Gaia mission extension science case

Following a successful launch in December 2013 and a 6-month commissioning phase, Gaia started its nominal 5-year operations phase in July 2014. This phase was completed at the end of June 2019, after which Gaia entered its extended mission phase. Mission extensions are confirmed through a standard process that includes the evaluation of the mission's scientific merits and technical status. Gaia underwent this process successfully several times (see e.g. https://www.esa.int/About_Us/ESAC/Extended_life_for_ESA_s_science_missions and  https://sci.esa.int/web/director-desk/-/extended-life-for-esa-s-science-missions) and its spacecraft operations were extended until 2025, when an onboard consumable critical for collecting scientific observations was exhausted. 

The science cases that were put forward in 2022 for the last mission extension are summarised below.

Science Case

Overall impact of Gaia

Gaia has so far had several data releases that have been received extremely well by the scientific community, as demonstrated by the extremely high rate of refereed articles making use of Gaia data appearing in the literature. The topics addressed range from the Solar System to Quasars with a natural emphasis on stellar astrophysics and Galactic structure. The accuracy and precision available in the releases are in line with the scientific performance predictions made both for the end of the nominal 5-year mission and for the 5-year extension. Therefore the science cases presented here for the extension period have a solid basis. Many of the extension cases have also been recognised by the scientific community which has been addressing the topics presented here based on Gaia data. The 5-year extension need has been explicitly mentioned in several articles as a requirement for addressing the scientific questions adequately.

Science case for the extension interval 2019–2025

The survey nature of the Gaia mission and the design characteristics of the satellite matched with its operational mode make the fundamental observational unit 5 years long to allow the best homogeneity possible across the sky. Therefore the science case for this extension proposal is based on another complete 5-year survey. This is also consistent with the expected lifetime of Gaia as far as critical consumables are concerned.

A major part of the science case for Gaia rests on its ability to measure the positions, parallaxes, and proper motions of stars to a very high precision and accuracy. Complemented with spectrometric measurements of radial velocities, using the on-board Radial Velocity Spectrometer (RVS), this gives complete information on the 3D source positions and velocities needed for modelling the structure and dynamics of our Galaxy. Complementing the astrometric and spectroscopic capabilities, Gaia also has a (spectro)photometer enabling astrophysical characterisation of the detected objects.

Based on simple but realistic assumptions about Gaia's measurement procedure, it is found that the precision (statistical uncertainty) of the fundamental quantities scales as T-0.5 for the (mean) positions, parallaxes, (mean) photometric magnitudes and (mean) radial velocities, if T is the mission length. While the basic parameters scale as T-0.5, for proper motions the scaling changes to T-1.5. Extending the operational lifetime from 5 to 10 years is therefore expected to improve the precision of the positions, parallaxes, photometry and radial velocities by a factor 1.4, and the proper motions by a factor 2.8. The improvement in e.g. parallaxes means that a three times larger volume of space is sampled at a given relative precision in distance, reaching more and possibly new kinds of objects. These considerations are based on the assumption that stars move uniformly through space relative to the solar system, which is a very good approximation for the majority of stars. It is manifestly wrong for many binary and multiple stellar systems and stars with unseen companions. The scaling relations above still hold if the proper motion is interpreted as the mean motion over the observed interval. However, the deviations from uniform space motion are in themselves highly interesting, as they provide dynamical information on the perturbing bodies, and they follow different scaling laws.

The astrometric method is particularly sensitive to long-period perturbations. The second derivative of position, i.e. the acceleration, gives the direction and size of the perturbing force, but is insufficient to distinguish a close-by low-mass companion from a more distant massive one. It turns out that the third derivative is also insufficient, owing to a degeneracy with the systemic proper motion. Unambiguous determination of the orbital period, mass, and distance of the perturbing body is only possible when the fourth derivative of the position can be measured. The precision of the n’th derivative of the position scales as T-(n+0.5), or as T-4.5 for the fourth derivative. A doubling of T therefore implies 22.6 times smaller uncertainty in quantities that are critical for a unique dynamical characterisation of long-period systems. This illustrates a very important and general fact, namely, that the ability to successfully interpret complex objects improves dramatically with increased mission length, very much more than for the basic astrometric data.

 

Structure and evolution of the Milky Way Galaxy

Gaia is set to revolutionise our understanding of the structure and evolution of our Galaxy, providing for instance tests of hierarchical structure formation, probes of the inner bulge/bar dynamics and disk/halo interactions. Regarding the halo, a mission extension will allow Gaia to accurately map the motions of individual stars over a much larger volume (by a factor of ~20 as a 10-year mission achieves a sufficient proper motion accuracy further away than the nominal 5-year mission) which in turn will enable three key science cases: 1) probing the dynamics of the unmixed young tidal debris, expected to be located beyond 30 kpc, which will allow the unravelling of the complex structure of the outer halo, and thus provide further insight into the accretion history of the Milky Way; 2) higher precision motions for sparsely populated tidal streams will allow placing significant new constraints on the dark sub-halo population and hence the characteristics of dark matter; 3) the internal kinematics of nearby dwarf galaxies can be resolved, which is mandatory for tying down their dark matter contents.

​​​​​Dynamics of the unmixed halo

​​​​​​​With its nominal 5 year mission, Gaia provides precision proper motion mapping of the Galactic stellar halo out to distance of ~20 kpc. Recent studies point to distances somewhat further than this being of significant interest, marking a transition region in our Galaxy halo. The inner halo appears to contain most of its stellar mass in old tidal debris accreted more than 8 Gyr ago, while the outer halo has a higher fraction of young debris accreted more recently. Models of stellar halo formation, with a range of accretion histories, predict varying fractions of this old to young tidal debris mix. Old and well-mixed tidal debris always dominates the central halo. However, the region with a significant fraction of young unmixed debris can be between 10 and 40 kpc from the Galactic centre. For accretion histories that currently best represent the Milky Way, i.e. without recent massive accretion, the smooth old halo extends beyond 20 kpc and often beyond 30 kpc. If true, this implies that to understand the accretion history of the Galaxy and to probe the granularity of the halo, we will have to map the motions of stars well beyond Gaia’s current horizon of 20 kpc. The stellar halo density was until recently thought to vary smoothly with distance. However, recent evidence from a range of tracers points to a break in the radial stellar density somewhere between 20 and 30 kpc. This break could show the existence of the transition region from the old smooth to the recent lumpy halo. Alternatively, this could also be the interface between the so-called ‘in-situ’ and the accreted halo. Only by extending Gaia for further 5 years we will be able to reach the stellar tracers beyond the break and understand the formation of the Galaxy and its halo.

Dark matter sub-halo properties from tidal stream gaps

The outer halo of the Milky Way contains numerous sparsely populated tidal streams with gaps along them caused by the impacts of dark matter sub-halos. With a five-year Gaia mission the study of these gaps will be limited as motions of the stars in the streams cannot be measured with enough precision. Extending the mission by five years would yield proper motions precise enough to study mean motions in sparse tidal-stream populations. This would open the way to characterising the properties of gaps in the tidal streams and hence to constrain the properties of the dark sub-halo population and of dark matter itself. This represents a very exciting science case and is one of the promising avenues for finding the myriads of missing satellites predicted by the LCDM cosmological model.

Another application of the study of the motions of tidal streams is that of measuring the gravitational potential, including the effects of the Magellanic Clouds. The streams that are most useful for determining the potential typically originate from small progenitor objects. One needs as high accuracy as possible in the individual measurements as there are not many bright enough stars available.

Internal kinematics of nearby dwarf galaxies

The nominal 5-year Gaia mission will not allow the detailed study of individual motions of stars in nearby dwarf galaxies. The studies of their internal kinematics will have to be restricted to averaging the measured proper motions of stellar populations in order to derive mean properties of the dark matter contents of these galaxies. However, the interpretation of such mean motions can be systematically biased if the dwarf galaxy is not in dynamical equilibrium. Consequently, the most effective approach to probing their internal kinematics is to obtain proper motions with sufficient precision and accuracy to enable modelling of the motions of individual stars. A 5-year extension will enable this.

For Carina, the Red Giant Branch tip falls just between the 2- and the 5-year extensions, so that only in the latter case will enough stars be observed with sufficient proper motion precision and accuracy to enable internal kinematics studies. In complex cases such as Carina, this is very important in order to avoid the dangers of averaging motions in a system that is not in equilibrium. The numbers above are basically the same for the Sextans, Sculptor, and Draco dwarf galaxies. For more populous dwarf galaxies, such as Fornax, the 5-year extension would allow for the study of the individual motions of around 100 stars.

Stars and star clusters

Apart from improvement of basic measurement precision and accuracy for all stars, a 10-year mission will open up new specific areas, which cannot be addressed with the nominal mission.

​​​​​​​For multiple stars, an extension is significant for binaries in 5-10 year orbits. These, typically unresolved, objects have proper motion and orbital motion degeneracy with 5 years of data. A 10-year mission will allow for breaking this degeneracy. Statistically it is precisely those orbital periods where the nominal 5-year Gaia is the weakest as unresolved shorter period binaries can often be solved and very long period binaries Gaia will already see as separate objects.

