What is a planet? A new definition based on physical properties

To be or not to be… a planet? For centuries, astronomers have grappled with how to define what counts as a planet, and despite our best efforts, the question is still surprisingly unsettled. The current official definition leaves us with some inconsistencies: even well-established planets like Venus, Earth, and Jupiter don’t fit neatly within its rules, and things get even murkier when we turn to exoplanets. In today’s astrobito, we look at a new framework that aims to bring clarity: one grounded in measurable physical properties.

Paper details:

Title: Discovery of a dwarf planet candidate in an extremely wide orbit: 2017 OF201

Authors: Madhu Kashyap Jagadeesh, Arkil D. P., Margarita Safonova, and Bernard Foing

First-author institution: Department of Physics, St. Josephs University, Bengaluru, India

Status: Acceso abierto en arXiv

For as long as we’ve looked to the night sky, we’ve been trying to make sense of planets. The word itself comes from the ancient Greek planētēs, which means “wanderer”, a nod to their shifting positions against the fixed stars. But the question of what exactly qualifies as a planet remains surprisingly unsettled. In fact, it may be more uncertain now than it was in ancient times.

Over the past few decades, discoveries of thousands of exoplanets have challenged long-standing assumptions. At the same time, controversial reclassifications within our own Solar System, like the demotion of Pluto in 2006, have exposed cracks in our current criteria. Today’s astrobito revisits this debate, offering not just an updated definition but an entirely new framework for understanding planets.

Rethinking the rules of planethood

In 2006, the International Astronomical Union (IAU) set the most widely accepted modern definition of a planet. According to the IAU, a planet must:

Orbit the Sun,
Be massive enough for its own gravity to pull it into a nearly round shape (a state called hydrostatic equilibrium), and
Have “cleared the neighbourhood” around its orbit, meaning it must be gravitationally dominant.

This definition famously led to Pluto’s reclassification as a dwarf planet, a move that sparked scientific and public debate. Since then, researchers have pointed out several limitations of the IAU criteria. For one, it only applies to objects orbiting the Sun, excluding the thousands of known exoplanets. It also doesn’t define exactly how “cleared” an orbit must be, and even hydrostatic equilibrium is more complicated than it seems; Mercury, for example, may not technically meet this requirement, yet no one disputes its planetary status.

These issues have led scientists to propose new definitions over the years. Some are based on orbital dynamics, others on mass thresholds or geological features. The latest proposal, built around the idea of a “fundamental planet plane”, adds a new dimension to the conversation by focusing on physical parameters that can be measured directly.

A planet by mass, radius, and moment of inertia

The fundamental planet plane is a conceptual diagram that plots objects based on three physical properties: mass, radius, and moment of inertia (a measure of how mass is distributed inside a body). The idea is similar to the way stars are classified using their temperature and luminosity on the Hertzsprung–Russell (HR) diagram. Just as the HR diagram reveals patterns in stellar evolution, the planet plane aims to reveal patterns in planetary identity.

According to the team behind this new definition, planets lie within a specific region on this plane, bounded by mass thresholds. Their proposed definition is simple in principle:

A planet is a celestial spherical object, bound to a star or unbound, that lies on the fundamental planetary plane, within a mass range of 10²³ kg (about 0.02 Earth masses) to 2.5 × 10²⁸ kg (13 Jupiter masses).

This redefinition explicitly includes rogue planets (those that drift through space without a host star) and avoids the often ambiguous “clearing the orbit” clause. Instead of relying on orbital context, it focuses on intrinsic properties that can be compared across the Solar System and exoplanetary systems alike.

Where is the line between planets and non-planets?

Using data from Solar System objects, rocky exoplanets, moons, and asteroids, the researchers constructed a “turn-off point” diagram—analogous to stellar turn-off points in the HR diagram. This turn-off point marks the lower mass limit for planets, clustering around objects like Mercury and Mars. Bodies below this line, such as large asteroids and small moons, don’t align with the fundamental planet plane.

