The discovery of the first confirmed exoplanet in 1995 forever changed our understanding of planetary systems. Planet 51 Pegasi b, located in the constellation Pegasus, proved to be a massive gas giant comparable to Jupiter, but with an orbit so tight that it circles its parent star in just a few days. Such a configuration was completely unexpected: in the Solar System, Jupiter orbits far from the Sun, deep in the region beyond the so-called snow line, where ice and gas are more easily retained. The appearance of "hot Jupiters" – massive gaseous planets in extremely close orbits – became one of the greatest challenges for planetary formation theories.

It is well known that hot Jupiters cannot easily be explained by formation "in situ," close to the star. Most models assume that these planets formed far from the star, in the colder parts of the protoplanetary disk, and only later migrated inward. However, exactly how they arrived where our telescopes observe them is still one of the key open questions of modern astrophysics. This is where two main hypotheses enter the story: disk migration and high-eccentricity migration.

What are hot Jupiters actually?

By the term hot Jupiters, astronomers usually mean gas giants similar to Jupiter in mass, but with orbital periods shorter than ten days. Due to their extreme proximity to the star, their temperatures on the day side are often several thousand degrees, their atmosphere is exposed to strong radiation and stellar winds, and gravitational forces cause intense tidal effects. In some cases, the outer layers of the atmosphere literally evaporate into interstellar space.

Such planets represent a laboratory for studying the extreme physics of planets and stars, but also a key to understanding how entire planetary systems develop. If we know how one gas giant arrived in a close orbit, we can much more easily reconstruct the history of other planets in the same system – especially smaller, potentially rocky worlds in the "habitable zone."

For hot Jupiters, two basic migration pictures dominate contemporary models. The first is migration through the protoplanetary disk: a young planet, still immersed in a dense disk of gas and dust, gravitationally interacts with the disk material and slowly spirals down toward the star. This process is relatively calm and gradual; it keeps the planet's orbit nearly circular and well-aligned with the plane of the disk from which it formed.

The second scenario is high-eccentricity migration. In this case, the planet experiences strong gravitational perturbations after formation – for example, due to a close massive planet or a distant stellar companion. Such interactions can throw the planet onto a highly elongated, eccentric orbit. Whenever it approaches the star at perihelion, strong tidal forces arise that dissipate orbital energy and slowly shorten the semi-major axis of the orbit, while eccentricity gradually decreases. After millions or even billions of years, tides finally circularize the planet's orbit and bring it close to the star.

The old problem: how to distinguish the two scenarios?

A seemingly simple question – did a specific hot Jupiter reach its close orbit through the disk or via high-eccentricity migration – proved to be extremely stubborn. One of the most frequently used clues is stellar obliquity, i.e., the angle between the star's rotation axis and the planet's orbital axis. High-eccentricity migration often leads to large inclinations and even retrograde orbits, so such extreme cases are strong candidates for a "violent" origin.

The problem arises with systems where the orbital inclination is small or not detected at all. Low obliquity could mean that the planet migrated calmly through the disk, but there is another possibility: over time, tidal forces can partially or completely align the orbit and the star's rotation. In other words, both disk migration and high-eccentricity migration can end up in a very similar, seemingly "orderly" configuration. Because of this, astronomers have long sought an additional, more reliable criterion that would separate these two populations.

A new approach: comparing circularization time and system age

A team led by doctoral student Yugo Kawai and Assistant Professor Akihiko Fukui from the Graduate School of Arts and Sciences at the University of Tokyo proposed an innovative way to break this degeneracy. Instead of relying solely on orbital geometry, they focused on circularization time, i.e., the time required for a highly eccentric orbit to turn into a nearly circular one through tidal action.

In the high-eccentricity migration scenario, the planet's path looks roughly like this: after some gravitational perturbation throws it onto an elongated trajectory, the planet spends most of the time far from the star, but encounters extreme tidal forces with every passage through perihelion. Each such passage little by little "extracts" energy from the orbit and shortens the semi-major axis, while eccentricity gradually decreases. After a sufficiently long time, a hot Jupiter on a close, nearly circular orbit is formed.

