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Signal Correction to Reveal other Earths

Periodic Reporting for period 2 - SCORE (Signal Correction to Reveal other Earths)

Berichtszeitraum: 2021-07-01 bis 2022-12-31

Searching for life signatures on other planets is one of the key endeavours of astrophysics, and today,
we are in a unique position to make this possible. The TESS satellite has detected interesting Earth candidates
and PLATO in a few years from now will detect real Earth-twins orbiting bright stars, which will allow follow-up
studies with JWST, ELTs and next generation space telescopes (LUVOIR, Habex, Life) to characterize the atmosphere
of those exoplanets.

However, TESS and PLATO will only measure the radius of the detected Earth-twins, which is not enough to interpret
the spectroscopic features in their atmospheres. The mass is also required, and it can be obtained using the
radial-velocity (RV) technique, which measures the gravitational influence of an exoplanet on its host star.

To measure the mass of the Earth-twins that TESS and PLATO will detect, the community have built incredible RV
instruments that can reach a RV precision of 0.3 m/s (ESPRESSO results on Proxima). Such an extreme precision
is required to measure the tiny signature of an Earth-twin, however, this is without considering the perturbing
signals induced by its host star, by Earth’s atmosphere and by instrumental noise. Indeed, we know that these
perturbing signals mask completely the signal induced by an Earth-twin, and now that the RV instruments have
the sensitivity to detect such planets, it is urgent to develop novel methods for mitigating the different perturbing
signals.

Understanding the different perturbing signals is extremely challenging and require incredible data. The PI
have built two telescopes that feed Sun-light into the best RV instruments. The obtained data are of
exceptional quality, and the goal of SCORE is to analyse them, explore novel promising methods for
mitigating the different perturbing signals and find the tiny signatures of Earth and Venus. This will
open the way towards the mass-measurement of Earth-twins, which is essential in the quest for finding
life elsewhere, but also to understand planetary formation and dynamics. SCORE will therefore benefit
the entire exoplanet community.
Techniques performed so far to mitigate the perturbing stellar activity signal have been based on the radial-velocity (RV) and a few other time series.
One of the main ideas of the SCORE project is to look for stellar signatures directly in the spectral time series, providing much more information.

The PI developed new data reduction software for HARPS-N and HARPS to mitigate at best the different instrumental signals. Published in Dumusque et al. 2021, the new data reduction applied to the HARPS-N solar RV data show an
improvement of up to 20% in RV precision. This new data reduction allows us to obtain more precise high-resolution spectra, which is needed to search for stellar activity signatures.

To look for stellar and instrumental signals in time series of high-resolution spectra, we developed new methods called RASSINE and YARARA (Cretignier et al. 2020, 2021). YARARA can also correct for the observed systematics
using physically-driven or data-driven approaches. After correction, we see a gain of 20-30% in RV precision and better stability on the long-term. When reanalysing all the data from HARPS-N, we discovered a dozen of very small
planetary candidate signals. Publication of 6 of those planets is currently in progress.

We also investigated the origin of the stellar signal in RV measurements. By evaluating at which physical depth in the photosphere each point of a high-resolution spectrum is formed, and then measuring the RV signal per physical depth
range, we can show that stellar activity is more affecting spectral regions that are formed deep into the photosphere. In Al Moulla et al. 2022, we show that the activity signal seen in RV, both at the rotational and the
magnetic cycle timescales significantly changes from deep into the photosphere towards the surface of the star.

Finally, to develop further stellar activity mitigation techniques that work at the spectrum level without being perturb by instrumental signatures, which is the case for real data sets, we needed a simulation that could model stellar activity
at the spectrum level. Postdoc Yinan Zhao therefore developed SOAP-GPU. Preliminary results using the observed position of solar active feature to model the solar RVs, and comparing with the HARPS-N solar RV measurements, show a very good
match between the simulation and observation. This shows that the main physics is included in SOAP-GPU and that the code can be used to simulate realistic solar activity affects at the spectral level.
The state of the art in the field of stellar signal mitigation in RV consist in going away for the RV time series and looking for extra information in the high-resolution
spectra available behind each RV measurement. In this sense, the SCORE project is really focusing on the state of the art, and is trying to push it further by investigating
new stellar signal proxies and developing clever ways of analysing spectral time series to disentangle stellar and planetary signals.

We already demonstrated that be carefully analysing spectral time series we can gain 20-30% in RV precision and a better long-term stability. This is particularly important
if we want to reveal small-mass planets orbiting in the habitable zone of their host stars, which would correspond to other Earths. There is still a lot of progress to make,
but it is likely that the result of the SCORE project, will lead to the detection of very interesting Earth-like candidates, that will be prime targets for further characterisation
space-based missions such as LUVOIR or HABEX or ground based observations such as the ones conducted with extremely large telescopes.
Spectral cleaning using YARARA