The star-planet connection

Introduction
 
The discovery and characterization of Earth-like planets with the eventual search for life is, arguably, one of the most exciting scientific endeavours of the coming years. Four centuries after the Copernican revolution, Humanity is now at the verge of closing the circle and finding its context in the living Universe. This will undoubtedly have a profound social and scientific impact. The route to this major breakthrough started with the first exoplanet discoveries and was followed by the identification of atomic and molecular species in their atmospheres. Recent exoplanet discoveries have deeply changed our understanding of the formation, structure, and composition of planets. Current statistics show that planets are common; data from the Kepler mission indicate that at least every other star has a planet, and a significant fraction occupy the habitable zone. The basic foundations are being laid out in the quest for the search and characterization of habitable rocky planets.
 
Most of the nearly 4000 exoplanets known so far have been discovered using indirect techniques, which aim at detecting variations on the position, brightness or velocity of the star caused by an invisible planet. Measuring the atmosphere properties of the planets (both by transmission and emission spectroscopy in transiting planets) also relies on measuring precisely the light emitted by the star. Given the stringent requirements on radial velocity, astrometric and photometric precision it is essential to understand if the star itself will be sufficiently stable to permit the acquisition of measurements with such exquisite precision. Sun-like stars have been studied in quite some detail thanks to the availability of solar data but lower-mass stars (M dwarfs) are a completely different story. They are known to possess relatively high levels of magnetic activity manifesting in the form of photometric variations and apparent radial velocity and position variations. Even small spot coverages can induce apparently stochastic changes causing noise levels ten times larger than the planet’s signature. Modelling the impact of stellar activity is the only way to turn the stellar “noise” into understandable “signal”.
 
In parallel with the discovery efforts, much interest lies in comprehending the properties and structure of the planets identified. The physical and chemical properties of the atmosphere are key elements in piecing together the formation history of the planet and, eventually, its potential to sustain life. Inevitably, questions such as 1) the history of stellar radiation of the parent star, 2) the budget of ionizing, evaporating and photodissociating radiation over the lifetime of the planet, or 3) the flux of eroding particles that have bombarded the planet atmosphere along its evolution, become strongly relevant. Surprisingly, little is known for Sun-like stars and virtually no information is available for M-type stars, except that they seem to be much more active. A global picture of the planetary population will necessarily be incomplete unless we can provide answers to these questions and precise knowledge of the star holds the key to them.
 
Proper characterization of stars is a key element to both detect the planets and understand their properties. This project sets out to provide a detailed picture of stars in the context of exoplanet hosts, both regarding the detection of planet signals and the interpretation of exoplanet data. Ultimately, only by collecting such information will we be able to address planet habitability in the most general way and potentially make unambiguous claims of biosignature detection. This is the star-planet connection.
 
Focus
 
The research activity is focused in two different aspects, namely 1) the modelling of stellar variations associated to magnetic activity and 2) the understanding of high-energy emissions of stars and their evolution over time.
 
  • We have devised and constructed a sophisticated code to invert light and radial velocity curves simultaneously and reconstruct the stellar photosphere including spot and facular regions, dubbed StarSim (now in version 2.0). StarSim 2.0 considers surface inhomogeneities in the form of (dark) starspots and (bright) faculae, takes into account limb darkening (or brightening in the case of faculae), and includes time-variable effects such as differential rotation and active region evolution. In the case of radial velocities, it includes full modelling of line profile changes (bisector spatial variability and convective blueshift). StarSim 2.0 is able to reproduce to good precision the simultaneous measurements of photometry and radial velocity of active stars and we can reconstruct the surface map of a star, showing active regions. The applications of StarSim 2.0 are various: correction of radial velocity time series, correction of photometric time series, simultaneous use of visible and near-infrared radial velocities, investigation of the chromatic Rossiter-McLaughlin effect, etc. We are working on such applications.
This research bears directly on various instruments and space missions, which are aimed at discovering terrestrial (potentially habitable) planets and measuring their atmospheric properties. We use our precise stellar modelling to maximize the scientific output from the CARMENES instrument, the CHEOPS mission, the PLATO mission and the ARIEL mission.
  • Over the past decade, we have been investigating the emissions of stars as the main source of energy shaping the structure, evolution, and even determining the mere existence of planetary atmospheres. “The Sun in Time” was a comprehensive program to study solar proxies and trace the high-energy evolution of Sun-like stars during their main-sequence lifetime. We found that high-energy emissions were orders of magnitude stronger in the past, which should have had strong influence on their planets.
To understand the distribution of life in the Universe in general, and to the design of terrestrial planet finding missions in particular, a detailed interdisciplinary study on the habitability of terrestrial planets in orbits around cool stars is crucial. The currently accepted definition of the habitable zone of a star is based on an Earth analogue with liquid water and a reservoir of CO2 to regulate the climate. Although this general climatological definition of the habitable zone can be applied to all stellar types, the evolution of the atmosphere and the planets water inventory of terrestrial planets in the habitable zone of cool stars may differ from that of planets around solar-type stars. Planets around low-mass stars have closer orbits, which makes them more vulnerable to the radiation and particle emissions from their parent stars. Radiation in the XUV is absorbed high in the planetary atmosphere and thus induces exospheric heating, expansion of the upper layers and eventual mass loss, thus altering the volatile content of the planet.