In the turn-off diagram, the moment of inertia increases exponentially with mass once objects reach a mass comparable to that of Mercury. Below this threshold, the moment of inertia remains relatively constant. The figure is adapted from Figure 1 of the main article.

Intriguingly, many of the IAU’s dwarf planet candidates do not fall on the planet plane. Pluto, for example, appears to lie more comfortably among the moons, both in structure and in its placement on the diagram. This doesn’t diminish Pluto’s scientific importance, as it remains a complex and geologically fascinating world, but it does raise questions about how we group celestial bodies.

In the same way, the upper mass limit is defined by the onset of deuterium fusion, the nuclear reaction that marks the beginning of brown dwarf status. At around 13 Jupiter masses, objects begin to burn deuterium, a process more star-like than planet-like. These limits help distinguish planets from both asteroids and stars, using observable physical data rather than theoretical or dynamical boundaries.

What about the neighbourhood?

One of the most contentious parts of the IAU’s definition is the requirement that a planet must “clear its neighbourhood.” This idea, more formally known as dynamical dominance, means that a planet should dominate its orbital region, sweeping up or ejecting other bodies over time.

But this is a murky metric. Jupiter, for instance, shares its orbit with over 100,000 Trojan asteroids near its L4 and L5 Lagrange points. Earth, too, has thousands of near-Earth objects (NEOs) and even temporarily captured moons; small rocks that orbit Earth for brief periods before drifting away. Venus has at least 20 known co-orbital asteroids. Does this mean none of these planets have “cleared” their neighbourhoods?

Some planets in the Solar System may not meet the current IAU definition of a planet, as they share their orbits with other celestial bodies. The green lines in the diagram represent the path of Lucy: NASA’s first mission to visit Jupiter’s Trojan asteroids. Figure adapted from https://science.nasa.gov/solar-system/navigating-nasas-first-mission-to-the-trojan-asteroids/.

Moreover, for exoplanets located hundreds of light-years away, it's nearly impossible to determine whether they have cleared their orbits. Our instruments simply aren’t sensitive enough to detect small nearby objects in those systems. So while dynamical dominance is a useful concept within our Solar System, it’s not a practical criterion for planets elsewhere.

The proposed planet plane sidesteps this by focusing on what we can measure: mass, size, and internal structure.

The Pluto problem (again)

Pluto has long been the poster child for the debate over what is or isn’t a planet. Under the IAU definition, it was demoted because it hadn’t cleared its neighbourhood. Under the fundamental planet plane model, Pluto again fails to qualify, but this time due to its physical properties.

According to the diagram, Pluto sits firmly on the “moon plane,” suggesting it has more in common with large satellites like Europa or Titan than with the classical planets. This isn’t meant as a slight; Pluto remains an active, icy world with a layered interior, a nitrogen atmosphere, and complex surface chemistry. But if we define planets by physical parameters, Pluto may not quite make the cut.

This raises an interesting philosophical point: should definitions be based on physical traits, dynamical context, formation history, chemistry, or something else entirely? There may not be a single “right” answer, just different ways to organise our understanding of an increasingly complex universe.

Shifting definitions, stable science

The new proposal to define planets using the fundamental planet plane doesn’t aim to settle the debate once and for all. Instead, it offers a compelling way to reframe the question, shifting focus from ambiguous orbital criteria to measurable physical parameters. It provides a framework that works for both Solar System and exoplanetary science, and lets us draw clearer boundaries between planets, moons, asteroids, and brown dwarfs.

And perhaps most importantly, it reminds us that science isn’t about drawing rigid lines. It’s about refining our models as new data comes in. Just as the HR diagram revolutionised our understanding of stars, the fundamental planet plane might one day do the same for planets. Because in the end, the question “what is a planet?” isn’t just about Pluto or orbits or semantics. It’s about how we structure knowledge and how we adapt to discovery.

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