Beyond exoplanets: Classifying extrasolar systems

The discovery of the first confirmed exoplanet in the 1990s marked the beginning of an astronomical revolution. Fast forward to January 2025, and we’ve confirmed nearly 6,000 exoplanets! While much of the focus has been on the diversity of these planets themselves, less attention has been given to the systems they belong to. So, is our solar system truly one-of-a-kind? Or are there extrasolar systems that share similarities with ours? Join us in today’s astrobito to find out!

Paper details:

Title: Architecture classification of extrasolar systems

Authors: Alex R. Howe, Juliette C. Becker, Christopher C. Stark, Fred C. Adams

First-author institution: The Catholic University of America, Washington DC, Estados Unidos

Status: Acceso abierto en arXiv y bajo revisión en The Astronomical Journal

It’s surreal to think that it’s been 30 years since the first detection of an exoplanet around a star like our Sun. At that time, we thought the only planets in the universe were the familiar ones of our own solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, and Uranus (and yes, we can throw Pluto into the mix if you’d like). But today, we know better. As of January 2025, nearly 6,000 exoplanets have been confirmed, with thousands more waiting to be verified.

In the years since that first discovery, we’ve made great strides in classifying these exoplanets. Terms like hot Jupiters, super-Earths, and sub-Neptunes are now commonplace in the astronomy community. However, there’s been less attention given to classifying the planetary systems themselves—where these exoplanets reside. The paper we’re exploring today tackles this gap, offering a new framework to categorise planetary systems based on a thorough survey of the current exoplanet population. Join us in today’s astrobito as we dive into this fresh approach and explore how our solar system compares (or contrasts) with these far-flung extrasolar systems!

Understanding the exoplanet population

To classify planetary systems, we first need to understand the full scope of the exoplanets we’ve discovered. By definition, a planetary system consists of a star and at least a planet (or other celestial bodies) that orbits it. So, before diving into how these systems are structured, the research team behind this study set out to answer some key questions: How many exoplanets are there? How were they detected? What are their physical and chemical properties? And most importantly, how are they arranged in their respective systems?

From their analysis, the researchers uncovered some fascinating insights:

A total of 5,686 exoplanets were included in the study.
Most of these (75%) were detected using the transit method, while 19% were found via radial velocity, and the remaining 6% came from other detection techniques.
These exoplanets vary wildly in size, ranging from planets smaller than Mercury to massive super-Jupiter worlds.
Their orbital periods are equally diverse, ranging from planets with incredibly short periods of just 4.3 hours to those that can take over 1.26 million years to complete an orbit!!
Of the 1000 multi-planet systems observed, more than 300 contain three or more exoplanets.

This preliminary analysis highlights just how diverse the exoplanet population is. While some of these planets bear similarities to those in our solar system, the majority of them are vastly different from what we’re familiar with. Yet, despite this diversity, the question remains: how similar are the planetary systems where exoplanets reside to our own?

A new classification framework for extrasolar systems

While it’s easy to imagine that extrasolar systems across the universe are wildly diverse, this research reveals that, in fact, there’s a surprising amount of order in how exoplanets are arranged across their own systems.

At the heart of this new classification system are two main concepts: where the planets are located in their systems, and how neatly their orbits and sizes align with each other. The framework divides planetary systems into two main categories: the inner systems, where planets are close to their star, and the outer systems, where planets are farther away. A gap between planets—if it's large enough—marks the boundary between these categories. If there’s no clear gap, a system is still considered an outer system if the first planet in the system is a massive Jupiter-like planet.

So, what do these systems actually look like? Well, most of the planetary systems considered in this study fall into the first category, inner systems, but this category can be further divided. The first is what the astronomy community calls peas-in-a-pod systems, where smaller planets (often similar to Earth or Neptune) are packed tightly together in regular orbits like our solar system. These systems are neat and orderly, with planets spaced out in a way that feels almost predictable. The second sub-category is warm Jupiter systems, which feature a large, Jupiter-like planet close to its star, often accompanied by smaller planets nearby. These systems tend to be more varied, with planets that have different sizes and orbital patterns.

