2024-04-22 Gaia's contribution to discovering distant worlds - Gaia

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Gaia's contribution to discovering distant worlds

Figure 1. Graphical representation of the mass (Jupiter Masses) versus orbital period (days) distribution of the population of known extrasolar planets (status 10 January 2024). The different colours and shapes with which the planets are identified indicate the different methods by which they have been detected. The light blue rectangle indicates Gaia's containment area, the purple one that of Cheops, the orange one that of PLATO and the dark blue one that of Ariel. Credits: Adapted from NASA Exoplanet Archive Caltech by Mariasole Agazzi for the Gaia mission. GIF by Tineke Roegiers.

The European Gaia space mission is primarily designed to map with extreme precision the positions, parallaxes, and proper motions of over a billion stars in the Milky Way and beyond, creating an exceptionally detailed three-dimensional stellar catalogue. Its characteristics have also proven optimal for contributing to one of the most promising fields of astrophysics in the last twenty years: the study and characterisation of extrasolar planets. Indeed, a wide range of applications exist for which Gaia data will be key for the ongoing exoplanetary ESA missions, Cheops, Plato and Ariel, as well as missions in collaboration, such as the James Webb Space Telescope. This story provides an overview of the contribution of Gaia to this research area and positions the mission in the landscape set by ESA's exoplanet missions, Cheops, PLATO, and Ariel, showing its synergies and complementarities.

The search for extrasolar planets

The field of exoplanets represents one of the most fascinating and dynamic areas of contemporary astronomy. With the growing interest in understanding the universe and its planetary diversity, the search for and study of planets outside our solar system has become crucial. Exoplanets offer an unprecedented opportunity to understand the formation and evolution of stellar and planetary systems, as well as to explore the conditions for the emergence and sustainability of life in the universe. The first discovery of an exoplanet in 1992 has indeed revolutionized our view of the universe, giving rise to many fundamental questions about the origin of life and its possible presence outside our solar system. In the last twenty years, we have witnessed an almost exponential increase in the number of detected planets: as of today (March 2024), over five thousand have been found.

Figure 2. Graphical representation of the number of confirmed extrasolar planets (y-axis) detected per year (x-axis) by type of detection. The different colours correspond to the different detection techniques used. The two predominant colours are green, which corresponds to the transit method, and red, which corresponds to the radial velocity method. Source: NASA Exoplanet Archive Caltech.

To detect these distant worlds, astronomers employ various methods. Among the main ones is the transit method, which detects the periodic decrease in the apparent brightness of a star when a planet passes in front of it. Another method is the radial velocity method (Doppler technique), which exploits small variations in the radial velocity of a star caused by the presence of a planet orbiting it. Others include gravitational microlensing, direct imaging, and astrometry. One of the most promising methods and newest kid on the block is astrometry. This technique uses the precise measurement of the positions of stars in the sky to detect the minute oscillatory movement caused by planets orbiting them. It is a method particularly suitable for detecting massive planets in orbits distant from their parent star.

Despite the potential of this technique, as we can see from the image above, only a few planets have been detected astrometrically so far. The reasons can be attributed to technical challenges, instrument limitations, and required resources. To utilize astrometry, extremely sensitive and precise instruments are necessary. Moreover, since deviations in stellar positions can be very subtle, repeated and prolonged observations are required. In this context, Gaia comes to the rescue, playing a significant role. In fact, the mission offers an unprecedented opportunity to expand the exoplanet knowledge through astrometry, providing extremely precise astrometric data on a vast number of stars in our galaxy.

"In terms of their geometric alignment, the planets discovered with Gaia will be orthogonal to those normally detected and characterised with the transit method by CHEOPS, PLATO, and ARIEL. This will open a completely new discovery space that we have not really been able to explore until now. It will be interesting to see whether the assumption holds true that most planetary systems lie on the same orbital plane just like in our Solar System. Maybe some exoplanet systems have been so perturbed that Gaia could find huge outer planets that are not in the same plane as their inner, transiting siblings. We can cross-match whether known transiting exoplanets have Gaia signals, or vice versa, follow the exoplanets detected by Gaia to detect if there are also transit signals. Such results could lead us to reassess our idea of planetary systems’ formation and evolution. In any case, we will open a new exploration sandbox."

Maximilian Günther, ESA Cheops Mission Project Scientist

The Gaia DR3 data release, for the first time, included astrometric orbital solutions for companions around stars with sensitivities extending into the planetary-mass regime. Through sophisticated methods like Markov chain Monte Carlo and genetic algorithms, it was possible to fit the orbits of 1162 sources, validating 198 of these, across the planetary, brown dwarf, and low-mass stellar companion regimes (Holl et. all 2023). Thanks to this technique Gaia DR3 showed the companion of HD114762 (Latham et al. 1989) which in hindsight could have been the very first super-Jupiter exoplanet, was a clear binary star. This achievement highlighted Gaia's capability to refine known orbital companions and discover new ones, promising significant advancements in our understanding of exoplanets and sub-stellar companions with future data releases.

