The Free-Floating Planetary-Mass Object PSO J318.5-22
As object with less than ~75 times the mass of Jupiter is referred to as a substellar object as it is not massive enough to burn hydrogen in its core and shine as a star. Since substellar objects cool as they age, it sets up a mass-age-luminosity degeneracy whereby a massive and old brown dwarf can have the same luminosity as a young planetary-mass object. In order to determine the mass for a substellar object of a given luminosity, the object’s age has to be constrained.
One way to determine the age of a substellar object is if the object is associated with a co-moving group of stars that were once part of the same natal cluster. PSO J318.5-22 is a planetary-mass object that has been identified to be a member of the β Pictoris Moving Group, a co-moving group of stars estimated to be 23 ± 3 million years old. Allers et al. (2016) present new observations on the physical properties of PSO J318.5-22. Measurements of the motion of PSO J318.5-22 through space indicate that it has a 99.98 percent probability of being a member of the β Pictoris Moving Group.
Assuming an age of 23 ± 3 million years, PSO J318.5-22 is estimated to have 8.3 ± 0.5 times the mass of Jupiter, an effective temperature of about 1127 K, and ~1.46 times the size of Jupiter. Interestingly, the gravity on PSO J318.5-22 is only slightly stronger than the surface gravity on Earth. Observations also indicate that the rotation period of PSO J318.5-22 is 5 to 10.2 hours, somewhat longer than the typical ~3 hour rotation periods of substellar objects that are similar in temperature to PSO J318.5-22. This is expected as PSO J318.5-22 is a young object and is still in the process of contracting.
Allers et al. (2016), “The Radial and Rotational Velocities of PSO J318.5338-22.8603, a Newly Confirmed Planetary-Mass Member of the β Pictoris Moving Group”, arXiv:1601.04717 [astro-ph.SR]