HARPS-N observes the Sun as a star

TL;DR

This study uses a solar telescope feeding sunlight into HARPS-N to observe the Sun as a star, aiming to understand and correct stellar surface noise affecting radial velocity measurements for exoplanet detection.

Contribution

It introduces a novel setup for long-term, high-precision solar radial velocity observations to study stellar noise and develop correction techniques.

Findings

Achieved 50 cm/s radial velocity rms over a few hours.

Reduced weekly radial velocity rms to 60 cm/s after correction.

Demonstrated potential to detect Venus and Earth-like planets using improved methods.

Abstract

Radial velocity perturbations induced by stellar surface inhomogeneities including spots, plages and granules currently limit the detection of Earth-twins using Doppler spectroscopy. Such stellar noise is poorly understood for stars other than the Sun because their surface is unresolved. In particular, the effects of stellar surface inhomogeneities on observed stellar radial velocities are extremely difficult to characterize, and thus developing optimal correction techniques to extract true stellar radial velocities is extremely challenging. In this paper, we present preliminary results of a solar telescope built to feed full-disk sunlight into the HARPS-N spectrograph, which is in turn calibrated with an astro-comb. This setup enables long-term observation of the Sun as a star with state-of-the-art sensitivity to radial velocity changes. Over seven days of observing in 2014, we show an average 50\cms radial velocity rms over a few hours of observation. After correcting observed radial velocities for spot and plage perturbations using full-disk photometry of the Sun, we lower by a factor of two the weekly radial velocity rms to 60\cms. The solar telescope is now entering routine operation, and will observe the Sun every clear day for several hours. We will use these radial velocities combined with data from solar satellites to improve our understanding of stellar noise and develop optimal correction methods. If successful, these new methods should enable the detection of Venus over the next two to three years, thus demonstrating the possibility of detecting Earth-twins around other solar-like stars using the radial velocity technique.