DISCOVERY OF THE COLDEST IMAGED COMPANION OF A SUN-LIKE STAR

We present the discovery of a brown dwarf or possible planet at a projected separation of 1 ''.9 = 29 AU around the star GJ 758, placing it between the separations at which substellar companions are expected to form by core accretion (similar to 5 AU) or direct gravitational collapse (typically greater than or similar to 100 AU). The object was detected by direct imaging of its thermal glow with Subaru/HiCIAO. At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B constitutes one of the few known T-type companions, and the coldest ever to be imaged in thermal light around a Sun-like star. Its orbit is likely eccentric and of a size comparable to Pluto's orbit, possibly as a result of gravitational scattering or outward migration. A candidate second companion is detected at 1 ''.2 at one epoch.