For stars in clusters, the huge gain will be achieved due to improved proper motions as kinematical and dynamical studies can be expanded from the Solar neighbourhood within about 1.5 kpc from the Sun to larger Galactic scales. Improving the proper motion precision by a factor of 2.8 would increase by a factor of about 20 the volume where open clusters can be investigated with a precision of ~0.3-0.5 km/s in tangential velocities, which is required to resolve cluster kinematics. This would allow characterizing the dynamical status of more distant (up to 2-3 kpc) and often more complex/sub-structured young regions, like e.g. the Scutum and Perseus arms in the Galactic center and anti-center directions, respectively, including the low mass populations. The expansion of the studies to Galactic scales will give a much better handle to understanding the star formation process in our Milky Way and the thin disc population.

For variable stars, Gaia will provide a unique census with its temporal sampling, photometric accuracy, full sky coverage and dynamic range. With a 10-year mission, the time span enabled by the mission length will provide a new, unmatched view on the statistics of variability on a decade time scale.

Exoplanets

Gaia will explore domains of the exoplanet parameter space that are difficult to access by other techniques, e.g. the transit or radial velocity (RV) methods. The astrometric method allows direct determination of the orbit inclination (i) and thus true planetary mass without the sin(i) ambiguity of the RV method. Unlike the transit method, it is sensitive to orbits of all inclinations, and it can be applied to almost all stellar types, including active young stars and binaries. The astrometric technique is best suited for finding gas and ice giants with periods from a fraction of a year up to 10-20 years (depending on the length of the mission) around stars within a few hundred pc from the Sun.

The sensitivity of the astrometric technique depends on the angular size of the star's reflex motion, the ‘astrometric signature’ a = avMp/(M*+Mp), where a is the semi-major axis (in au) of the planet's orbit, v the parallax of the star, and Mp, M* the masses of the planet and star, respectively. Given N astrometric observations of precision s, spread out over T years, the system with orbital period P is likely to be detected and the orbital parameters determined to useful precision if aN0.5/s ≥ 20 and P ≤ T. Systems with longer periods (up to about 3T) may be detected but are difficult to characterise completely without complementary information, e.g. from direct imaging or radial-velocity data. Extending T from 5 to 10 years will decrease the detection limit for Mp by a factor 1.4 for the census of giant exoplanets with P ≤ 5 year (by doubling N), and enlarge the domain of detected periods by a factor 2. Such an extension will increase the number of exoplanets found by Gaia by a factor three to four, up to around a total of several tens of thousands.

70% of the planets that are found with T = 10 year, but not with T = 5 year, have P > 5 year. The chances of successfully resolving more complex systems (e.g. multiplanet and circumbinary systems) drastically improve with longer T, allowing mutual inclinations and other dynamically important parameters to be measured in many more systems. Accessing the period range above 5 years (corresponding to a ≥ 3 au for solar-type stars), where giant planets are believed to be formed before some of them migrate to positions much closer to the star, is important for understanding the dynamical evolution of exoplanetary systems in different environments, including the uniqueness of our own solar system. Gaia will be able to explore this domain efficiently for large and varied samples of stars, but only if the mission length is significantly longer than five years. Although Gaia is unable to find Earth mass planets, it is capable of pointing out systems where a giant planet ‘guards’ the habitable zone closer to the star. These may be the most interesting objects for Earth like planet searches in the future.

Solar System science

With a 5-year extension, we will have twice as many individual observations for each minor body within the sensitivity reach of Gaia. In astrometry, this direct benefit means an orbit reconstruction improved by a factor higher than the mere sqrt(2) for the following reasons:

  • The orbit phase coverage will be more regular and this will impact on the remaining correlations between the orbital parameters.
  • A typical main belt object has an orbital period of about 5 years and in general with the nominal mission the largest interval of time between the first and last observations is shorter than the mission. With the 5-year extension we will have nearly two orbits covered and the constraint on the orbital period (therefore on the semi-major axis) will improve.
  • For the more distant solar system objects like the Trojans and the brightest members of the Kuiper belt, this is even more impressive. A Trojan has an orbital period of 11 years and the orbital elements are not well constrained with the 5-year mission, while an almost full orbit will be covered with the extension. The improvement will be close to one order of magnitude. For a Kuiper belt object whose orbital period is very long, the improvement should be similar to the coefficient of the proper motion (T1.5) and its first derivative (T2.5).
  • More difficult to quantify is the time sampling of the Near Earth Objects, which are not always detectable with Gaia due to the varying distance to the Earth, and with favourable observational epochs that are spaced by few years. Therefore, some will finally have a sufficient coverage to fit an orbit of good quality. In any case when this improvement will take place, this won't be a T0.5, but a qualitative improvement between a very poor, if any, orbit and a useful one.
  • Thanks to close encounters between big and small asteroids, about 50-100 asteroid masses can be determined from Gaia observations. A longer mission increases the performance at least in proportion of the mission duration since the number of favourable occurrences increases linearly with the time and the visibility of the orbit change is better as we have observations over a longer time span.

