# Radio Astrometric Detection and Characterization of Extra-Solar Planets: A White Paper Submitted to the NSF ExoPlanet Task Force

## TL;DR

Radio interferometry's high astrometric precision offers a promising avenue for discovering and characterizing exoplanets, especially at larger orbital radii, complementing other detection methods and enabling future Earth-mass planet detection.

## Contribution

This paper introduces RIPL, a new radio astrometric program using VLBA and Green Bank telescopes, and discusses future prospects with the SKA for exoplanet detection.

## Key findings

- Current VLBA achieves better than 100 microarcsecond accuracy.
- Upgrading VLBA bandwidth could improve accuracy by an order of magnitude.
- Future SKA could reach 1 microarcsecond accuracy, enabling Earth-mass planet detection.

## Abstract

The extraordinary astrometric accuracy of radio interferometry creates an important and unique opportunity for the discovery and characterization of exo-planets. Currently, the Very Long Baseline Array can routinely achieve better than 100 microarcsecond accuracy, and can approach 10 microarcsecond with careful calibration. We describe here RIPL, the Radio Interferometric PLanet search, a new program with the VLBA and the Green Bank 100 m telescope that will survey 29 low-mass, active stars over 3 years with sub-Jovian planet mass sensitivity at 1 AU. An upgrade of the VLBA bandwidth will increase astrometric accuracy by an order of magnitude. Ultimately, the colossal collecting area of the Square Kilometer Array could push astrometric accuracy to 1 microarcsecond, making detection and characterizaiton of Earth mass planets possible. RIPL and other future radio astrometric planet searches occupy a unique volume in planet discovery and characterization parameter space. The parameter space of astrometric searches gives greater sensitivity to planets at large radii than radial velocity searches. For the VLBA and the expanded VLBA, the targets of radio astrometric surveys are by necessity nearby, low-mass, active stars, which cannot be studied efficiently through the radial velocity method, coronagraphy, or optical interferometry. For the SKA, detection sensitivity will extend to solar-type stars. Planets discovered through radio astrometric methods will be suitable for characterization through extreme adaptive optics. The complementarity of radio astrometric techniques with other methods demonstrates that radio astrometry can play an important role in the roadmap for exoplanet discovery and characterization.