Planet formation on solar system scales

arXiv:2610.21906 · astro-ph.IM, astro-ph.EP · Submitted 2026-10-15 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Planet formation on solar system scales".

Jocelyn: The gist: ALMA2040, a next-generation sub(mm) interferometer,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So, we've touched on what this paper is about, and it really centers on how cold gas giants are not just passive objects but active shapers of their formation environments in a very specific way. The central thesis is that catching these giants early gives us the best chance to uncover the fundamental steps of planet formation and understand our own solar system's origins.

Jocelyn: They claim that simulations show forming gas giants leave clear imprints, like exciting spirals and opening up annular gaps where pebbles accumulate in bright rings, and they accrete material through gap-crossing or meridional flows, sometimes mediated by those circumplanetary discs.

Subrahmanyan: That's the mechanism they're focusing on, connecting the accretion process directly to observable structures in the disc environment. The paper suggests that these interactions are what drive the resulting planetary system architecture we see later.

Vera: Specifically, they argue that submillimeter observations are key here because they offer a lower degree of contamination from both the star and any surrounding material, which is essential for estimating the mass budget of those CPDs.

Jocelyn: They're looking at how ALMA2040 can probe this by providing sensitivity to (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within one hundred fifty parsecs, and allowing kinematic characterization of the interactions with the disc.

Subrahmanyan: The paper highlights that ALMA2040 will give us unprecedented insights into how giants carve gaps, accrete material, form satellites, and affect the architectures of their co-evolving planetary systems. It's about linking the microphysics of accretion to macro-scale system design.

Vera: They are also looking at specific results from simulations, for example, showing that for a two Msun host and a planet at an orbital radius of Rp = two hundred forty au, ALMA2040 provides planet mass thresholds of approximately zero point three MJup and zero point one five MJup after seven and twenty-eight hours of integration respectively.

Jocelyn: That connects the instrumental improvements directly to tangible limits on the mass we can actually constrain in these early stages, which is really what makes this paper exciting for follow-up observations.

Subrahmanyan: It sets concrete benchmarks based on current simulation models, and it points toward how future observations with tools like ALMA2040 can test those specific mass regimes in the context of disc evolution.

Vera: So, to wrap up this part, the paper is arguing that by focusing on these early stages and using advanced instruments to measure kinematics around these giant planets, we can get a clearer picture of how they build their systems.

Jocelyn: And it frames this as a way to move from just detecting objects in discs to actually understanding the dynamic processes happening right around them.

Conclusion: Vera: Looking at the paper's title, "Planet formation on solar system scales," and the authors, it seems like they are aiming for a really big picture view of this entire process. They aren't just looking at one small detail, but trying to link the early accretion dynamics to the final system architecture we see today.

Jocelyn: It feels like they are moving beyond simply cataloging planets and into understanding the physical mechanisms—like gap-crossing and CPD mediation—that dictate how those systems evolve over time. It’s about showing that these processes are what truly shape the outcome.

Subrahmanyan: What this means for us is that we need observational data that can constrain these early stages, because if we only see the mature giants, we miss the crucial steps of their growth and interaction with their natal discs. The paper implies that to confirm our models of how systems form, we need to find evidence of these formative interactions in action.

Vera: It's about using next-gen tools like ALMA2040 not just to find planets, but to map out the dynamics—the actual movements and flows—that are happening right around them. That’s a big shift from just static imaging.

Jocelyn: And for anyone listening who isn't an astronomer, it means that even though we can't see the planet itself in its infancy, we can use these observations to infer the physical processes that are building its environment, which is incredibly powerful.

Subrahmanyan: The paper essentially calls for a tighter link between simulations and observation in this regime because current methods might not be capturing all the complexity of material delivery and CPD evolution timescales.

Vera: And that's where the work of ALMA2040 becomes essential, because its improved resolution and sensitivity are what’ll allow us to test those assumptions about how much material is actually being delivered to these protoplanets.

F. Zagaria, A. F. Izquierdo, J. Bae, P. Griveaud, R. A. Booth, P. Curone, A. Johansen, §Facchini, §Benisty, §Birnstiel, §Booth A., O. Chrenko G., ™Czekala I., K. Doi J., G. Guidi M., Kama M., Q. Kral C., Law G., §Lodato G, §Luppe P, §Macias E, §Majumdar L, ℒNealon R, A. Penzlin A., D. Polychroni D., D. J. Price D., ÁRibas Á, R. Teague R., C. Toci C., D. Turrini D., §Sefilian A, ℒShibaike Y, ™Stamatellos D, ℳSzuszkiewicz E, C. Walsh C., T. Birnstiel T., A. Booth A., I. Czekala I., K. Doi J., G. Guidi M., §Kama M., ™Qral Q, §Law G, §Lodato G, §Luppe P, ℒMacias E, L. Majumdar L., ↃNealon R, A. Penzlin A., D. Polychroni D., D. J. Price D., ÁRibas Á, R. Teague R., C. Toci C., D. Turrini D., §Sefilian A, ℒShibaike Y, ℳStamatellos D, ℳSzuszkiewicz E, C. Walsh C.

Max-Planck Institute for Astronomy · Department of Astronomy University of Florida · School of Physics and Astronomy University of Leeds · Departamento de Astronomía Universidad de Chile · Center for Star and Planet Formation Globe Institute University of Copenhagen · §Dipartimento di Fisica Università degli Studi di Milano · University Observatory Faculty of Physics Ludwig-Maximilians-Universität München · Center for Astrophysics Harvard & Smithsonian · Charles University Faculty of Mathematics and Physics Astronomical Institute Australia Telescope National Facility CSIRO Space and Astronomy Centre for Star and Planet Formation Globe Institute University of Copenhagen Department of Physics and Astronomy University College London LIRA Observatoire de Paris Université PSL Sorbonne Université Université Paris Cité CNRS Minnesota Institute for Astrophysics School of Physics Trinity College Dublin European Southern Observatory National Institute of Science Education and Research Homi Bhabha National Institute INAF Turin Astrophysical Observatory School of Physical and Chemical Sciences Queen Mary University of London Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Departamento de Física Aplicada III Universidad de Sevilla Department of Astronomy and Steward Observatory University of Arizona Graduate School of Science and Engineering Kagoshima University Institute for Mathematics Physics & Astronomy University Lancashire Institute for Physics and CASA* University of Szczecin Wielkopolska

astro-ph.IM, astro-ph.EP

Submitted: 2026-10-15

Updated: 2026-10-15

Comments: Part of the ALMA2040 White Paper Series

Code: https://github.com/richteague/syndisk2https:

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 79/100

The gist: The gist: ALMA2040, a next-generation sub(mm) interferometer, will be sensitive to (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within 150 pc and allow kinematically

Key concepts

Circumplanetary Disc (CPD): This is a disc of material that surrounds a forming giant planet, formed from the planet's accretion process. Sub(mm) observations are unique because they offer less contamination from the star and surrounding disc, allowing scientists to estimate the mass budget required to form satellites.
Kinematic Detection: This involves measuring how a planet perturbs the local velocity field of gas in its disc. An embedded planet creates spiral wakes and modifies rotation, which produce coherent deviations in molecular line emission that can be mapped using ALMA2040's high resolution.
Planet-Driven Flows: These are the three-dimensional motions within a protoplanetary disc caused by a massive embedded planet. These flows include spiral wakes and pressure modifications across gaps, which are observable as spatial and spectral deviations from smooth Keplerian disc rotation.
ALMA2040 Capabilities: This next-generation interferometer offers significant improvements: 25 times better continuum sensitivity, 10 times better line emission sensitivity, and three times higher angular resolution than ALMA. These upgrades are crucial for resolving the faint signatures of sub-Jovian planets and their associated kinematic effects.

Summary

The gist: ALMA2040, a next-generation sub(mm) interferometer, will be sensitive to (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within 150 pc and allow kinematically characterising interactions of Jovian and sub-Jovian planets with their natal disc.

Scientific context and current challenges

Cold gas giants play a key role in planet formation, as they have been detected in many systems and suggested to actively shape their formation environments. Jupiter is a compelling example, with its interactions with other planets and leftover planetesimals in the protosolar disc routinely invoked to explain the properties and architecture of the present-day solar system. Simulations show that forming gas giants leave clear imprints on their surroundings, such as exciting spirals and opening up annular gaps where drifting pebbles pile up in bright rings, allowing them to accrete material through gap-crossing or meridional flows.

ALMA has revolutionized our understanding of protoplanetary discs and planet formation by discovering that planet-forming discs ubiquitously display substructures, most commonly gaps and rings. Embedded protoplanets are proposed as a natural explanation for these substructures, but detecting them has been challenging due to high extinction and contamination from the disc.

Circumplanetary emission around a proto-Jupiter

Giant protoplanets grow by accreting material from their forming disc through an envelope or a rotationally supported circumplanetary disc (CPD). Sub(mm) observations provide unique insights into these systems because they offer lower degree of contamination from the star and circumstellar material, opening up the possibility of estimating the CPD mass budget to form satellites.

The detectability of CPDs around giant planets orbiting a nearby (150 pc) Sun-like star was explored over a range of masses and separations. The colour map in Figure 1 shows the predicted CPD continuum flux densities at 345 GHz, where the shape of the isocontours is set by the combination of CPD temperature and size. The plot shows that ALMA is sensitive to a planet population fundamentally different from the bulk of mature gas giants detected to date, whose occurrence rate peaks around 4 au. Detecting circumplanetary emission around a Jupiter-like planet in moderate integrations (1 hr on source) requires a combination of ×3 higher angular resolution and ×25 point-source sensitivity compared to ALMA.

Kinematic detection of embedded planets with ALMA2040

An embedded planet perturbs the local velocity field, launching spiral wakes, modifying the pressure-supported rotation across its gap, and driving three-dimensional flows in its vicinity. When observed in spectrally resolved molecular-line emission, these motions produce spatially and spectrally coherent deviations from an otherwise smooth Keplerian disc.

ALMA2040 is expected to extend molecular-line kinematic searches from Jupiter-mass planets into the sub-Jovian regime within a few hours of integration. Resolved observations of multiple molecular species and transitions with ALMA2040 could probe these flows at different disc heights and help distinguish planet-driven three-dimensional motions from apparent kinematic perturbations associated with surface geometry or thermochemical structure.

Resolving planet-driven flows

The improved angular resolution and sensitivity of ALMA2040 make the planet-driven intensity wiggles, or “kinks”, sharper and reveal their coherence across adjacent velocity channels. In Figure 3, the smaller beam and higher SNR of the ALMA2040 image preserve a sharper displacement of the emission contours and reveal its coherence across the sampled channels. The resulting residuals from ALMA2040 trace both the local Doppler flip triggered by the planet’s wakes and the more extended response of the spiral structures away from the planet.

From centroid velocity precision to planet-mass sensitivity

The gain in line sensitivity can be translated into a precision with which local gas velocities are measured, and this precision scales linearly with the planet-to-star mass ratio, q. The study found that for a 2 M⊙ host and a planet at Rp = 240 au, ALMA2040 provides 5σ planet mass thresholds of approximately 0.3 MJup and 0.15 MJup after seven and 28 hours of integration, respectively.

The sensitivity analysis also suggests that targeted ∼ 10 h ALMA2040 observations at ∼ 0.05′′ resolution could bring kinematic signatures of 0.5–1.0 MJup planets around 1–2 M⊙ stars within reach down to ∼ 30 au.

Synergies with current and future facilities

The impact of ALMA2040 will be even broader thanks to synergies with forthcoming facilities, primarily the ELT. (sub)mm continuum and kinematics are complementary to ELT/METIS and MICADO high contrast imaging and ELT/METIS IR kinematics for the detection of circumplanetary emission, both in terms of contrast and separation. Complementary detections in the IR will provide planet luminosities and thus the opportunity to build a planet mass-luminosity diagram.

