, and it is also denser than any planet, substellar mass object or main-sequence star discovered so far. The host star of EPIC201702477b has 0.870 ± 0.013 times the mass and 0.901 ± 0.057 times the radius of the Sun. Its effective temperature is 5517 ± 20 K, and its estimated age is 8.8 ± 4.1 billion years old. Currently, there are only 12 known brown dwarfs (i.e. objects with 13 to 80 times the mass of Jupiter) that transit main-sequence stars.
Figure 2: Transit light curves indicating the presence of EPIC201702477b, phase-folded to the best fitting orbital period of 40.73691 ± 0.00037 days. Bayliss et al. (2016)
Figure 4: The density-mass relationship for the known transiting brown dwarfs. EPIC201702477b stands out as the highest density object yet discovered, very near to the peak density predicted by the model. Bayliss et al. (2016)
Reference:
Bayliss et al. (2016), "EPIC201702477b: A Long Period Transiting Brown Dwarf from K2", arXiv:1606.04047 [astro-ph.EP]