The most widely accepted explanation for the formation of planets is the accretion theory. This theory states that small particles in a protoplanetary disk accumulate due to gravity and over time form larger and larger bodies, known as planetesimals. Eventually, many planetesimals collide and combine to form even larger bodies. In gas giants, these become cores, which then attract huge amounts of gas over millions of years.
However, the accretion theory can hardly explain the formation of gas giants far away from their stars or the existence of ice giants such as Uranus and Neptune.
The accretion theory dates back to 1944, when Russian scientist Otto Schmidt proposed that rocky planets like Earth formed from “meteor material.” A further advance came in 1960, when English astronomer William McCrea proposed the “protoplanet theory,” which states that planets form in the solar nebula cloud. In the decades since, the accretion theory has been refined and expanded, and in modern times astronomers have collected further observational evidence to support it.
However, the theory has some gaps that still need to be closed.
The theory suggests that it takes several million years to form a core large enough to become a gas giant, and protoplanetary disks break up too quickly for that to happen. Protoplanets also tend to migrate toward their star as they grow, and they may not accumulate enough mass before the star swallows them.
The accretion theory faces another problem that has become apparent since the discovery of additional exoplanets in other solar systems: it is difficult to explain hot Jupiters and super-Earths.
Over the years, the developmentThe theory of flow instability and pebble accretion has solved some of these problems. Flow instability explains how particles in a gas disk face air resistance and clump together into clumps that then collapse under gravity. Pebble accretion explains how particles from centimeters to meters in diameter face air resistance and form planetesimals. Both theories have strengthened the accretion theory, but astronomers still hunger for a complete theory of planet formation.
Researchers have developed a new model that takes into account all the physical processes of planet formation. Their work, 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 student at the Ludwig Maximilian University in Munich.
The new model shows that substructures in a protoplanetary disk, called ring-shaped perturbations, can trigger the formation of several gas giants in rapid succession. