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The formation of circumbinary planets through disc fragmentation

This study demonstrates that gravitational instability in realistic circumbinary discs is a viable formation pathway for gas giant planets, as these discs fragment more efficiently and produce a higher number of planetary-mass objects compared to circumstellar discs.

Original authors: Matthew Teasdale, Dimitris Stamatellos

Published 2026-04-28

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CC BY 4.0

Original authors: Matthew Teasdale, Dimitris Stamatellos

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Pizza Dough" Theory: How Planets Form Around Double Stars

Imagine you are watching a chef prepare a pizza. Usually, we think of a star as a single, steady oven in the middle of a kitchen, with a smooth swirl of dough (the disc of gas and dust) spinning around it. This is how most planets, including Earth, are thought to form.

But what happens when you have two ovens sitting side-by-side in the middle of the kitchen? This is a binary star system. Instead of one steady heat source, you have two competing temperatures, two gravitational pulls, and a lot of chaotic energy.

A new study by Matthew Teasdale and Dimitris Stamatellos explores a wild way that planets might form in these "double-oven" systems: Disc Fragmentation.

1. The "Wobbly Dough" Effect (Gravitational Instability)

In a normal system, the "dough" (the gas disc) is relatively calm. But in a binary system, the two stars act like two heavy spoons stirring the dough at different speeds and angles. This creates massive wobbles and ripples.

If the dough gets thick enough and the wobbles get violent enough, the dough doesn't just stay a smooth swirl—it starts to clump together into heavy lumps. This is fragmentation. Instead of a slow, steady build-up of a planet (like adding grains of sand to a pile), the disc essentially "breaks" into several large chunks of gas all at once.

2. The "Cooler Kitchen" Advantage

You might think two stars would make everything too hot for planets to form. However, the researchers found something surprising. Because the two stars are moving around each other, they actually help create a "cooler" environment in certain parts of the disc compared to a single star.

The Analogy: Imagine trying to bake a delicate pastry. If you have one giant, roaring bonfire in the center, the heat is intense and uniform. But if you have two smaller, moving heat sources, you create "pockets" of different temperatures. In these cooler pockets, the gas can clump together much more easily.

Because of this, the study found that circumbinary discs (around two stars) are actually better at making planets than single-star discs. They produce more "protoplanets" (baby planets) per disc.

3. The "Cosmic Pinball" Game (Migration and Ejection)

Once these baby planets form, the "kitchen" becomes a dangerous place. Because there are two stars tugging in different directions and multiple baby planets competing for space, it becomes a game of Cosmic Pinball.

The Migrators: Some planets get bumped and sent sliding inward toward the stars, or flung outward toward the edges of the system.
The Outcasts (Free-Floating Planets): Some planets get hit so hard by the gravitational "slingshot" of the two stars that they are kicked out of the system entirely. They become "free-floating planets"—lonely wanderers drifting through the dark void of space, belonging to no star at all.

The Big Picture

For a long time, scientists weren't sure if gas giants (like Jupiter) could form in these chaotic double-star systems. This paper provides a "Yes!"

It suggests that the chaos of two stars isn't just a hurdle; it’s actually a factory. The gravitational wobbles act like a cosmic stirrer, breaking the gas disc into chunks that quickly become massive, wide-orbit gas giants.

In short: While a single star builds planets like a slow, careful sculptor, a binary star system builds them like a high-speed, chaotic explosion of dough!

1. Problem Statement

While over 50 circumbinary exoplanets have been discovered, many exist on wide orbits ( $>10$ au). Traditional planet formation models, such as core accretion, struggle to explain the presence of gas giants on wide orbits because the growth timescale (approx. 10 Myr) often exceeds the lifetime of the protoplanetary disc (3–5 Myr).

The authors investigate whether gravitational instability (GI)—specifically the fragmentation of circumbinary discs—serves as a viable alternative formation pathway. They aim to determine if the presence of a central binary system alters the efficiency, mass distribution, and orbital properties of resulting protoplanets compared to those forming around single stars (circumstellar discs).

2. Methodology

The study employs 3D Smoothed Particle Hydrodynamics (SPH) simulations using the SEREN code, incorporating a polytropic radiative transfer approximation. The researchers compared three distinct disc models:

Circumstellar (CS): Discs around a single star ( $0.7 \, M_\odot$ ).
Circumbinary Fiducial (CBF): Discs around a binary with the same temperature profile as the CS model.
Circumbinary Realistic (CBR): Discs around a binary where the temperature profile is determined by individual heating from each star.

Simulation Parameters:

Binary Properties: Separations of 5 and 10 au; eccentricities of 0.2 and 0.5; and varying mass ratios ( $q_b$ ) from 0.1 to 1.
Disc Properties: Initial mass of $\sim 0.2 \, M_\odot$ ; extending from 5 to 120 au.
Criteria: Fragmentation was identified when condensation reached a density of $10^{-9} \, \text{g cm}^{-3}$ . Simulations ran until 70% of the disc mass was accreted.

3. Key Results

A. Fragmentation Efficiency and Timing

Binary Influence: Discs around binaries fragment earlier and more efficiently than circumstellar discs. The asymmetric gravitational potential of the binary acts as a "seed" for gravitational instability.
Model Comparison: The CBR model was the most efficient, producing an average of $9 \pm 0.9$ protoplanets per disc, compared to $7.5 \pm 0.8$ in CS discs and $6.5 \pm 0.6$ in CBF discs.
Separation Effect: Wider binary separations (10 au) promoted faster and more frequent fragmentation than closer separations (5 au).

B. Protoplanet Mass and Orbital Distribution

Mass Regime: Protoplanets in the CBR model have lower initial masses due to the cooler temperatures of realistic circumbinary discs. Crucially, a much higher fraction of these objects ( $71\%$ ) fall within the planetary-mass regime ( $< 20 \, M_J$ ), whereas CS models produce more brown dwarfs and low-mass stars.
Orbital Radii: Fragmentation occurs primarily beyond the binary’s "forbidden region" ( $\sim 50$ au). Final orbital radii typically peak at $\sim 100$ au.
Migration: Protoplanet movement is driven by stochastic dynamical scattering (planet-planet or planet-binary interactions) rather than standard disc-planet migration. Approximately 56% of protoplanets migrated outward.

C. Ejection and Free-Floating Objects

The simulations showed that dynamical interactions eject a significant fraction of protoplanets.
Ejection Drivers: Higher binary mass ratios, higher eccentricities, and tighter binary separations all increased the ejection fraction.
Velocity: Ejected objects (potential Free-Floating Planets) reached velocities of $2\text{--}6.5 \, \text{km s}^{-1}$ .

4. Key Contributions & Significance

New Formation Pathway: The paper establishes that gravitational fragmentation of circumbinary discs is a highly viable and potentially dominant pathway for forming wide-orbit gas giants.
Predictive Power for Planet Types: It demonstrates that realistic circumbinary environments are uniquely suited to forming planets rather than brown dwarfs, because the cooler discs distribute mass among a larger number of smaller fragments.
Explaining Observational Data: The results align with observed populations of high-mass circumbinary planets and provide a mechanism for the production of free-floating planets via dynamical ejection from binary systems.
Theoretical Advancement: The work extends the understanding of how binary stellar evolution and non-axisymmetric gravitational potentials fundamentally alter the physics of planet formation compared to the standard single-star paradigm.