The unprecedented precision of stellar brightness measurements achieved by the planet-hunting space telescopes initiated a new era in stellar photometric variability investigations. Understanding stellar brightness variations is of great interest to the solar, stellar, and exoplanetary communities, for the following reasons: Stellar brightness variations can provide constraints on the historical solar variability and solar role in climate change, as well as they allow to determine stellar magnetic cycles' properties. Moreover, stellar brightness variations are a limiting factor for detection and characterisation of the exoplanets via transit photometry. Recently, a plethora of observational data have pushed ahead theoretical studies aiming at developing methods for extracting information about stars and their planets from the available records of brightness variations. These studies can greatly profit from knowledge acquired by studying the Sun. Thus the way forward is to focus on the solar-stellar comparison and examining how the solar paradigm can help us to explain variability of other stars and develop criteria for distinguishing between typical photometric signatures of intrinsic stellar variations and exoplanet transits.

1. Observing stellar photometric variability
2. Advances in modelling stellar photometric variability
3. Exoplanet detection and limiting factors
4. State-of-the-art in solar irradiance modeling

Gibor Basri (Department of Astronomy, University of California, USA)
Joe Llama (Lowell Observatory, Flagstaff, USA)
Nadège Meunier (Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France)
Benjamin Montet (Department of Astronomy and Astrophysics, University of Chicago, USA)

• Gibor Basri - Department of Astronomy, University of California, USA
• Natalie Krivova - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
• Alexander Shapiro - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
• Sami Solanki - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
• Yvonne Unruh - Blackett Laboratory Imperial College London, UK
• Veronika Witzke - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany

First observations of brightness variations of Sun-like stars have been performed with ground- based instrumentations. These observations were able to reliably detect long-term variations, which led to the discovery of magnetic activity cycles in brightness of Sun-like stars. The variations show a wide variety of patterns. For example, brightness and magnetic activity (as indicated by the emission in the Ca II H and K lines) change mainly in phase for stars older than the Sun and in anti-phase for younger stars.

The unprecedented precision of stellar brightness measurements achieved by the planet-hunting space telescopes initiated a new era in stellar photometric variability investigations. In particular, Kepler allows detailed studies of stellar brightness variations on timescales from minutes to weeks for more than a hundred thousand of stars. The interest in this topic is further enhanced by the anticipation of the TESS, CHEOPS, and PLATO missions.

A plethora of observational data have pushed ahead theoretical studies aiming at developing methods for extracting information about stars and their planets from the available records of brightness variations. Interestingly, these studies can greatly profit from knowledge acquired by studying a very special star - our Sun.

Supported by the available high resolution observations of the Sun, our understanding of solar brightness variations has dramatically improved over the last few years. It was recently found (Shapiro et al., 2017, Nature Astronomy, 1, 612S) that observed solar brightness variations (excluding oscillations, which form a separate topic) can be explained with remarkable accuracy by the joint action of only two sources, the surface magnetic field and granular convection. Whereas variations with periods shorter than a day (which is the timescale of interest for hunting planetary transits) are caused by convection and evolution of magnetic features, main contribution to the variability on timescales from a day to a year comes from transits of magnetic features across the solar disc as the Sun rotates. The variability on longer timescales is linked to changes in the overall solar magnetic activity. Concurrently, new models of solar brightness variations, based on realistic 3D magnetohydrodynamic (MHD) simulations of near-surface convection, have appeared (Yeo et al., 2017, Phys. Rev. Let., 119, 9). These models can now be directly extended to other Sun-like stars (Norris et al., 2017, Astron. Astrophys., 605, 45), bringing the effort in understanding their variations to a completely new level.

Moreover, the recovery of the long-term variations of Sun-like stars from the full-frame Kepler images (Montet et al., 2017, Astrophys. J., 851, 116) together with the recent progress in modelling of stellar variations, allow to address the fundamental question how Sun-like stars compare to the Sun. Previous observations found that the majority of Sun-like stars with near solar magnetic activity show significantly higher brightness variability on the activity cycle timescale compared to the Sun (see Lockwood et al. 2013, ASPC,472, 203 for the recent update). This posed the oxymoronic question of whether the Sun is actually a Sun-like star. One possible explanation for this anomaly is due to the incidental combination of solar fundamental parameters and its magnetic activity (Shapiro et al. 2016, Astron. Astrophys., 589, 46; Karoff et al. 2018, Astrophys. J., 852, 46). If confirmed, this hypothesis will put strong constraints on the possible range of solar variability over millennia.

Recently the focus was shifted also to brightness variations on timescales of days to weeks, where a large number of comparison investigations of Sun-like stars to the Sun were performed based on the Kepler data. These studies led to contradictory results and an unambiguous answer to this question is still not found. The key ingredient to the present controversy might be the difficulty to identify proper comparison stars in the Kepler sample. In particular, the complex configuration of magnetic features and their evolution render the period determination of stars with near-solar magnetic activity to be very difficult. The new surveys and methods for determining rotational periods of Sun-like stars (e.g. Angus et al. 2018, MNRAS, 474,2, 21) can potentially remove (or at least to identify) the main biases affecting the comparison of solar and stellar variability on timescales from days to weeks.

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