Exactly how long this process will take depends on a number of parameters: planet mass, radius, density, distance from the star, initial eccentricity, and, very importantly, the so-called tidal quality factor, a quantity that describes how efficiently the planet dissipates energy under the influence of tidal forces. If astronomers can estimate all these quantities, they can calculate how long it should take for the orbit of a hypothetical proto–hot Jupiter to circularize to currently observed conditions.

The key idea of Kawai and colleagues is simple but powerful: if the circularization time for the given parameters is longer than the age of the observed planetary system, high-eccentricity migration simply did not have enough time to finish the job. In that case, it is more likely that the planet arrived at today's close, circular orbit via calmer migration through the disk.

How they calibrated tidal processes on hundreds of planets

To turn their approach into a concrete diagnostic tool, the researchers first had to determine what the typical tidal quality factor is for gas giants. They did this by analyzing a large sample of more than 500 known exoplanets ranging in mass from approximately one-fifth to thirteen Jupiter masses, for which both masses and radii are known. By combining the observed eccentricity distribution and models of tidal energy dissipation, they obtained a tidal factor value comparable to that estimated for Jupiter itself in the Solar System.

Based on such a calibrated model, they calculated for each planet with a nearly circular orbit the time that would be required for high-eccentricity migration to lead to the observed state. Then they compared that value with the age of the corresponding system, which astronomers usually estimate from the properties of the parent star – color, brightness, spectroscopic features, and evolutionary models.

The result was surprisingly clear. While for many hot Jupiters they found that high-eccentricity migration could indeed lead to today's orbits within the star's lifetime, for a part of the population it turned out that the circularization process would take longer than the age of the entire system. Despite this, these planets are observed on almost perfectly circular orbits.

About thirty candidates for disk migration

Ultimately, roughly thirty hot Jupiters were singled out whose orbital eccentricities are very small, and the calculated circularization time significantly exceeds the age of their stellar systems. According to the logic of the new model, these planets almost certainly could not have gone through the full phase of high-eccentricity migration. The most natural explanation is that they slowly descended toward the star while still immersed in the protoplanetary disk of gas and dust.

When the researchers looked more closely at their sample of candidates for disk migration, three interesting trends appeared. First, a clear boundary was observed in stellar obliquity exactly around the ratio where the circularization time equals the system age. Above that threshold are mostly planets with good alignment, while markedly inclined orbits are more common in systems where high-eccentricity migration had enough time to do its work.

Second, among hot Jupiters identified as candidates for disk migration, the occurrence of neighboring planets in relatively close orbits is surprisingly common. In the high-eccentricity migration scenario, strong gravitational perturbations that elongate the orbit of one giant usually lead to the scattering or even ejection of other planets. Therefore, the presence of additional planets in the plane and on stable orbits further supports the picture of calmer, disk migration.

Third, the authors discern in the data an intriguing "notch" in the distribution of candidates of a certain planet-to-star mass ratio. In that mass range, it seems there is a lack of planets that would fit the criteria for disk migration, which could point to the phenomenon of so-called uncontrolled or "runaway" migration. In such a scenario, the planet, once it crosses a certain threshold, sinks extremely quickly through the disk toward the star, leaving only a narrowed window in which we can find it at a transitional distance.

Aligned orbits and multiple systems as traces of a peaceful past

The candidates that the team from Tokyo singled out share several characteristics that fit naturally into the picture of disk migration. Their orbits are mostly well-aligned with the star's spin, which is expected if they formed and grew in a thin gas disk whose plane defined the reference geometry of the entire system. At the same time, there is no need to resort to additional alignment mechanisms that would "iron out" the configuration again after a turbulent high-eccentricity phase.

An even stronger argument comes from the fact that a significant portion of these planets live in multiple systems. In them, alongside the hot Jupiter, we find additional planets, sometimes of only slightly smaller mass or located on slightly more distant orbits. Preserving such an architecture is difficult to reconcile with a scenario in which one gas giant went through a phase of extreme eccentricities and strong close encounters with other bodies. Disk migration, conversely, naturally allows a whole series of planets to be jointly dragged inward, without dramatic collisions and ejections.

Together, these indicators point to the fact that within the population of hot Jupiters there exists a recognizable subset that arrived at its tight orbits via a "soft" path, while other examples most likely bear the signature of a more violent, high-eccentricity history. The new approach does not rule out either scenario, but enables us for the first time to statistically separate those systems in which disk migration was the dominant process.

