From ESPRESSO to PLATO: detecting and characterizing Earth-like planets in the presence of stellar noise Tese de Doutouramento Luisa Maria Serrano Departamento de Fisica e Astronomia do Porto, Faculdade de Ciências da Universidade do Porto Orientador: Nuno Cardoso Santos, Co-Orientadora: Susana Cristina Cabral Barros March 2020 Dedication This Ph.D. thesis is the result of 4 years of work, stress, anxiety, but, over all, fun, curiosity and desire of exploring the most hidden scientific discoveries deserved by Astrophysics. Working in Exoplanets was the beginning of the realization of a life-lasting dream, it has allowed me to enter an extremely active and productive group. For this reason my thanks go, first of all, to the ’boss’ and my Ph.D. supervisor, Nuno Santos. He allowed me to be here and introduced me in this world, a distant mirage for the master student from a university where there was no exoplanets thematic line. I also have to thank him for his humanity, not a common quality among professors. The second thank goes to Susana, who was always there for me when I had issues, not necessarily scientific ones. I finally have to thank Mahmoud; heis not listed as supervisor here, but he guided me, teaching me how to do research and giving me precious life lessons, which made me growing. There is also a long series of people I am thankful to, for rendering this years extremely interesting and sustaining me in the deepest moments. My first thought goes to my parents: they were thousands of kilometers far away from me, though they never left me alone and they listened to my complaints, joy, sadness...everything. Thank you, without your sustain I would not be here writing this thesis. I also have to thank my historical friends, Federico and Silvia. I went away from Trieste, still they kept on being always present and getting updated with my life. A special thank goes to other Ph.D. students and researchers who shared with me nice moments: Akin, Raquel, Solene, João, Fatima and Elisa more than everybody, but I should mention a long list of people here. For this reason, I will just say: thank you CAUP, for the friendly environment you offered me. Thank you Nuno, Júlia and Jorge for sharing with me my other passion, archery, my best stress relies. And thanks to Alessia, Nicoló, Irene and all those, who in the last 2 years ’stucked with me until the very end’. Without all of you these years would have been completely different and, probably, less interesting. Finally, I have to thank someone who entered my life silently and slowly, becoming unexpectedly important to me. Zé, you were there as a friend, you are still here as my love, and you sustained me through these last months of thesis. I hope our future is going to be bright. ii Abstract The search for extra-solar planets dates back to the mid 20th century, when the Doppler effect was proposed as a possible detection method (Struve 1952). As the time passed, a deeper understanding of the stellar physics and its manifestation and the spectroscopic improvements, allowed the discovery of the first exoplanet, by Mayor & Queloz (1995). Their work represented a fundamental milestone for the field, which grew faster as new detection methods were adopted and the instrumental precision improved. Nowadays, this field, among the other objectives, heads towards a precise characterization of exoplanets and their atmospheres and the identification of an Earth-twin. Reaching these aims can only be possible by adopting very precise instruments and accounting for several sources of stellar noise. In this thesis, we specifically analyze the measurability with the current and future instruments oftwo planetary parameters, the albedo and the spin-orbit angle. The albedo of an exoplanet represents the fraction of stellar light reflected by the planetary atmo- sphere. Since reflection depends on the structure and composition of the layers crossed by photons, knowing the albedo helps to probe the presence of clouds and specific molecules in the atmosphere. Measuring this parameter is challenging and it requires the detection of the reflected light through opti- cal photometric observations. This detection is possible in the context of phase curve analysis. A phase curve is the flux variation from the target star and its orbiting planets as a function of time. Itinvolves, in optical wavelengths, the primary transit, the secondary eclipse and 3 more effects, the beaming effect, the ellipsoidal modulation and the reflected light component. While the beaming and ellipsoidal areneg- ligible, the reflected light might dominate the out-of-transit flux if there were no additional noise.The presence of instrumental noise and stellar activity may cause difficulties and obstacles in the detection of the planetary signal, even accounting for a precise knowledge of the planetary properties derivable from the transit feature. While the instrumental noise can be reduced with better instruments or