Overcoming stellar activity in radial velocity planet searches

Project Information

GPRV

Grant agreement ID: 865624

DOI

10.3030/865624

EC signature date 13 January 2020

Start date 1 January 2021

End date 31 December 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 999 833,00

EU contribution

€ 1 999 833,00

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Periodic Reporting for period 3 - GPRV (Overcoming stellar activity in radial velocity planet searches)

Reporting period: 2024-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

This project aims to improve the methods used to extract and model stellar Radial Velocity (RV) observations to enable the detection of exoplanets, including those that could potentially support life, around nearby stars. The main challenge in the field is the intrinsic variability of the host stars, which causes complex apparent RV variations that are considerably larger than the planetary signals of interest. As it is difficult to model these stellar nuisance signals from first principles, we use data-driven models based on Gaussian Processes (GPs) developed by the PI and her team. Over the course of the project we will test and refine these models on archival data for a wide range of stars, use them to find and characterise new planets (in particular young planets orbiting active stars, and comparatively small and cool planets), and take part in the preparation and early operations phase of the Terra Hunting Experiment (THE).

In this report, we summarise the work done during the first 30 months (2.5 years) of the 5-year grant period. During that time, we have focused mainly on methodological developments, improving upon the state of the art methods previously developed in the PI's group to disentangle between stellar and planetary signals in the time domain, by also modelling them in the wavelength domain, and starting to couple the two. We have also successfully applied these methods to observations of young, planet hosting stars, allowing us to measure, for the first time, the masses of several young transiting hot Netpunes and super-Earths, which are key laboratories of planet formation and evolution. In the background, we are also preparing for the THE survey, contributing to the consortium-wide effort to select targets and optimize the observing strategy.

Over the remainder of the grant, we will focus increasingly on analysing new and archival observations of Sun-like stars, pushing the detection limits towards Earth analogues, and exploring the "warm super-Earth" or "Super-Venus" regime which is very much unchartered so far. In parallel, we will continue to deepen our understanding of the physical processes that give rise to the "nuisance" activity signals, probing inhomogeneous, 3-D stellar atmopsheres in increasing details using a suite of spectroscopic diagnostics tailored on the Solar case (for which simultaneous resolved observations can be compared directly with the disk-integrated spectra).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Over the first 30 months, we have:
- published a publicly available software implementation of our GP framework, available on github and documented in Barragan et al. (2021)
- analysed the data from an intensive RV monitoring campaign on the young M-dwarf AU Mic, which has enabled us, for the first time, to measure the mass of the second (outer) transiting planet in the system (Zicher et al. 2022), despite the star's extreme activity level (the planet signals have amplitudes 60 times below the activity signals). We were also able to detect signs of magnetic interaction between the star and the innermost planet, and to repeat our mass measurement using an independent method (Doppler imaging), which boosts our confidence in the results (Klein et al. 2022).
- organised a very successful workshop on the topic of stellar activity in RV planet searches, in All Souls College Oxford (https://sites.google.com/view/gprv-workshop-2022/) with 45 in-person and 35 online participants
- applied our framework to >350 stars observed by the HARPS spectrograph over the last 15 years, with the goal of exploring how effective the method is at removing activity signals depending on factors such as stellar type, rotation rate, activity level and the time-sampling of the observations. The first publication based on this work, focusing on the measurement of stellar rotation periods from activity indicators, will be submitted for publication in the next few weeks.
- re-analysed observations of the K2-233 system to obtain more precise masses for the 3 small planets in this system, which is vital to understand observations of the planets atmospheres via transmission spectroscopy. This work is being prepared for publication (Barragan et al. 2023.).
- Used observations of the Sun-as-a-star with the HARPS-N spectrograph to test observing strategies for the future Terra Hunting Experiment (THE) and to develop and test novel methods to model spectral line profiles in order to reduce the impact of stellar activity on the extracted RVs. This work will form the basis of two papers in the coming year (Aigrain et al. in prep., Klein et al. in prep.)
- Written an Annual Reviews article on Gaussian Processes for time-series astronomy
- presented our research at a number of international conferences and given a number of invited seminars and lectures.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have now demonstrated that it is possible to detect planet signals with amplitudes 50 to 100 times smaller than the stellar signals they are "buried" in, for young and active stars. This level of "activity suppression" is what is needed to achieve the overarching goal of the project, namely to enable the detection of Earth-like planets around Sun-like stars. In Sun-like stars, however, the activity signals are smaller but less coherent, and hence harder to correct as effectively. Current state-of-the-art methods presented a the recent GPRV workshop we organised in March 2022 use a range of approaches, from the purely data-driven to the very theoretical, and from the time-domain to the wavelength and velocity domains. The next step, which will be key to reaching the required perfomance in Sun-like stars, is to combine these different approaches, and that is what we will aim to do in the next couple of years. We have already made a very promising start, developing a method to model time-series of "mean spectral line profile" observations which, to the best of our knowledge, is the only one to simultaneously address the time- and the velocity-depedence of activity signals.

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