Planet Populations as a Function of Stellar Properties

Exoplanets around different types of stars provide a window into the diverse environments in which planets form. This chapter describes the observed relations between exoplanet populations and stellar properties and how they connect to planet formation in protoplanetary disks. Giant planets occur more frequently around more metal-rich and more massive stars. These findings support the core accretion theory of planet formation, in which the cores of giant planets form more rapidly in more metal-rich and more massive protoplanetary disks. Smaller planets, those with sizes roughly between Earth and Neptune, exhibit different scaling relations with stellar properties. These planets are found around stars with a wide range of metallicities and occur more frequently around lower mass stars. This indicates that planet formation takes place in a wide range of environments, yet it is not clear why planets form more efficiently around low mass stars. Going forward, exoplanet surveys targeting M dwarfs will characterize the exoplanet population around the lowest mass stars. In combination with ongoing stellar characterization, this will help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet

The PLATO 2.0 mission

PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence) providing a wide field-of-view (2232 deg 2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmosphere. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science

Future Astrometric Space Missions for Exoplanet Science

High-precision astrometry at the sub-microarcsecond level opens up a window to study Earth-like planets in the habitable zones of Sun-like stars, and to determine their masses. It thus promises to play an important role in exoplanet science in the future. However, such precision can only be acquired from space, and requires dedicated instrumentation for a sufficient astrometric calibration. Here we present a series of concepts designed for handling this task. STARE is a small satellite concept dedicated to finding planets in the very nearest stellar systems, which offers a low-cost option toward the study of habitable planets. The NEAT concept is a set of two formation-flying satellites with the aim to survey the 200 nearest Sun-like stars for Earths in the habitable zone. Finally, THEIA is a proposal for an ESA M-class mission, with a single-unit telescope designed for both dark matter studies as well as a survey for habitable Earth-like planets among the 50 nearest stars. The concepts illustrate various possible paths and strategies for achieving exquisite astrometric performance, and thereby addressing key scientific questions regarding the distribution of habitability and life in the universe.Comment: 12 pages, 3 figures. Invited review to appear in 'Handbook of Exoplanets', Springer Reference Works, edited by Hans J. Deeg and Juan Antonio Belmont

Space Astrometry Missions for Exoplanet Science: Gaia and the Legacy of Hipparcos

Astrometry as a technique has so far proved of limited utility when employed as either a follow-up tool or to independently search for planetary-mass companions around stars in the solar neighborhood. However, the situation is bound to change soon. In this chapter, we provide a brief overview of past and present efforts to detect planets via milli-arcsecond (mas) astrometry, with a special focus on the legacy of the Hipparcos mission. We then focus on the Gaia mission that is poised to become a game changer in the field of exoplanets by unleashing for the first time the power of micro-arcsecond (μas) astrometry. We start by briefly describing the mission status and operation. Next, we address some of the relevant technical issues associated with the precise and accurate determination of astrometric orbits of planetary systems using Gaia data. We then present and discuss the Gaia planet-finding capabilities. We conclude by putting Gaia astrometry in context, illustrating its potential for crucial contributions to exoplanetary science in synergy with other indirect and direct methods for the detection and characterization of planetary systems