Planetary system classification based on the total population of confirmed exoplanets

Interestingly, there’s a third, rarer type of system that has some planets with more chaotic orbits—these are called irregular or gapped systems, where planets are more spread out and don’t follow a consistent pattern in their distances or orbital periods. The new framework helps classify these irregular systems, too, by pinpointing where the largest gaps between planets are located, offering a deeper understanding of how planetary systems evolve.

One of the coolest aspects of this new framework is how it shows the remarkable similarities between many planetary systems. Despite the vast diversity of exoplanet types—ranging from small, rocky worlds to gas giants—many planetary systems share similar patterns in how their planets are arranged. And while we’ve long known that our own solar system is unique, this research suggests that many extrasolar systems might have a more familiar structure than we thought.

The future of planetary systems classification

This new classification framework is an important first step, but it’s not the final word. As our ability to detect and study exoplanets improves, especially with upcoming missions and advanced telescopes, we can expect this framework to evolve. For example, the boundaries between inner and outer planets might shift as we discover more planets with longer orbits, extending the range of systems we can study. Surveys like the upcoming microlensing mission from the Roman Space Telescope could help us fill in the missing gaps in our understanding by revealing new types of planetary systems that we haven’t been able to detect before.

The Nancy Grace Roman Space Telescope will revolutionise the way we study exoplanets by directly looking at them. Image taken from: https://science.nasa.gov/mission/roman-space-telescope/

There’s also the possibility of adding more categories to the classification system as we uncover new types of exoplanets, such as Trojan planets (which share an orbit with another planet) or exomoons (moons orbiting planets in other systems). As these discoveries are made, we may need to update our framework to account for these additional features.

One of the key points this framework brings up is how our own solar system fits into the bigger picture. When we consider the four planets most easily detected with current methods—Venus, Earth, Jupiter, and Saturn—our solar system appears to have a clear division between inner and outer planets. However, with only these planets considered, it’s hard to definitively classify it. But when we include all eight planets, our solar system fits into the "peas-in-a-pod" classification, like the majority of extrasolar systems considered in this study (~80%). Our system does have some irregularities, like the less uniform sizes of and longer orbital periods of its inner planets, which might seem unusual due to detection biases. Uranus and Neptune, although not yet commonly detected in exoplanet studies, may turn out to be more common with better detection tools. But, overall, this study suggests that our solar system isn’t nearly as unusual as we once thought, despite being unique in some ways.

Our solar system fits into the “peas-in-a-pond” category, where there’s a clear distinction between inner and outer planets, and the planets are somewhat regularly spaced between them. Image taken from: https://www.nasa.gov/stem-content/scale-of-the-solar-system/

The real challenge going forward is understanding why planetary systems form the way they do. Why do some systems have planets that are evenly spaced like "peas-in-a-pod," while others have more chaotic arrangements? And what’s missing from the systems we’ve observed so far? This opens the door to exciting future research that could reveal deeper insights into planet formation, from why super-Earths are so common to what shapes the orbits of gas giants like Jupiter.

Lastly, this new classification system has important implications for the search for habitable planets. Most planetary systems we’ve studied have planets that orbit too close to their stars to fall within the habitable zone—the sweet spot where liquid water can exist on a planet's surface. However, systems around smaller stars, like M-dwarfs, could place planets within their habitable zones more easily. Still, we’ll need to consider challenges like radiation from these stars, which could impact a planet’s ability to support life as we know it.

The NASA Exoplanet Archive

For anyone eager to explore these findings further, the data that powered this research is freely available to the public. The NASA Exoplanet Archive (a treasure trove of exoplanet data) is an invaluable resource for anyone curious about the hundreds of exoplanets we've discovered. With just a few clicks, you can dive into the data, explore the latest discoveries, and even conduct your own analysis! This framework is just the beginning of a much larger conversation about the nature of planetary systems, and the tools to explore it are at your fingertips.

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