Tracking Solar Supergranulation: How the Sun’s Surface Patterns Evolve Over Its Magnetic Cycle

Understanding the Sun's surface is key to finding Earth-like planets around other stars. In this study, O’Sullivan et al. explore how a particular solar phenomenon called supergranulation--large, slow-moving patterns of gas on the Sun’s surface--affects our ability to detect exoplanets using the radial velocity (RV) technique. These patterns, while subtle, can mimic or hide the faint signals of planets orbiting distant stars.

Data Collection and Activity Correction Methods

The team used eight years of high-precision solar observations from the HARPS-N spectrograph to measure how the Sun’s RV signal changes over time. By observing the Sun as if it were a distant star (a method called “Sun-as-a-star” observing), they could study the influence of supergranulation without the confusion caused by sunspots or other magnetically active regions. To isolate the “quiet Sun” signal, the authors used two independent methods: one based on solar images from the Solar Dynamics Observatory (SDO) and another using YARARA, a software tool that corrects the spectral data directly.

Modeling Solar Surface Signals with Gaussian Processes

To analyze the data, the team applied a statistical technique called Gaussian Process regression. This allowed them to separate out the effects of granulation (smaller, shorter-lived surface flows) and supergranulation, and to measure their properties--specifically their strength and timescale. While the granulation signal remained fairly stable, the timescale of supergranulation varied significantly. It was found to be longest when solar activity was at a minimum and shortest during periods of high activity, indicating a strong anti-correlation with sunspot numbers.

Robustness Across Methods

The researchers observed this trend regardless of the method used to clean the data, suggesting that it is a genuine physical effect. These changes in supergranulation over the Sun’s 11-year magnetic cycle could complicate RV-based planet searches if similar behavior occurs in other stars. Choosing to observe stars during low-activity phases (a common strategy to reduce sunspot noise) may unintentionally amplify the impact of supergranulation noise.

Planning Observations Beyond the Sun

To help prepare for future exoplanet searches, the authors also explored how much telescope time is needed to detect supergranulation patterns in other stars. Their simulations showed that a carefully designed 23-night observing campaign using current technology could measure these patterns for up to 10 stars. Strategies that frequently switch between stars each night yielded the most efficient results.

Implications for Future Planet Searches

In conclusion, this work highlights the importance of accounting for stellar surface variability--like supergranulation--when searching for Earth-like planets. The findings emphasize that even the Sun, our most familiar star, still holds surprises that challenge our ability to look beyond it.

Source: O’Sullivan