What can these planets tell us about conditions in protoplanetary disks?

Spotting planets that still bear a clear stamp of their migration process is extremely valuable because it opens a window into the early stages of planetary system development. If we know that a certain hot Jupiter reached its orbit by migration through the disk, then its current chemical and dynamic properties become traces of the conditions that prevailed in that disk.

For example, ratios of elements like carbon, oxygen, nitrogen, and metals in the atmosphere of such a planet can reveal in which part of the disk it formed – above or below the freezing line of water, carbon monoxide, or other key compounds. If it turns out that disk–candidates systematically carry a different chemical signature from planets that likely went through high-eccentricity migration, that would mean that not only was their path to the star different, but their "birthplaces" within the disk were also different.

Besides atmospheric composition, valuable information is hidden in the internal structure of the planet. Possible differences in core mass, heavy element fraction, and total density are linked to the conditions in which the planet accreted. While detailed models require a combination of observations in multiple wavebands and sophisticated numerical simulations, precisely the selected group of disk–candidates represents an ideal sample for such studies.

Future observations: from TESS to large ground-based telescopes

The new criterion based on the comparison of circularization time and system age comes at a moment when the number of known exoplanets is rapidly increasing thanks to missions like TESS and Gaia, as well as numerous dedicated radial-velocity and transit surveys from Earth. Every new hot Jupiter for which mass, radius, and orbital parameters are known immediately becomes a candidate for the application of the same diagnostic method.

In the coming years, an increasing number of detailed atmospheric measurements of hot Jupiters is expected using next-generation space telescopes, but also high-resolution spectrographs on large ground-based telescopes. The combination of such observations with information about the probable migration path of individual planets could turn disk–candidates into sorts of "fossils" that preserve the chemical record of the early disk.

Additionally, statistical comparisons between disk–candidates and hot Jupiters with clear signs of high-eccentricity migration can help in determining the share that each scenario contributes to the total population. Already now, based on the existing database, a picture is emerging in which neither mechanism is exclusive: it seems that nature uses both peaceful and violent paths to bring large gaseous planets into hot, tight orbits.

The bigger picture: what hot Jupiters say about our Solar neighborhood

Although there is no hot Jupiter in our Solar System, insights about these exotic worlds directly affect the understanding of our own cosmic neighborhood. Jupiter and Saturn, according to modern models, likely also migrated – albeit much more moderately – and by their movement strongly influenced the distribution of material in the inner system. Thereby they shaped the conditions for the formation of Earth and its neighbors.

If we know under what conditions gas giants end up as hot Jupiters, and when they remain at moderate distances, we can better estimate how rare or common systems like ours are in the galaxy. Disk migration that leaves room for stable, multiple planetary configurations could favor the formation of rocky planets in habitable zones. Conversely, violent high-eccentricity migration that disrupts the inner system likely reduces the chances for long-term stable, Earth-like worlds.

In this context, the work of Kawai, Fukui, and their colleagues is not just a technical advance in modeling tidal processes, but also an important step toward a bigger picture: what part of the galaxy consists of "peaceful" systems in which planets grow and migrate harmoniously, and what part of those in which one gas giant takes on the role of a cosmological destroyer?

Planets as time capsules of the early disk

Hot Jupiters which, according to the new method, most likely arrived at their orbits by migration through the disk, can be observed as time capsules. Their current configuration is the result of a long-lasting, but relatively smooth process in which they changed their position over millions of years while the disk slowly dissipated. Proper alignment of orbits, presence of other planets, and specific chemical signatures in their atmospheres make them unique witnesses of an epoch we cannot otherwise directly observe.

As exoplanet databases expand, and models of tidal evolution are refined, this kind of approach could be applied to other classes of planets as well – from mini-Neptunes to massive super-Earths on tight orbits. Every new group of candidates with "impossible" circularization times will become valuable for reconstructing the history of protoplanetary disks and recognizing dominant migration mechanisms.

For now, the thirty identified hot Jupiters provide the first firmer statistical sample that links migration through the disk to concrete observable characteristics. They are just the beginning of the story, but already now clearly show that behind the simple term "hot Jupiter" hides a diversity of cosmic journeys – from peaceful spirals through the disk to dramatic elliptical leaps that nearly destroy entire planetary systems.