In order to better characterize the noise introduced by stars when searching for new exoplanets, the HARPS-N spectrometer, mounted on the Telescopio Nationale Galileo, was equipped in 2015 with a small dedicated solar telescope. This solar feed, in tendem with HARPS-N, acquires disc-integrated high-resolution spectra of our Sun for several hours every possible days, with a cadance of 5-minutes.

The Radial Velocity (RV) technique was the first efficient method to detect worlds orbiting stars other than the Sun. The detection of the first exoplanet using this method led Michel Mayor and Didier Queloz, two scientists from the University of Geneva, to win the Physics Nobel Prize in 2019. Today, this technique remains widely used. Enormous progress has been made on the instrumentation side and the latest generation of hardware should be capable of detecting other Earths, even though the induced signal is very weak. At present, however, perturbing signals from the stars themselves limit our sensitivity. The next breakthroughs will come from understanding how to separate stellar signals from planetary motion.

The key difficulty is clearly seen on the Sun, whose visible surface seethes with millions of rising and falling pockets of gas whose pattern changes every few minutes. The magnetic fields surrounding dark sunspots suppress their motion. The sunspots themselves block light, changing their velocity as the Sun spins on its axis once a month. And all those phenomena vary in time with the overall level of activity rising and falling with the 11-year sunspot cycle. These processes change the Sun’s apparent velocity by amounts hundreds of times greater than the signal of an Earth-like planet. Understanding these complex processes requires exceptional data if we are to develop techniques to compensate for them. Transferring these techniques to other stars, whose surfaces we cannot observe directly, will enable the detection of other Earths, aiding the long-term goal of the search for life on alien worlds.

In order to obtain the required data set, an international team composed of scientists from the University of Geneva (Switzerland) and the Harvard-Smithsonian Center for Astrophysics (USA) built a low-cost solar telescope (LCST) in 2015 [1,2] and connected it to the HARPS-N spectrograph, the most precise instrument to discover exoplanets at the time [3]. On July 18th 2015, the team started observing the Sun every clear day with a 5-minute cadence from the Telecopio Nationale Galileo (TNG) facility in La Palma, Spain.

This data set already enabled the examination of the physical processes that drive intrinsic stellar radial-velocity variations [4,5,6]. However, reaching and surpassing the precision needed to detect other Earths requires open, international, and cross-disciplinary collaboration and it is in that spirit that the team releases every few years carefully curated data to be used by the community. The latest release, done in 2025, gives access to a decade of solar observations [7], which are accessible here thanks to the Data & Analysis Centre for Exoplanets web-interface (https://dace.unige.ch/) hosted at the University of Geneva.

[1] Dumusque et al. 2015 ApJL, 814, 2, id. L21
[2] Phillips et al. 2016, SPIE, 9912, id. 99126Z
[3] Cosentino et al. 2012, 8446, article id. 84461V
[4] Collier Cameron et al. 2019, MNRAS, 487, 1082
[5] Milbourne et al. 2019, ApJ, 874, 107
[6] Dumusque et al. 2021, A&A, 648, 103
[7] Dumusque et al. 2025, in prep.