Rajat Saxena is a graduate student at Ludwig Maximilian
University of Munich. He is interested in Theoretical and
Computational Physics problems in statistical mechanics
and astrophysics. Besides scribbling equations and
coding, you can find him running and hiking through and reading philosophy.
Title: Sequential giant planet formation initiated by disc substructure
Authors: Tommy Chi Ho Lau, Til Birnstiel, Joanna Drążkowska and Sebastian Markus Stammler
First Author’s Institution: University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München
Access: Published in Astronomy and Astrophysics [open access]
Astronomers have discovered more than 5,000 exoplanets. Yet, despite these staggering numbers, we still don’t understand how planets form. We’re currently able to simulate independent phases of planet formation on their own, such as dust growth, planetesimal formation, or gas accretion, but we haven’t yet integrated them into a cohesive model.
Many models still rely on smooth, featureless disks for simulations. However, recent high-resolution images from Atacama Large Millimeter/submillimeter Array (ALMA) have revealed that protoplanetary disks—cradles of planet formation—often have substructure, like gaps, bands, spirals and rings. Importantly, these substructures are more than just eye candy; they are the ideal locations for a planetary nursery.
Rising from the Dust
In today’s paper, the authors present an end-to-end planet formation model that starts from micron-sized particles and builds up to several Earth radii. They achieved this by coupling the dust-gas evolution code DustPy with SyMBAp, an N-body integrator. This combo offers the best of both worlds: initially, the disk is evolved in DustPy where, if the right conditions are achieved, gravitational collapse of dust leads to the formation of planetesimals, which are then treated as N-body particles with gas drag, planet-disk interactions, and pebble or gas accretion in SyMBAp.
The simulation is initialized with a protoplanetary disk around a sun-like star and an artificially included pressure bump at ~5.5 AU, motivated by Jupiter’s current location. This pressure bump traps dust and can be visualized as a thin bright circular band in the smooth protoplanetary disk. The trapped dust creates a high-density region that sets the stage for planetesimal formation. Once planetesimals are formed, they start gobbling pebbles, leading to rapid growth.
Eventually, one of the cores becomes massive enough to start accreting gas, entering a runaway phase and transforming into a gas giant. In doing so, it opens a deep gap in the disk, stopping other planetesimals from accreting more pebbles around it.
“Bumpy” Origins of Others
The gap opened by the first gas giant creates a new pressure maximum at its outer edge, which then serves as a secondary pressure bump that again starts trapping dust. The exact process repeats, giving rise to a new generation of planet embryos. This is called “sequential planet formation”—the formation of one planet sets up the conditions needed to trigger the formation of the next.
The authors performed five simulations–each taking three to four weeks to execute–and found two Jupiter-mass gas giants. The second core also accretes gas rapidly, but much later than the first one. As the second becomes a gas giant, it also opens a gap in the disk, pushing the material outwards. Another generation of planetesimals forms from this material, giving rise to the ice giants. Hence, by the end of two million years, there were pairs of gas giants seen in three of the simulations. Two simulations show pairs of ice giants and one showed a single ice-giant. Rest two of the simulations didn’t show any ice giants instead, they had a swarm of planetesimals and planet embryos which could serve as the build blocks of the ice giants later.
Efficiency and Architecture
The goal of such simulations is not just to form planets, but also to do it efficiently. This simulation does this well; in one of the simulations, over 85% of the solid mass in the outer disk was converted into massive bodies. In fact, the first gas giant acts as a barrier, preventing the dust from drifting inwards and preserving material for the second generation to come. This mechanism can explain the chemical partitioning in the Solar System. A proto-Jupiter could have blocked the material exchange between the inner and outer reservoirs, leading to the observed dichotomy between carbonaceous and non-carbonaceous meteorites.
Although this model wasn’t designed to simulate the Solar System, some results are strikingly similar. The first generation of gas giants resembles Jupiter and Saturn, while the delayed formation of the ice giants could mirror the evolution of Uranus and Neptune. Moreover, the results of the simulations lead to starting conditions and the resonances seen in the Nice Model.
What More?
While these results show a promising picture of planet formation, the simulations have some limitations. One important phenomenon that was not included in the simulation is disk photoevaporation, a process by which gas from the disk is lost due to the higher-energy radiation of the central star. This process is crucial because it can shut down gas accretion and may limit the mass of later-forming planets. Further, the initial substructure/pressure bump was “put in by hand” and the authors don’t explain its origin.
Nonetheless, this work presents a powerful idea: one planet’s formation sets the stage for the next, and the second generation of planets are formed standing on the shoulders of giants–gas giants. The initial pressure bump triggers a domino effect, creating a self-propagating sequence of planet formation. Remember, those gaps might not just mark where the planet is, but also where the next one might be!
Disclosure: The author of this article is affiliated with one or more of the authors of the paper discussed.
Astrobite edited by Caroline von Raesfeld
Featured image credit: ALMA/DSHARP https://almascience.eso.org/almadata/lp/DSHARP/