even by binning the data, the stellar activity cannot be removed, especially in the context of a space telescope, such as CHEOPS, which will offer a limited time-span for observations (20 days). In our work, we explored how the stellar activity could limit the detection of the planetary albedo, accounting for an increasing observational time and imposing CHEOPS precision as instrumental noise. In detail, we built mock light curves, including a realistic stellar activity pattern, the reflected light component of the planet and white noise, averagely on the level of CHEOPS noise for different stellar magnitudes. Afterwards, we fit our simulations with the aim of recovering the reflected light component and assuming the activity patterns could be modeled with a Gaussian process. The main conclusion of such analysis was that at least one full stellar rotation is necessary to retrieve the planetary albedo. iii This result, independent of the level of noise, is a consequence of the adopted methodology to model the stellar activity, the Gaussian process, which needs to detect the full stellar rotation to describe the activity pattern. We found as well that, for a 6.5 magnitude star and accounting for CHEOPS noise level, it is possible to detect the planetary albedo up to a lower limit of RP = 0:03R∗. These results can represent a starting point for phase curve analysis not only with CHEOPS, but also with future photometric missions, such as PLATO and TESS. They also show that detecting the albedo for Earth- like planets will only be possible with an increased photometric precision and long observations, as they will be offered by PLATO. The projected spin-orbit angle is the angle between the planetary orbit and the stellar rotational axis. It can be measured through the Rossiter-McLaughlin (RM) effect, the radial velocity signal generated when a planet transits a rotating star. Stars rotate differentially and this affects the shape and amplitude of the RM signal, on a level that can no longer be ignored with precise spectrographs. Highly misaligned planets provide a unique opportunity to probe stellar differential rotation via the RM effect, asthey cross several stellar latitudes. In this sense, WASP-7, and its hot Jupiter with a projected misalignment of ∼ 90◦, is one of the most promising targets. Although Albrecht et al. (2012a) measured the RM of WASP-7b, they found no strong detection of the stellar differential rotation, which suggests us the possibility of an imprecise measurement of the spin-orbit misalignment as well. For this reason, we decided to explore the main hurdles which prevented the determination of WASP-7 differential rotation, adopting the tool SOAP3.0, updated in way it accounted as well for non-rigid stellar rotation. Furthermore, we investigated whether the adoption of the new generation spectrographs, like ESPRESSO, would solve these issues. We finally assessed how instrumental and stellar noise influence this effect and the derived geometry of the system. We found that, for WASP-7, the whitenoise represents an important hurdle in the detection of the stellar differential rotation, and that a precision of at least 2 m s−1 or better is essential. However, we noticed that the past observations of WASP-7b show unusually high residuals, which cannot be justified with any of the additional stellar noise sources explored in our analysis and, thus, they require further exploration. Such exploration would be well suited to the ESPRESSO spectrograph for WASP-7-like systems, as it will provide the radial velocity precision necessary to disentangle the instrumental and stellar noise sources. Unluckily this kind of measurement in the case of Earth-like planets appears to be a quite far achievement. As an overall result we can conclude that the detailed description of the planet, especially with the current and new instruments, is only possible when properly accounting for the stellar noise sources. Moreover, the presence of a planet can help as well in understanding better certain stellar properties, as it is the case of stellar rotational pattern explored in our works. With this thesis, we can thus strongly iv stress the importance of exoplanets in the frame of stellar analysis and vice-versa. v Resumo A pesquisa por exoplanetas foi pela primeira vez considera no século 20, quando o efeito Dopler foi proposto como um possível método de detecção (Struve 1952). Assim que o nosso conhecimento de Física estelar evoluiu e o campo da espectroscopia avançou o suficiente, foi possível descobrir o primeiro exoplaneta, por Mayor & Queloz (1995). Esta descoberta marcou o nascer do campo de exoplanetas, que cresceu cada vez mais rápido assim que novos métodos de detecção e análise foram descobertos e a precisão instrumental melhorou. Hoje em dia, um dos vários objectivos deste campo consiste em realizar caracterizações precisas dos exoplanetas e suas atmosferas, assim como a identificação de planeta gémeo da Terra.