The most widely recognized explanation for planet formation is the accretion theory. It states that small particles in a protoplanetary disk accumulate gravitationally and, over time, form larger and larger bodies called planetesimals. Eventually, many planetesimals collide and combine to form even larger bodies. For gas giants, these become the cores that then attract massive amounts of gas over millions of years.
But the accretion theory struggles to explain gas giants that form far from their stars, or the existence of ice giants like Uranus and Neptune.
The accretion theory dates as far back as 1944 when Russian scientist Otto Schmidt proposed that rocky planets like Earth formed from ‘meteoric material.’ Another step forward happened in 1960 when English astronomer William McCrea proposed the ‘protoplanet theory,’ stating that planets form in the solar nebula. In the decades since then, the accretion theory was refined and added to, and in modern times, astronomers have gathered more observational evidence that supported it.
However, the theory has some holes that still need plugging.
According to the theory, forming a core large enough to become a gas giant takes several million years, and protoplanetary disks dissipate too soon for that to happen. Protoplanets also tend to migrate toward their star as they grow, and they may not gather enough mass before the star consumes them.
The accretion theory faces another problem that’s surfaced since we’ve discovered more exoplanets in other solar systems. It struggles to explain hot Jupiters and super-Earths.
Over the years, the development of streaming instability and pebble accretion has overcome some of these problems. Streaming instability explains how particles in a gas disk experience drag and accumulate into clumps, which then collapse gravitationally. Pebble accretion explains how particles from centimetres to meters in diameter experience drag and form planetesimals. Both of these have strengthened the accretion theory, but astronomers still hunger for a complete theory of planet formation.
Researchers have developed a new model that incorporates all the physical processes involved in planet formation. Their work, which is published in the journal Astronomy and Astrophysics, is titled “Sequential giant planet formation initiated by disc substructure.” The lead author is Tommy Chi Ho Lau, a doctoral candidate at Ludwig-Maximilians-University in München, Germany.
The new model shows that substructures in a protoplanetary disk called annular perturbations can trigger the formation of multiple gas giants in rapid succession. Critically, this model matches up with some of the most recent observations.
Planets form in unstable gas disks around stars. The researchers show how small, millimetre-sized dust particles accumulate in the disk and become trapped in the annular perturbations. The authors call these migration traps. Since they’re trapped, the particles can’t be gravitationally drawn toward the star. A lot of material from which planets form accumulates in these compact regions in the disk, which creates the conditions for rapid planet formation.
“We find rapid formation of multiple gas giants from the initial disc substructure,” the researchers write in their paper. “The migration trap near the substructure allows for the formation of cold gas giants.”
This is an image of the HL Tau planet-forming disk taken with the Atacama Large Millimeter Array (ALMA). ALMA has imaged many of these protoplanetary disks with gaps. The gaps have been interpreted as rings carved out of the disk by forming planets, but this new model has a different explanation. Credit: ALMA (ESO/NAOJ/NRAO)The process creates a new pressure maximum at the outer edge of the planetary gap, which triggers the next generation of planet formation. This results in a compact chain of giant planets, which is what we see in our Solar System. The process is efficient because the first gas giants that form prevent the dust needed to form the next planet from drifting inward toward the star.
“When a planet gets large enough to influence the gas disk, this leads to renewed dust enrichment farther out in the disk,” explains Til Birnstiel, co-author and Professor of Theoretical Astrophysics at LMU and member of the ORIGINS Cluster of Excellence. “In the process, the planet drives the dust—like a sheepdog chasing its herd—into the area outside its own orbit.”
These panels are snapshots from five different times in one of the simulations that show sequential planet formation. The solid line represents gas density, and the dashed line represents dust density. Each dot is a formed planet. As time passes, the dust density peak moves further from the star, shepherded along by newly formed planets. Image Credit: Lau et al. 2024. The process then repeats itself. “This is the first time a simulation has traced the process whereby fine dust grows into giant planets,” said Tommy Chi Ho Lau, the study’s lead author.
The Atacama Large Millimetre-submillimetre Array (ALMA) specializes in observing protoplanetary disks. It can see through the dust that obscures planet formation around young stars. It’s found gas giants in young disks at a distance beyond 200 AU. In our Solar System, Jupiter is at about 5 AU, and Neptune is at about 30 AU. The authors say that their model can explain all of these different architectures. It also shows how our Solar System stopped forming planets after Neptune because the material was all used up.
“This work demonstrates a scenario of sequential giant planet formation that is triggered by an initial disc substructure,” the authors write in their conclusion. “Planetary cores are formed rapidly from the initial disc substructure, which can then be retained at the migration trap and start gas accretion.” The results show that “… up to three cores can form and grow into giant planets in each generation.”
How the substructures form is beyond the scope of this work. More research is needed to investigate this.
This work can explain how gas giants form, but it can’t explain how the timing worked in our Solar System. That requires more research into how gas accretion works, which the astronomical community is actively pursuing.
“Further investigations specifically on gas accretion are required to model the formation time of the Solar System’s giant planets,” the authors conclude.
