1.1.2 Mission history and science case

The fundamental scientific-performance requirements for Gaia stem, to a large extent, from the main scientific target of the mission: the Milky Way galaxy. Gaia is built to address the question of the formation and evolution of the Galaxy through the analysis of the distribution and kinematics of the luminous and dark mass in the Galaxy. By also providing measurements to deduce the physical properties of the constituent stars, it is possible to study the structure and dynamics of the Galaxy. Although the Gaia sample ‘only’ covers about 1% of the stars in the Milky Way, it consists of almost 2000 million stars covering a large volume (out to many kpc, depending on spectral type), allowing thorough statistical analyses to be conducted. The dynamical range of the Gaia measurements facilitates reaching stars and clusters in the Galactic disk out to the Galactic centre as well as far out in the halo, while providing extremely high accuracies in the Solar neighbourhood. In addition to using stars as probes of Galactic structure and the local, Galactic potential in which they move, stars can also be used to map the interstellar matter. By combining extinction deduced from stars, it is possible to reconstruct the three-dimensional distribution of dust in our Galaxy. In this way, Gaia addresses not only the stellar contents, but also the interstellar matter in the Milky Way.

The current understanding of galaxy formation is based on a combination of theories and observations, both of (high-redshift) extragalactic objects and of individual stars in our Milky Way. The Milky Way galaxy provides the single possibility to study details of the processes, but the observational challenges are different in comparison with measuring other galaxies. From our perspective, the Galaxy covers the full sky, with some components far away in the halo requiring sensitivity, while stars in the crowded Galactic centre region require spatial resolving power. Both these topics are addressed with the Gaia data. Gaia distances allow the derivation of luminosities for stars which, when combined with metallicities, allow the derivation of accurate individual ages, in particular for old subgiants, which are evolving from the main-sequence turn-off to the bottom of the red giant branch. By combining the structure and dynamics of the Galaxy with the information of the physical properties of the individual stars and, in particular, ages, it is possible to deduce the star formation histories of the stellar populations in the Milky Way.

Distances are of fundamental importance for understanding and interpreting astronomical observations of stars. Yet, direct distance measurement using trigonometric parallax of any object outside the immediate Solar neighbourhood or not emitting in radio wavelengths is challenging from the ground. The Gaia revolution is in the parallaxes, with hundreds of millions being accurate enough to derive high-quality colour-magnitude diagrams and to make significant progress in stellar astrophysics. The strength of Gaia is also in the number of objects that are surveyed as several phases of stellar evolution are fast. With a billion parallaxes, Gaia covers most phases of evolution across the stellar-mass range, including pre-main-sequence stars and (chemically) peculiar objects. In addition to parallaxes, the homogeneous, high-accuracy photometry allows fine tuning of stellar models to match not only individual objects but also star clusters and populations as a whole. The combination of Gaia astrometry and photometry also contributes significantly to star formation studies.

On average, each star is measured astrometrically $\sim$70 times during the five-year nominal operations phase (with a further $\sim$70 observations expected for a five-year extended mission). At each focal-plane transit, photometric measurements are also made: ten in the Gaia $G$ broadband filter and one each with the blue and red photometer (Section 1.1.3). For the variable sky, this provides a systematic survey with the sampling and cadence of the scanning law of Gaia (Section 1.1.4). This full-sky survey provides a census of variable stars with tens of millions of new variables, including rare objects. Sudden photometric changes in transient objects are captured and the community is alerted to enable follow-up observations (e.g., Delgado et al. 2019). Pulsating stars, especially RR Lyraes and Cepheids, can easily be discovered from the Gaia photometric data stream, allowing, in combination with the parallaxes, calibration of the period-luminosity(-metallicity) relations to high accuracies, thereby improving the quality of the cosmic-distance ladder and scale.

Gaia is a powerful mission to improve our understanding of multiple stars. The instantaneous spatial resolution, in the scanning direction, is comparable to that of the Hubble Space Telescope and Gaia is surveying the whole sky. In addition to resolving many binaries, all instruments in Gaia complement our understanding of multiple systems. The astrometric wobbles of unresolved binaries, seen superimposed on parallactic and proper motion, can be used to identify multiple systems. Periodic changes in photometry can be used to find (eclipsing) binaries and an improved census of double-lined systems follows from the Gaia spectroscopic data. It is again the large number of objects that Gaia provides that allows addressing fundamental questions of mass distributions and orbital eccentricities among binaries.

