Exoplanet discovered by astrometry

Two JPL astronomers have found another exoplanet, which is the first to be found using astrometry. That is, the presence of the planet was inferred by careful study of the host star to detect a very faint wobble (with respect to nearby stars) caused by the planet’s mass as it orbits the star. Unsurprisingly, this detection method works best when the mass of the planet is large relative to that of the star, and indeed, the VB 10 star (a red dwarf) is very small as stars go (1/10 the size of our Sun), and its planet is estimated as being nearly the same size as the star, although less massive. Surprisingly, this may actually be the first time the astrometry technique has borne fruit. All previous claims of planet detection by astrometry could not be verified using other methods. If this one succeeds, it will be the first. The challenge is that extremely high precision and multiple observations over the course of years (ideally, multiple orbits of the planet) are required to detect the extremely small planet-induced stellar motion. In this case, the discovery comes as the result of 12 years of observations by the Palomar Observatory.

You can read more details in the pre-print of the scientific paper, “An Ultracool Star’s Candidate Planet,” by Pravdo and Shaklan. I particularly like Figure 7, in which a Keplerian orbit for the planet is shown, modeled from the collected observations of stellar perturbations. The figure includes both error bars on the observations and lines connecting the observations to the corresponding points on the model. You can even watch a video of the observations of the star’s motion with an accompanying view of where in its orbit the planet would be (although this is a little confusing because the orbit is represented off to the side instead of traveling with the star). The effect is subtle enough, and the observations are spaced far enough apart, that I don’t see it with my eye (even stepping frame by frame), but that’s to be expected. Still, error bars and all, this is a fascinating hint at what might be going on in the vicinity of VB 10, and a definite motivation to obtain followup observations with other techniques (although it is a difficult target for the radial velocity and transit methods since the planet’s orbital plane is likely close to perpendicular from our perspective). The paper notes that it’s possible that other planets lurk in the same system — perhaps even in the habitable zone.