Since we’ve recently been discussing astrometry, the discipline that measures star distances and movements, now would be a good time to look at two significant projects that go beyond optical methods to use radio astrometry in planet hunting. The Radio Interferometric Planet Search (RIPL) will draw on the Very Long Baseline Array, ten dish antennae spanning more than 5000 miles, and the 100-meter Green Bank telescope in West Virginia. The target: 29 active low-mass stars to be examined in a three-year planet hunt.
The targets are significant because they’re a kind of star that’s currently out of reach for radial velocity techniques. All are M dwarfs that are active, meaning they display ‘starspots’ (analogous to sunspots), flares or other activity in their chromospheres. The more active a faint star like this, the more likely that radial velocity measurements will be distorted with a ‘jitter’ that disturbs the precision of the measurement. RIPL ought to be able to sort out the situation and bring planet hunting to this niche.
The new observing project ought to be useful, then, in fleshing out our slowly growing knowledge of M dwarf systems. The authors of a recent paper outlining the project go so far as to say this:
Radio astrometric searches can determine whether or not M dwarfs, the largest stellar constituent of the Galaxy, are surrounded by planetary systems as frequently as FGK stars and if the planet mass-period relation varies with stellar type. The population of gas giants at a few AU around low mass stars is an important discriminant between planet formation models.
We’ve seen that issue in play recently with the planet discovered around GJ 674, which seems to reinforce the core accretion model of planet formation. What RIPL brings to the table is the ability to find lower mass planets in long-period orbits, and to define precise astrometric positions for these stars and their planets. Interestingly, radio astrometry is quite useful in making measurements of distant objects, including the proper motion of some pulsars and even the motion of Sagittarius A*, the apparent black hole at the center of the Milky Way. So employing radio techniques in the planet hunt is complementary to existing searches.
Beyond RIPL, the proposed radio telescope called the Square Kilometer Array (SKA) would have a collecting area roughly 200 times that of the VLBA. Here we achieve the possibility of detecting Earth mass planets around such stars, although the instrument will be sensitive enough to extend the search from M dwarfs to stars much like the Sun. For this one, though, we wait. Construction is not planned for more than a decade.
The overview paper is Bower et al., “Radio Astrometric Detection and Characterization of Extra-Solar Planets: A White Paper Submitted to the NSF ExoPlanet Task Force,” available here.