From the entire spectrum of scientific topics that Gaia addresses, the exoplanet research area has been the most dynamic in the past two decades. The field has expanded from detecting hot, giant planets to characterising smaller planets further away from their host star, including multi-planetary systems. These advancements have been achieved both with space- and ground-based facilities. Nevertheless, the Gaia astrometric capabilities remain unique, probing a poorly explored area in the parameter space of exoplanetary systems and providing astrophysical parameters not obtainable by other means. A strong point of Gaia in the exoplanet research field is the provision of an unbiased, volume-limited sample of Jupiter-mass planets in multi-year orbits around their host stars. These are logical prime targets for future searches of terrestrial-mass exoplanets in the habitable zone in an orbit protected by a giant planet further out. In addition, the astrometric data of Gaia allow actual masses (rather than lower limits) to be measured. Gaia data of course also contributes to the characterisation of exoplanet host stars. Finally, the data of Gaia provide the detailed distributions of giant exoplanet properties (including the transition regime between giant planets and brown dwarfs) as a function of stellar-host properties with unprecedented resolution.

Although Gaia is designed to detect and observe stars, it provides a full, magnitude-limited census of all sources that appear point-like on the sky. The movement of solar-system objects with respect to stars smears their images and makes them less point-like. As long as this smearing is modest, Gaia still detects the object. The most relevant solar-system object group for Gaia comprise asteroids. Unlike planets, which are too big in size (and, in addition, sometimes too bright) to be detected by Gaia, asteroids remain typically point-like and have brightnesses within the dynamical range of Gaia. Gaia astrometry and photometry provide a census of orbital parameters and taxonomy in a single, homogeneous photometric system. The full-sky coverage of Gaia also provides this census far away from the ecliptic plane as well as for locations inside the orbit of the Earth. Through the Gaia follow-up network for Solar-system objects (FUN-SSO), alerts are being made of newly discovered asteroids to trigger ground-based observations to avoid losing the object again (e.g., Tanga et al. 2016). For near-Earth asteroids, Gaia’s census is not complete as the high apparent motion of such objects often prevents Gaia detection (de Bruijne et al. 2015). Gaia provides fundamental mass measurements of those asteroids that experience encounters with other solar-system bodies during the Gaia operational lifetime. Finally, the high precision and accuracy of stellar positions and motions provided by Gaia allow significant improvements in occultation studies of moons, asteroids, and Kuiper-Belt objects in the Solar system.

In the Local Group, the spatial resolution of Gaia is sufficient to resolve and observe the brightest individual stars. Tens of Local Group galaxies are routinely observed, including the Andromeda galaxy and the two Magellanic Clouds. While for the faintest dwarf galaxies only a few dozen of the brightest stars are observed, this number increases to thousands and millions of stars in Andromeda and the Large Magellanic Cloud, respectively. In dwarf spheroidals such as Fornax, Sculptor, Carina, and Sextans, thousands of stars are covered. A major scientific goal of Gaia in the Local Group concerns the mutual, dynamical interaction of the Magellanic Clouds and the interaction between the Clouds and the Galaxy. In addition to providing absolute proper motions for transverse-velocity determination, needed for orbits, it is possible to explore internal stellar motions within dwarf galaxies. These data reveal the impact of dark matter, among other physical processes in the host galaxy, to the motions of its stars.

Gaia provides a homogeneous, magnitude-limited sample of unresolved galaxies. For unresolved galaxies, the sampling function is complicated as on-board detection depends on the contrast between the point-like, central element (bulge) and the extended structure (disk). For unresolved galaxies, the most valuable Gaia measurements are the photometric observations. Millions of galaxies across the whole sky are measured systematically. As the same Gaia system is used for stellar work, the astrophysical interpretation of the photometry of extragalactic objects can be based on statistically sound fundamentals obtained from Galactic studies. Quasars form a special category of extragalactic sources for Gaia: not only their intrinsic properties can be studied but they can also be used to construct an inertial, optical reference frame.

Relativistic corrections are part of the routine data processing for Gaia. Given the large number of measurements that Gaia collects, it is possible to exploit the redundancy in these corrections to conduct relativity tests or to use (residuals of) the Gaia data in more general fundamental-physics experiments. Specifically for light bending, it is possible to determine the $\gamma$ parameter in the parametrised post-Newtonian formulation of gravity with high precision. Another possible experiment is to explore light bending of star images close to the limb of Jupiter in order to measure the quadrupole moment of the gravitational field of the giant planet. A common element in all fundamental-physics tests using Gaia data is the use of large sets of measurements. This is meaningful only when all systematic effects in the data are under control, down to micro-arcsecond levels. Therefore, Gaia results for relativistic tests are expected only towards the end of the mission and data processing, when all calibration aspects have been handled successfully.