How do planetary systems like our Solar System form – and how early in the life of a star do planets begin to shape their orbits? The latest data analysis from the European Space Agency's (ESA) Gaia space telescope, combined with observations from the ALMA radio telescope in Chile, provides the most detailed statistical insight yet into what happens in the inner parts of protoplanetary disks around very young stars. In 31 out of 98 analyzed young systems, astronomers observed a subtle “wobble” of the stars that reveals a hidden society: planets, brown dwarfs, and additional stars.

From gas clouds to a “baby” stellar system

Every stellar system begins as a vast, cold cloud of gas and dust in interstellar space. Under its own gravity, the cloud begins to collapse. As material flows toward the center, the cloud speeds up, flattens, and takes the shape of a rotating disk. In the center, a dense, glowing core forms – the future star – while around it stretches a protoplanetary disk, a reservoir of material from which planets, moons, and smaller bodies are born.

In some of these disks, astronomers observe a large internal “empty” space, a cavity in the dust at distances of several to several dozen astronomical units (AU) from the star. These objects are called transition disks. The gaps are not truly completely empty – they still contain gas and fine particles – but the lack of dust is visible at infrared and millimeter wavelengths. One of the main explanations for a long time was that the cavities are carved out by a massive planet or several planets “clearing” their orbits.

Thanks to the ALMA interferometer, astronomers have been imaging transition disks with incredible spatial resolution in recent years. In collages obtained by ALMA, the disks are often shown in shades of orange and purple, with brighter rings indicating accumulations of millimeter dust. In the latest study, some of these ALMA images – 31 “baby” stellar systems – are assembled into a composite image where, in the lower right corner, there is also a reconstruction of the young Solar System at an age of about a million years, with Jupiter's orbit indicated as a reference.

Gaia: the space telescope that measures the “stumbling” of stars

The Gaia space telescope was launched in 2013 with the primary task of creating a three-dimensional map of our Galaxy. Over more than ten years of operation, it has measured the positions, distances, and movements of about 2.5 billion stars with precision previously considered unattainable. Instead of a classic “image,” Gaia repeatedly scans the entire sky and records tiny movements of stars in the sky – astrometry – in angular sizes of the order of micro-arcseconds.

If there is a massive companion around a star, whether it is a planet, a brown dwarf, or another star, the gravity of that object will move not only the satellite but also the star itself. Instead of gliding peacefully through space, the star describes a tiny spiral around the common center of mass of the system. Gaia can register exactly this “stumbling” – astrometric wobble – as a deviation from the expected straight-line motion. The more massive the companion and the wider its orbit, the easier it is to detect this signal.

This technique has already been used to discover massive planets and brown dwarfs around older stars. In 2025, for example, analysis of Gaia data confirmed the existence of the super-Jupiter Gaia-4b and the brown dwarf Gaia-5b around two low-mass stars, with Gaia for the first time independently discovering an exoplanet solely through astrometry. But the new study went a step further: the same method has now been applied to stars that are only in the formation phase and are still embedded in protoplanetary disks.

Statistical hunt for companions in 98 transition disks

The team led by Miguel Vioque from the European Southern Observatory (ESO) focused on 98 young stars with transition disks. These are objects whose structure has already been studied in detail with ALMA, which means that the size and shape of their dust cavities are well known. The goal was to answer two related questions: how often such disks even have massive companions in the inner parts of the system and whether these companions can indeed be responsible for the formation of the observed dust cavities.

Using the latest Gaia data, the researchers calculated so-called proper motion anomalies – the difference between the expected and measured movement of each star across the sky – and analyzed the RUWE parameter, which measures how well Gaia's simple point-parameter model describes the observations. A significant deviation usually means that a more complex system with a companion is hidden behind a “simple” star.

Analysis shows that 31 of the 98 transition disks (approximately 32% of the sample) show convincing astrometric anomalies that are best explained by the presence of a hidden companion. By modeling the combination of mass and orbital semi-major axis that can produce the observed signal, the authors showed that Gaia in this sample can typically detect companions with a mass ratio greater than about 1% of the star's mass, at distances of approximately 0.1 to 30 AU. In translation, we are talking about objects with masses ranging from a few Jupiters up to low-mass stars in orbits that cover the region where Earth, Jupiter, and Saturn orbit in our system.

Seven planetary-mass candidates, brown dwarfs, and additional stars

The most exciting result of the study is the identification of seven systems in which the astrometric signal is compatible with a planetary-mass companion, smaller than about 13 Jupiter masses. These are the stars HD 100453, J04343128+1722201, J16102955-3922144, MHO 6, MP Mus, PDS 70, and Sz 76. Some of these objects are already known for their interesting disks or even previously discovered planets, but Gaia now provides independent confirmation that massive gravitational “architects” are indeed active in their inner zones.

In eight other systems, the data match best with the existence of brown dwarfs – objects with masses between the most massive planets and the smallest stars, which do not have enough mass to sustain long-term hydrogen fusion in their core. Such “failed” stellar embryos are particularly interesting because they blur the boundary between planets and stars: do they form like stars, through the direct collapse of a cloud, or like planets, through the accumulation of material in a disk?

The remaining part of the detections – an estimated sixteen cases – likely represents additional low-mass stars in binary or multiple systems. In these cases, the companion is so massive that it falls into the stellar domain by all criteria, even though it continues to develop surrounded by the remaining disk of gas and dust. Taken together, most of the observed companions are more massive than 30 Jupiter masses, which means that transition disks often hide an unexpectedly “heavy” society.

Cavities in disks: where are the planets that dug them?