Solar system science will also benefit from Gaia indirectly, thanks to the quality of the astrometry of the star catalogue and that of the reference frame. The quality of the star catalogue in the past is determined almost exclusively by that of the proper motion. They improve as T1.5, meaning factor 2.8 for a 10-year mission. When the astrometry of the reference stars is the limiting factor in the re-analysis of old observations, we will have a comparable improvement in the small field astrometry in 20, 50 and even 100 years old images and photographic plates. Therefore, Gaia will allow improvement also of past observations.

Reference frame

The Gaia Celestial Reference Frame (Gaia-CRF) is composed of the core materialisation with the QSOs and the densified access with the stars. Both are important for the astronomical community and detailed below.


The overall quality of the Gaia-CRF is determined by the number of the sources and the astrometric solution. The overall spin and cosmic acceleration are constrained by the noise on the proper motion of the QSOs. With a mission twice as long, this noise decreases by a factor 2.8 and ultimately this will be the improvement factor on the inertiality of the Gaia-CRF at the mission completion. Considerably better proper motions will also help to improve the purity of the QSO set used for Gaia-CRF.

In addition, the Gaia frame will come with a 3D vector to represent the cosmic acceleration (or galactic aberration) acting like a systematic proper motion field for every QSO. Being similar to a proper motion, its determination improves as T1.5, or again this factor 2.8 for a 10-year mission, slowing down in the same proportion the degradation of the frame with the time.


This vector has a deep physical meaning since it represents the overall acceleration of the solar system with respect to the most distant background sources. With the 5-year mission it is expected to be determined to a 20-sigma level and this should reach almost the 60-sigma level with a 10-year mission. A level sufficient to investigate the direction of this acceleration in comparison with the direction of the Galactic centre determined by totally independent means. Considerably improved proper motion will allow us also to better characterize other physical effects resulting in apparent proper motions of QSOs - secular light deflection, cosmic parallax, etc. - and thus improve the long-term stability of the Gaia-CRF.


The secondary, densified access to the frame is achieved using stars, which are much more numerous in a small (random) field. The frame materialised by the stars degrades because of the imperfection in the knowledge of their proper motion. Again with a 10-year mission, the degradation rate is smaller by a factor 2.8 and for close binary stars less polluted by the orbital motion ‘noise’. The densified reference frame is essential for future ground-based giant telescopes with small field of view.

Fundamental physics

Several parameters of interest for fundamental physics are fitted as global parameters from all observational data. For a mission extension of 5 years, the statistical uncertainties of these parameters can be expected to improve by a factor 1.4. These parameters include e.g., parameter γ of the Parameterized Post-Newtonian formalism, parameters describing possible violation of the Local Lorentz Invariance or the upper boundary of the energy flux of primordial gravitational waves with frequencies below 2-3 nHz.


Special attention should be paid to possible higher-frequency gravitational wave signals in the Gaia astrometric data. Gaia is in principle sensitive to gravitational waves from continuous sources with frequencies between 3 nHz and 30 µHz. The possible sources of such continuous signals are binary supermassive black holes in the centre of galaxies. The sensitivity of Gaia after 10 years of operations is again expected to become 1.4 times better than
for the 5 year duration mission. Obviously, this leads to an increase of the space volume in which a given binary system can be detected by a factor of 2.8. The lower frequency limit for a 10 year mission drops to 3 nHz from about 6 nHz for a 5 year duration mission.

Complementarity and uniqueness

Gaia is a unique astrometry mission. There is nothing else of similar scope on the agenda of any space agency and Gaia observations will remain as the fundamental astrometric reference data for decades to come. In addition to astrometry the (spectro)photometric survey will remain unmatched in its full sky coverage, high dynamical range and milli-magnitude accuracy. Furthermore, the radial velocities provided for more than 100 million stars dwarf all ground-based spectroscopic surveys with similar aims. In its uniqueness the Gaia mission is practically a must pre-requisite for all photometric exoplanet missions. Gaia will also provide Euclid with the knowledge of stars that will be present in every Euclid field. In the future, essentially all ground-based telescopes and satellites using star trackers will use the Gaia catalogue as the astrometric basis.