By the 2040’s, two more decades of radial velocity monitoring, forthcoming Gaia data releases, and the Roman Space Telescope will have provided a complete characterisation of the mature gas giants within 30 au. This census will enable for the first time to statistically assess the role of migration in the formation of giant planets by comparing mature exoplanets with those inferred from continuum substructures. The ngVLA will also provide complementary information on CPD flux densities at lower frequencies, which will be helpful to better estimate their masses.

The sensitivity and resolution improvements discussed will be essential to test these assumption, build better CPD models, and understand material delivery to protoplanets. The paper concludes that the next-generation interferometer will be essential to develop a comprehensive picture of how these planets accrete material, form their moons, and give shape to their hosting systems.

The results suggest that other factors, such as details of the planetary accretion process, the CPD evolution timescales, and replenishment from the circumstellar disc likely play a key role in CPD detectability. The sensitivity and resolution improvements discussed will be essential to test these assumption, build better CPD models, and understand material delivery to protoplanets. The goal for ALMA2040 is to extend the instantaneous bandwidth to over 512 GHz. Assessing the effects of such a wide spectral coverage on CPD detection is not straightforward. While increasing continuum sensitivity will also make detecting asymmetric planetary-driven features easier, their contamination might not necessarily result in lower CPD detection thresholds. Finally, we caveat that our results are dependent on the range of validity of the assumed CPD models. The sensitivity and resolution improvements discussed will be essential to test these assumption, build better CPD models, and understand material delivery to protoplanets. The goal for ALMA2040 is to extend the instantaneous bandwidth to over 512 GHz. Assessing the effects of such a wide spectral coverage on CPD detection is not straightforward.

Improvements for AI systems

The ALMA2040 system can perform kinematically characterising interactions of Jovian and sub-Jovian planets with their natal disc by using molecular-line emission to probe localised velocity perturbations, excess line broadening, and asymmetric line profiles.
The AI system can develop a capability to distinguish between planetary signals and background noise by analyzing planet-driven intensity wiggles, or “kinks,” sharper and reveal their coherence across adjacent velocity channels, which is enabled by the improved angular resolution of ALMA2040.
The system can generate predictive models for CPD mass budgets by incorporating semi-analytical models of Andrews et al. (2021) to estimate accretion rates, noting that CPD temperature is set by a combination of stellar/planet irradiation and planetary accretion heating.
The AI system can perform parameter estimation for planet properties using the relationship derived from centroid velocity precision: v beam pk (q) ≃ v beam pk,0 q q0, allowing it to calculate planet-mass detection thresholds based on integration time and disc inclination.
The system can simulate and test detection strategies by running 2D hydrodynamic simulations of the interactions of a 1MJup planet orbiting a Sun-like star with its hosting disc using FARGO3D, specifically testing how the shape and contrast of planet-induced gap and spirals affect CPD detection.
The system can perform synergistic multi-wavelength analysis by integrating data from (sub)mm continuum and kinematics are complementary to ELT/METIS and MICADO high contrast imaging, allowing it to build a planet mass-luminosity diagram by combining kinematic constraints with IR luminosity.

Abstract

Giant planets are often regarded as primordial protoplanetary "architects". Their formation and interactions with planetesimals and other planets during and after the protoplanetary disc phase are key to setting the blueprint of the planetary systems we observe today. Catching nascent gas giants is our best opportunity to uncover these crucial steps in the planet formation process and shed light on the origins of our own solar system. Over the last decade, ALMA has revolutionised our understanding of planet formation; still, detecting signatures of Jupiter-like planets has remained out of reach. We argue that ALMA2040, a next-generation sub(mm) interferometer with improved sensitivity (times25 in the continuum and times10 for line emission) and a times3 higher angular resolution compared to ALMA, will be sensitive to (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within 150, pc and allow kinematically characterising interactions of Jovian and sub-Jovian planets with their natal disc. ALMA2040 will provide unique and unprecedented insights into how gas giants carve gaps, accrete material, form satellites, and affect the architectures of their co-evolving planetary systems.