We are opening a new arena in this context by studying stars less massive than the Sun. The strategy is based on characterizing the emissions of a stellar sample and generalizing the results to any low-mass star. Initial findings indicate that cool stars possibly have stronger and longer-lived XUV emissions than solar-like stars. Whether a terrestrial planet in the habitable zone around a low-mass star can keep an atmosphere and sustain a biosphere remains to be seen. UV radiation is absorbed deeper in the planet’s atmosphere leading to photodissociation of some key molecules thus giving rise to a rich photochemistry. Our present view of the stellar habitable zone is certainly incomplete. Only by characterising the emissions of the entire population of low-mass stars and by understanding the interactions of their radiation and particle fluxes with planet atmospheres will we be able to complete a true picture of the habitable zone around a star.
 
Selected publications
 
  • "The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present", Ribas, Ignasi; Bolmont, Emeline; Selsis, Franck; Reiners, Ansgar; Leconte, Jérémy; Raymond, Sean N.; Engle, Scott G.; Guinan, Edward F.; Morin, Julien; Turbet, Martin; Forget, François; Anglada-Escudé, Guillem, Astronomy & Astrophysics, Volume 596, id.A111, 18 pp. (2016)
  • "Modelling the photosphere of active stars for planet detection and characterization", Herrero, Enrique; Ribas, Ignasi; Jordi, Carme; Morales, Juan Carlos; Perger, Manuel; Rosich, Albert, Astronomy & Astrophysics, Volume 586, id.A131, 19 pp. (2016)
  • "Photospheric activity, rotation, and magnetic interaction in LHS 6343 A", Herrero, E.; Lanza, A. F.; Ribas, I.; Jordi, C.; Morales, J. C., Astronomy & Astrophysics, Volume 553, id.A66, 14 pp. (2013)
  • "The Evolution of Solar Flux from 0.1 nm to 160 ?m: Quantitative Estimates for Planetary Studies", Claire, Mark W.; Sheets, John; Cohen, Martin; Ribas, Ignasi; Meadows, Victoria S.; Catling, David C., The Astrophysical Journal, Volume 757, Issue 1, article id. 95, 12 pp. (2012)
  • "Time evolution of high-energy emissions of low-mass stars. I. Age determination using stellar chronology with white dwarfs in wide binaries", Garcés, A.; Catalán, S.; Ribas, I., Astronomy & Astrophysics, Volume 531, id.A7, 10 pp. (2011)
  • "Evolution of the Solar Activity Over Time and Effects on Planetary Atmospheres. II. ?1 Ceti, an Analog of the Sun when Life Arose on Earth", Ribas, I.; Porto de Mello, G. F.; Ferreira, L. D.; Hébrard, E.; Selsis, F.; Catalán, S.; Garcés, A.; do Nascimento, J. D., Jr.; de Medeiros, J. R., The Astrophysical Journal, Volume 714, Issue 1, article id. 384-395 (2010)

Senior Institute members involved

I. Ribas
Institute of Space Sciences (IEEC-CSIC)

Campus UAB, Carrer de Can Magrans, s/n
08193 Barcelona.
Phone: +34 93 737 9788
Email: ice@ice.csic.es
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An institute of the Consejo Superior de Investigaciones Científicas

An institute of the Consejo Superior de Investigaciones Científicas
Affiliated with the Institut d'Estudis Espacials de Catalunya

Affiliated with the Institut d'Estudis Espacials de Catalunya