Description of the radial velocity detection method for exoplanets

iLocater: A Diffraction-Limited Radial Velocity Spectrograph

Exoplanets, or planets outside of our solar system, is one of the most fascinating areas of modern astrophysics. Thanks to the Kepler mission, we have discovered thousands of new worlds, ranging from the desolate and uninhabitable to ones that have the potential to be much like Earth. One way these planets are discovered is the radial velocity method (shown above). This relies on detecting small shifts in stellar absorption lines – if these shifts are periodic, they can signal the movement of the star that would occur if a planet orbits. This is because of Newton’s Third Law – though we traditionally think only of planets orbiting stars, the stars actually orbit the planets as well due to a gravitational force towards the center of mass.

In order to detect these shifts, a very high resolution is necessary. Current spectrographs achieve this resolution by using state-of-the-art detectors, fiber optic cables, and cryogenically cooled environments. However, the fiber optic cables are multimodal in existing spectrographs – iLocater will be the first spectrograph to use a single mode fiber. Single mode fibers offer substantial improvements in resolution over multimodal fibers as they eliminate constructive and destructive interference within the fiber. This resolution comes at a cost though – it is substantially harder to couple light into a single mode fiber.

I worked with this team, led by Justin Crepp (Notre Dame), on control software for our “demonstrator” system, a proof-of-concept system that was recently tested at the LBT. It showed that light could successfully be coupled into a single-mode fiber for this purpose (Bechter et al 2016) , and work is being done to develop the final system that will go online within the next few years. This system will obtain sub-m/s radial velocity measurements and herald a new era of precision in exoplanet discovery and classification.