But that’s not all, Gaia makes for an essential tool in the research and understanding of planetary systems with its detailed catalogue of host stars. Gaia stellar data are widely used by current exoplanetary missions, significantly contributing to our understanding of the universe.

"Knowing the characteristics of exoplanet host stars is fundamental. In fact, stars and planets interact directly with each other. The star plays a fundamental role in the evolution of the planet: the same planet around a different star would have developed in a completely different way. For this reason, it is important to know the characteristics of the star, and this knowledge provided by Gaia is increasingly recognised within the exoplanet community."

Theresa Lüftinger, ESA Ariel Mission Project Scientist

Gaia's role in exoplanet detection

Despite the field of extrasolar planets developing concurrently with the Gaia mission’s inception, the initial documents outlining Gaia's objectives already hinted at a significant contribution to this emerging research area. Over time, this possibility became a certainty: to date, Gaia significantly contributes to the science of extrasolar planets through various means.

"Exoplanet science wouldn’t be the same without Gaia."

Maximilian Günther, ESA CHEOPS Mission Project Scientist

Characterisation of host stars

By creating an exceptionally detailed multi-dimensional stellar catalogue, Gaia provides information about host stars around which extrasolar planets orbit. The data obtained with the space telescope are employed to derive stellar masses, stellar radius, temperatures, luminosity, and other characteristics crucial to understanding the environment in which planets orbit. A comprehensive characterisation of the fundamental properties of host stars is essential, as the formation and evolution of exoplanets are directly influenced by host stars at various points in time. Stellar information also allows to determine characteristics of the planet itself. Accurate stellar data are essential for deriving the planet's radius, mass, and age, but additional information may be required to fully determine these parameters. This is particularly intriguing because some theoretical models, using knowledge of radius and mass to derive density, allow us to make certain assumptions about the planet's composition and structure.

Starting with the Data Release 2 in 2018, Gaia data have allowed to re-derive very accurate and precise stellar radii that in turn made for a critical re-assessment of the bimodality of the distribution of the radii of thousands of small planets uncovered by the Kepler mission: Gaia's first key contribution to the exoplanet field.

Gaia’s star catalogue constitutes a fundamental reference for other missions, which use it both initially to select the targets to observe and, subsequently, to deduce planet information from their host stars. Also the catalogue of future missions currently being developed, such as HPIC: The Habitable Worlds Observatory Preliminary Input Catalog, is based on the use of Gaia data.

"If we desire to know and characterise the planet, we must know the star. For PLATO, our input catalogue has been fundamentally based on the Gaia catalogue, which was then complemented with additional information."

Ana Heras, ESA Plato Mission Project Scientist

Release of an unbiased planets catalogue

Although Gaia's primary goal isn't the detection of exoplanets, its ability to monitor variations in stellar positions reveals the presence of planets orbiting around them. The mission surveys a vast number of stars, all-sky, covering a time baseline of around 10 years, measuring their positions, radial velocities, and other features with high precision. This data can be used to detect the gravitational influence of exoplanets on the position of their host stars, enabling planet detection.

Gaia is primarily dedicated to high-precision astrometry but also provides spectroscopy and photometry, allowing to study various types of photometric variability, including exoplanetary transits. So far, transit photometry has been the most effective method for detecting exoplanets with over 3000 of the over 5000 exoplanets discovered to date, in majority thanks to the Kepler mission.

Gaia’s photometry has some limitations though, making it less optimal for transit planet detection: it is sparse and irregularly sampled, requiring a longer observation span to distinguish the signals by extrasolar planets. Nonetheless, it has already been possible to confirm the detection of two previously discovered exoplanets: WASP-19b (Hebb et al. 2009) and WASP-98b (Hellier et al. 2014). Furthermore, in 2022, Gaia found its first extrasolar planets with the transit method (Panahi et al. 2022) which were never detected before and published 214 of such candidates for follow-up. With the upcoming fourth data release, we can anticipate the discovery of hundreds of new planets by Gaia using the transit method.

However, Gaia's true paradigm shift lies in the astrometric method, which has so far allowed the discovery of only a limited number of extrasolar planets. The Gaia mission is set to become a game-changer in this context, unleashing the power of micro-arcsecond astrometry.

Like the spectroscopic technique, astrometric measurements can reveal periodic changes of the star’s position around the system's barycenter due to the gravitational attraction of orbiting planets. With the two major data releases still come, Gaia DR4 (expected no sooner than the end of 2025) and Gaia DR5, Gaia will produce all-sky exoplanet catalogues containing thousands of new exoplanets candidates. In addition, the complete pool of collected data will be made available, which can be analysed by the researchers to detect further candidate exoplanets. This might double the number of extrasolar planets discovered to date, which currently stands at over five thousand: a real revolution.

"With the two upcoming data releases, DR4 and DR5, Gaia's full astrometric planet finding capabilities will be unveiled. Leveraging the 10-yr extended mission, thousands of giant planets beyond the snowline will be detected, including true Jovian analogues (same mass and period of Jupiter). Gaia is particularly 'democratic', as it will provide a census of cold gas giants that will be unbiased across mass, chemical composition, and age of the host stars. Gaia's crucial contributions to many aspects of the formation, physical and dynamical evolution of planetary systems will be fully realized exploiting its huge synergy potential with ongoing and planned exoplanet detection and (atmospheric) characterization programs, such as those that are being and will be carried out by Cheops, PLATO and Ariel."

