Astronomers have discovered thousands of planets beyond our solar system, but one question remains surprisingly difficult to answer: why do planetary systems develop so differently, even when they form around stars of the same galaxy?
A study published in Nature Astronomy on October 13, 2026, offers a new perspective. Researchers identified two distinct families of multi-planet systems in the Milky Way, finding that the properties of a star and the conditions in which its planets form can influence how that system develops over time. Read the original research in Nature Astronomy.
The findings suggest that planetary systems are not simply collections of worlds orbiting a star. Their present-day arrangements may preserve clues about their beginnings, including whether their early histories were relatively calm or marked by gravitational upheaval.
For scientists searching for potentially habitable planets, understanding these differences could help explain why some planetary systems remain orderly while others undergo dramatic rearrangements.
How Scientists Identified Two Families of Planetary Systems
The international research team, led by Diego Turrini and colleagues, analysed hundreds of known multi-planet systems using measurements of planetary masses, orbital properties and the characteristics of their host stars.
Their reference sample contained 298 planetary systems with sufficiently detailed measurements. The researchers also examined additional samples to test whether their findings remained consistent when different data-selection criteria were applied.
Rather than classifying systems according to a single feature, the team investigated their overall architecture, including how planetary mass is distributed and how widely the planets orbit their stars.
The analysis revealed a bimodal distribution in system mass, meaning that the observed population showed two broad groupings rather than one continuous pattern.
The researchers identified a lower-mass family and a higher-mass family. Each displayed different characteristics associated with its host star and the dynamical history of its planets.
This does not mean every planetary system fits neatly into one category. Some systems fall into an uncertain region between the two groupings, and observational limitations continue to affect which planets astronomers can detect.
Nevertheless, the results provide evidence that the diversity of planetary systems may follow recognisable patterns rather than being entirely random.
Family One: Planetary Systems That Experienced More Turbulent Histories
The first family contains relatively massive planetary systems, many of which orbit stars more massive than the Sun and have higher concentrations of elements heavier than hydrogen and helium.
In astronomy, these heavier elements are collectively described as metals, even when they are not metals in the everyday chemical sense.
The researchers found that this family tends to have more dynamically excited configurations. In practical terms, its planets are more likely to occupy orbital arrangements that indicate a history of gravitational disturbances.
When several planets form within the same system, their gravitational forces can influence one another. If their orbits become unstable, planets may migrate, collide, or even be ejected into interstellar space.
Such events can substantially alter a system’s original architecture.
A planet that formed far from its star, for example, might eventually move into a much closer orbit. Other planets could be displaced, leaving behind a system very different from the one that existed during its earliest stages.
The study suggests that some high-mass systems may have started out unstable because their planet-forming environments contained abundant gas and solid material. Under those conditions, planets can grow rapidly, potentially making it harder for the original orbital arrangement to remain stable.
This offers a possible explanation for some of the unusual planetary configurations astronomers have discovered.
Family Two: Smaller Worlds With More Compact Arrangements
The second family consists of lower-mass planetary systems, frequently hosted by stars with masses and metallicities comparable to or below those of the Sun.
These systems tend to contain planets arranged in relatively compact configurations. Their neighbouring planets can also have similar masses, a pattern astronomers call peas in a pod.
The expression describes a tendency for planets within the same system to resemble one another in size or radius. It does not mean that every planet in these systems is identical.
The researchers found that this family generally exhibits lower levels of dynamical excitation than the high-mass family.
That distinction matters because a system’s orbital arrangement can preserve evidence of its past. A relatively orderly configuration may indicate a different evolutionary pathway from one in which planets experienced substantial gravitational disturbances.
However, compactness does not automatically mean that a system is stable, nor does a less turbulent history guarantee that its planets are habitable.
The study found that both families share some characteristics that distinguish them from our own solar system. The researchers therefore caution against treating the Solar System as a universal template for how planets must form and evolve.
Which Family Is Most Like Our Solar System?
This is where the findings become particularly interesting.
The Solar System does not fit perfectly into either family. Its planets span a wide range of masses and occupy orbits extending from Mercury’s close path around the Sun to Neptune’s much more distant orbit.
The research found that the lower-mass family has a typical level of dynamical excitation comparable to that of the Solar System. This makes it a useful point of comparison when considering the gravitational histories of planetary systems.
However, the researchers also found that both families have orbital-spacing characteristics that differ from those of our system.
In other words, sharing a similar level of dynamical excitation does not make two planetary systems identical.
Our Solar System contains small rocky planets in its inner region and much larger planets farther out. Jupiter and Saturn also exert significant gravitational influences on the system’s long-term evolution.
The lower-mass family offers some useful parallels, but it should not be described as an exact collection of Solar System twins.
For readers interested in how planetary architecture varies across the galaxy, the growing catalogue of newly discovered exoplanets illustrates just how diverse worlds beyond our Sun can be.
What Does This Mean for the Search for Alien Life?
The discovery could influence how scientists evaluate planetary systems when searching for environments that might support life.
A planet’s habitability depends on many factors, including its temperature, atmosphere, water availability, radiation environment and geological conditions. Its host star and the gravitational behaviour of neighbouring planets can also influence its long-term environment.
A system that experiences repeated close encounters or major orbital rearrangements may undergo substantial changes. Such events could alter planetary climates, orbital periods or the conditions under which atmospheres remain stable.
By contrast, a system with a less turbulent history might preserve more consistent orbital conditions, although that alone would not establish whether life could develop there.
This is one reason astronomers continue investigating potentially habitable worlds, including planets that may have very different environments from Earth. Research into whether alien life could exist on Europa also highlights how the search for life extends beyond planets that closely resemble our own.
The new study does not identify a planet with life, nor does it establish that one family is inherently more habitable than the other.
Instead, it provides a framework that future investigations can use to explore how a planetary system’s formation and evolution may influence its planets.
Why the Findings Matter for Planet Formation
One of the study’s most important conclusions is that a planetary system’s history may be linked to the properties of the star around which it formed.
Stars develop within environments containing different quantities of gas, dust and heavier elements. These conditions influence the raw materials available for planet formation.
The research suggests that the combination of these initial conditions and subsequent gravitational evolution can help explain the differences between the two families.
This is significant because astronomers have long struggled to reconcile the Solar System’s architecture with the wide variety of exoplanetary systems observed elsewhere.
The findings also highlight an important limitation: scientists can only study the planetary systems their instruments are capable of detecting and characterising. Observational biases may therefore influence the picture astronomers have of the galaxy’s overall population.
Future observations and improved measurements will be needed to determine how representative the two families are of all planetary systems in the Milky Way.
A New Way to Understand Our Place in the Galaxy
The identification of two planetary-system families offers a fresh way to investigate how worlds form, evolve and sometimes undergo dramatic changes.
It also challenges the assumption that our Solar System represents the standard arrangement against which every other planetary system should be measured.
For astronomers, the next challenge is to understand how these broad families emerge and whether their differences can help predict the long-term behaviour of newly discovered worlds.
The findings do not establish that one family is better suited to life, and they do not place our Solar System neatly into either category. What they reveal is that the history of a planet may be inseparable from the history of the system around it.
As astronomers discover more exoplanets and refine their measurements, the Milky Way’s planetary population may become easier to understand. Each newly characterised system provides another opportunity to investigate the conditions that create planets, shape their orbits and determine what happens to them over billions of years.