One of the key questions that motivated this analysis is: can the observed companions explain the large dust cavities we see in ALMA images of transition disks? Intuitively, a massive planet or a brown dwarf in a suitable orbit should “clear” material in its surrounding area, creating ring-like structures and holes in the disk. However, the results show that the story is more complex.

For approximately half of the detected companions – the authors state about 53% of cases – simple models fail to reconcile their orbital parameters and mass with the size and shape of the dust cavities. In other words, even when we know a companion exists, it seems it cannot alone create the disk structure we observe. In these cases, the cavities are likely formed by the action of other, so far undiscovered companions at greater distances, or in combination with processes such as disk photoevaporation by high-energy radiation from the star, magnetic fields, and turbulence in the gas.

These results build on the broader “transition disk revolution” that has been going on for more than a decade. Systematic observations with ALMA have shown that rings, arcs, and spirals in disks are not the exception, but the rule. For some of these structures, the planets creating them have been directly discovered, but in many cases, the “culprit” is still missing. Gaia's astrometric view now confirms that there is a rich inventory of massive companions in transition disks, but also that their presence alone does not always provide a simple explanation for everything we see.

The young Solar System as a benchmark

In the visualization of the new study, the panel dedicated to our own system is particularly interesting. Researchers have reconstructed what the Solar System might have looked like at an age of about a million years, when the planets were just forming from the protoplanetary disk. The Sun is placed in the center of the image (though not explicitly shown), and Jupiter's orbit is marked by a blue (cyan) ring. This ring is also used in other panels as a reference for scale comparison: how much larger or smaller the “baby” stellar systems are than the one from which our home originated.

Such a display allows the reader to intuitively understand the distances involved. While some transition disks have cavities smaller than Jupiter's orbit, others extend far beyond it, into the region where Uranus and Neptune orbit in our system. Understanding how planets form and migrate in these disks ultimately means understanding why our Solar System turned out exactly like this, with four rocky planets in the inner part and four giants in the outer part.

What the new discovery says about planet formation

The combination of Gaia astrometry and ALMA images of transition disks opens a new phase in the study of planet formation. Unlike individual spectacular discoveries, this study provides statistical insight into a whole sample of nearly a hundred young stellar systems. When these results are combined with theoretical models, it becomes clearer that dust cavities cannot always be explained by a single giant planet: it is often more likely that several planets are involved, a combination of planets and brown dwarfs, or planets hiding at greater distances than those to which Gaia is currently most sensitive.

On the other hand, Gaia's success in finding companions in transition disks confirms that massive objects do indeed form very early, while the disk still exists. This fits into the broader picture provided by other recent discoveries. For example, observations of the young HOPS-315 system with the James Webb telescope and ALMA during 2025 showed the first traces of hot mineral grains hardening in a disk only a few hundred thousand years old – the earliest “seeds” of future planets. Such results suggest that the process of forming solid bodies begins exceptionally early, and Gaia now adds evidence that massive companions can significantly reshape the disk even then.

In the global picture, transition disks represent a transitional phase between a “young” disk filled with gas and dust and a later stage dominated by planets and smaller bodies like asteroids and comets. Understanding the role of companions in this phase is key to answering the question of how similar or different typical planetary systems in our Galaxy are to the Solar System.

Gaia has finished observations, but the data is just starting to “work”

Although the Gaia space telescope finished collecting scientific data in early 2025, its astrometric revolution will last for years to come. Routine observations ceased on January 15, 2025, and the mission was formally concluded in the spring of the same year. By then, a database had been collected with information on the positions, velocities, and physical properties of approximately 2.5 billion sources, from asteroids in the Solar System to distant quasars.

So far, the third major data release (DR3) has been publicly released, but the next step is already being prepared in the background. The fourth major data release, Gaia DR4, is expected in 2026 and will be based on the first 5.5 years of observations. It is expected to include the first large collections of exoplanet candidates discovered by astrometry, potentially thousands of new planets and brown dwarfs around stars in our cosmic neighborhood.

The study of transition disks led by Vioque builds exactly on that upcoming wave. Although this is a targeted sample of “only” 98 stars, the methods developed in this work have shown that Gaia data can be successfully applied to young, variable sources, where additional processes – such as accretion jets, starspots, and light scattering in the disk – potentially spoil the astrometric signal. The authors conclude that these effects do not dominate and that astrometry can be robustly used even in the earliest stages of a star's life.

Synergy with future telescopes

The list of companions Gaia has now identified in transition disks represents an ideal catalog of targets for further observations. The James Webb telescope, with its sensitive instruments in the infrared, can “peek” into the inner parts of the disks and try to directly detect the thermal radiation of young planetary masses or study the chemical composition of the gas in their vicinity. ALMA can further resolve the structure of dust and gas around the candidates Gaia has discovered, while future terrestrial giants like the Extremely Large Telescope (ELT) will be able to directly image some of these objects.

Such synergy is key to understanding planet formation. Gaia provides a global, “dynamic” view – it shows us how the star reacts to the gravity of a companion – while ALMA, James Webb, and other telescopes provide a “snapshot” of the disks and planets themselves. Together, they will separate cases in which one massive companion dominates the dynamics from those where many, perhaps even less massive planets distributed at different distances are responsible for the disk structure.

As the publication of Gaia DR4 approaches, astronomers expect the number of known companions – from planets to brown dwarfs – in young and mature systems to increase dramatically. The latest results from transition disks show that there will likely be a whole population of hidden objects among them operating within dust cavities, precisely in the regions where, at least in our case, the key building blocks for the formation of potentially habitable worlds are located.