Alessandro Sozzetti, Gaia Data Processing and Analysis Consortium

A crucial characteristic is that Gaia is impartial in its examination of the entire sky, monitoring stellar and sub-stellar objects without discrimination based on spectral type, age, evolutionary status, stellar environment, or multiplicity status. We can therefore expect Gaia not only to monitor millions of main-sequence stars with sufficient sensitivity to detect brown dwarf and/or exoplanet companions within a few astronomical units of their host stars but also to provide accurate astrometric time series for thousands of very low-mass stars in the solar neighborhood (Sozzetti et al. 2014). For these targets, the mission's astrometry could be sufficiently precise to reveal any orbiting companion with masses even below one Jupiter mass in addition to planets belonging to binary systems. Obtaining an impartial catalogue is a crucial result as it will allow us to better understand the distribution of different types of planets across the Milky Way and to study the impact of stellar environment.

"For those working in this field, an important goal is to understand the demography and diversity of the systems out there - what can exist? Sub-Neptune-type planets, for example, are one of Cheops' main targets. Although they do not exist in our solar system, we know that they are the most common planet type in our galaxy. This poses a new puzzle to solve: why do we find ourselves in a system with life without this kind of planet? Could we have life with this kind of planet? With Gaia exploring a completely new parameter space, a range we have never had the opportunity to look at before, similarly new insights could be discovered, giving us more clues about the formation and evolution of extrasolar planets. Such pieces of the puzzle from different angles are truly fundamental for completing the picture."

Maximilian Günther, ESA Cheops Mission Project Scientist

Figure 3. Graphical representation of the mass (Jupiter Masses) versus orbital period (days) distribution of the population of known extrasolar planets (status 10 January 2024). The different colours and shapes with which the planets are identified indicate the different methods by which they have been detected. The light blue rectangle indicates Gaia's containment area, the purple one that of Cheops, the orange one that of PLATO and the dark blue one that of Ariel. Credits: Adapted from NASA Exoplanet Archive Caltech by Mariasole Agazzi for the Gaia mission.

Gaia will not be able though to detect planets with an Earth-like mass unless the object is extremely close to the Sun, which is unlikely. In an era where exoplanetary research is focused on the search for Earth-like planets, we may therefore ask: does it really make sense to try to detect a sample of planets that deviate so much from the target of greatest interest? Yes, absolutely! In fact, it is essential to understand the variety of the extrasolar planet zoo, to possess a large sample of planets with different characteristics. For instance, the presence of Jupiter in the solar system, acting as cometary vacuum cleaner preventing Earth from being heavily bombarded, may well have been instrumental for the development of life on Earth.

"If we want to understand Earth-like planets, how they form, how they evolve, what their atmospheres are composed of, we absolutely also need to understand the formation and evolution of the largest planets. Even if we look for habitability in other systems, it is necessary to understand the whole system and its chemistry and dynamics, including the gas giant planets. Take our own solar system as an example: Jupiter and Saturn are important for the orbit of the Earth, and thus for the development of life. If only Saturn had been on a more elliptical orbit or closer to the Sun, Earth's climate would have developed in a very different way - and this is just one example. The large planets therefore also play a crucial role in the evolution and the habitability of the smaller ones."

Theresa Lüftinger, ESA Ariel Mission Project Scientist

Finally, Gaia contributes to validating extrasolar planetary candidates discovered by other missions such as Kepler, TESS, and ground-based telescopes. Gaia's precise measurements of stellar positions can confirm the existence and characteristics of suspected exoplanets and reveal the structure and dynamics of extrasolar planetary systems.

What can we expect for the future?

In conclusion, Gaia emerges as a crucial catalyst for the study of extrasolar planets. With its ability to perform extremely precise astrometric measurements, the mission will soon bring about a paradigm shift in this field of research. With the upcoming fourth data release, Gaia is expected to unveil thousands of new planets. Its non-discriminatory astrometry against spectral type, age, or multiplicity of the host stars offers the opportunity to detect planets even in binary systems and will provide a comprehensive view of the exoplanet zoo. Moreover, Gaia will continue to contribute to the fundamental characterisation of host stars of exoplanets, adding to the understanding of the formation and evolution of planetary systems. In summary, Gaia opens new avenues in the search for and study of exoplanets, enriching our understanding of the universe and its diverse planetary facets.

Figure 4. Gaia discovery space. Shown here are the planetary mass of exoplanets versus the semi-major axis of the orbits of exoplanets about their host star. The upper purple curves indicate the sensitivity of Gaia with respect to objects out to a distance of about 150 parsecs (so out to various star formation regions), as compared to the lower purple curves which indicate the sensitivity of Gaia to detect extrasolar planets out to a distance of about 20 parsecs. While the dotted purple curves correspond to the nominal Gaia mission lifetime of 5 years, the full purple curves correspond to an extended mission for a total of 10 years. Credits: Sozzetti and de Bruijne 2018.