In Chaps. 10 and 11 , I surveyed the various ways in which a host star’s radiation and wind can erode an exoplanet’s atmosphere, change its chemistry, and thereby determine whether the exoplanet could be habitable. I now turn to the question of whether an exoplanet can change the properties of its host star, in particular, its rotation rate, UV and X-ray radiation, and the properties of its wind. The study of this feedback of an exoplanet on its host star is usually called star-planet interactions (SPI), although a more accurate term would be planet-star interactions.
In addition to photons at all wavelengths, stars eject ionized and neutral particles to space. These typically magnetized winds interact with the upper layers of exoplanet atmospheres producing charge-exchange and other reactions that facilitate mass loss. In this chapter, I describe the different types of winds observed in hot and cool stars. For some star types, it is relatively easy to measure diagnostics of mass loss but in solar type stars it is more difficult. I describe the different techniques for measuring or estimating stellar mass-loss rates and the diverse models developed to explain stellar mass loss.
The lower layer of a star’s atmosphere, its photosphere, has a thermal structure that decreases outward controlled by the balance of radiative and convection heat from below and the loss of radiation to space. With increasing height in a stellar atmosphere, magnetic heating processes become important in the energy balance, forcing the temperature to increase with height in a region called the chromosphere. Magnetic heating processes include the damping of different types of magnetic waves and the reconnection of magnetic fields.
The term “stellar activity” is routinely used to characterize phenomena that are more energetic, brighter, time-variable or otherwise different from the normal behavior of a star. Stellar activity plays a central role in the evolution of exoplanet atmospheres, but the term is rarely defined or tied to underlying causes of active phenomena on stars. In this chapter, I will describe the different types of stellar activity whose underlying cause is heating and particle acceleration by magnetic processes.
Measurements of activity indicators including stellar X-ray and UV emission and wind flux are essential for calculating the photochemistry and hydrodynamical outflow rate in exoplanet atmospheres at the present time, and it is also important to estimate what the stellar fluxes were at earlier times and will likely become in the future. Assessing the habitability of an exoplanet requires knowledge of the host star’s emission back to the protoplanetary stage.
Unfortunately, the two of the most important features in stellar spectra, the Lyman-α line and the EUV, are the most difficult to observe. The Lyman-α emission line dominates the UV spectra of stars cooler than the Sun. For the Sun, the intrinsic Lyman-α line flux is about equal to the rest of the FUV flux, whereas for an M star like GJ 876 (M4 V) the Lyman-α line flux is more than twice as large as the rest of the FUV and nearly as large as the entire FUV+NUV flux excluding this line (France et al. 2012).
The spectral energy distribution (SED) emitted by a host star’s photosphere, chromosphere, and corona is essential input for models of the chemistry and mass loss from exoplanet atmospheres. This chapter describes typical SEDs of different types of stars including spectral line and continuum radiation from the X-ray region to the infrared.
These pithy quotes highlight the main reasons for discussing stellar magnetic fields in the context of exoplanet atmospheres and possible habitability.
Photochemical reactions often dominate over collision-based equilibrium chemistry at atmospheric pressures less than 1 mbar, corresponding to about 0.1% of the Earth’s surface pressure and heights above 60 km for Earth-like rocky planets.
Exoplanets have intimate relationships with their host stars as the Earth does with its host star. While the Earth’s environment produced by the Sun is usually benign, exoplanets located close to their host stars, especially active M dwarfs, must suffer through powerful flares, stellar winds, CMEs, and very high energy radiation. The environment in which exoplanets must live is now called “stellar space weather” in analogy with the extensively monitored “space weather” that is the environment of the Earth. In this chapter I describe flares and superflares on the Sun and stars and how repeated flares destroy O3 in the atmospheres of exoplanets possibly leading to the sterilization of their surfaces and loss of habitability.
This book is the perfect primer on star-planet interaction for astrobiologists and planetary scientists investigating the habitability of exoplanets. Authored by a leading expert in the field this text is self-contained and includes many figures and tables.
The temperature distribution in a star decreases monotonically from the nuclear burning core to the top of its photosphere as heat leaks out into cold space. However, the hottest region of a star can be its outermost layer, the corona with temperatures of 106 K to 107 K or even hotter. The heating mechanism responsible for hot coronae has been debated since the existence of hot coronal gas was first identified in the early 1940s.
Whether an exoplanet retains its atmosphere and surface water depends on many factors. I will first describe the two main mass-loss mechanisms for exoplanet atmospheres. The main thermal process is hydrodynamical outflow driven by extreme ultraviolet and x-radiation from the host star that heat and inflate an exoplanet’s exosphere leading to mass loss. Non-thermal mass loss occurs when ions and electrons in the host star’s wind remove neutrals and ions in the exoplanet’s upper atmosphere through charge exchange, ion pick-up, and other processes. Although there are simulations of thermal and non-thermal processes occurring in isolation, these processes can work together to enhance the mass-loss rate. There are, however, many unknowns or poorly known parameters including the exoplanet’s initial water inventory, its orbital migration, the strength of its magnetic field, its outgassing rate, the history of the host star’s UV, EUV and x-radiation, and the present and past flaring and coronal mass ejection (CME) rates. Thus for an exoplanet to be habitable today, it must survive a gauntlet of inadequately known challenges. Latter in the chapter, I describe what the term “habitable zone” is intended to be and include case studies of several exoplanets for which there are estimates of mass loss, surface liquid water, and habitability. Finally, I consider which types of host stars are most likely to support exoplanets that satisfy the wide variety of conditions that appear to be required for habitability.
Like planets in the solar system, exoplanets form, evolve, and interact with their host stars in many ways. Exoplanets form out of protostellar disks, which contain positive ions (H+, H $$^+_2$$ , and H $$_3^+$$ ) and other radicals produced when molecules in the disk are photo-dissociated by stellar UV and X-ray photons and then charge-exchanged by protons in the stellar wind. These positive ions become the formation seeds of complex molecules including simple organics.
Now that exoplanets are being discovered almost daily through transit, radial velocity, and other techniques, the direction of exoplanet research is changing from discovery to characterization of exoplanet properties. Of particular importance is the existence and composition of exoplanet atmospheres and whether these atmospheres are supportive of life forms on the surfaces of rocky planets. It is now recognized that the emissions of host stars, both their radiation and wind properties, determine whether their exoplanets retain their initial atmospheres and any secondary atmospheres that may later emerge. While there is only one present example of an inhabited planet, there are now more than a handful of possibly habitable rocky exoplanets and the number of such planets will surely increase rapidly in the near future. Although the presence of surface water was the original criterion for the term habitable zone, or more precisely “liquid water habitable zone” (LWHZ), there are additional factors that also determine habitability and many of these factors involve the present and past properties of the host star.
