Regarding that lonely planet, a reader asks:
How do they know how old it is? Without a star as a spatial age reference point, I have no idea how that can be done.
According to this paper on arXiv, while the planet is not orbiting any particular star, it is associated with a group of stars:
A member of the 23±3 Myr β Pic moving group, PSO J318.5-22 has Teff = 1160+30−40 K and a mass estimate of 8.3±0.5 MJup for a 23±3 Myr age.
A trifle obscure. I’m thinking Myr means millions years. “Teff” is “effective temperature“, and I will guess MJup is Mass of Jupiter. And the assumption is the planet was created at the same time as the stars, out of the same dust cloud, although they also use a formula (with which I’m not familiar) based on measurements of the planet to independently confirm the age.
Star ages are estimated using the method gyrochonology. From Wikipedia:
The basic idea underlying gyrochronology is that the rotation period P, of a main-sequence cool star is a deterministic function of its age t and its mass M (or a suitable proxy such as color). The detailed dependencies of rotation are such that the periods converge rapidly to a certain function of age and mass, mathematically denoted by P = P (t, M), even though stars have a range of allowed initial periods. Consequently, cool stars do not occupy the entire 3-dimensional parameter space of (mass, age, period), but instead define a 2-dimensional surface in this space. Therefore, measuring two of these variables yields the third. Of these quantities, the mass (or a proxy such as color) and the rotation period are the easier variables to measure, providing access to the star’s age, otherwise difficult to obtain.
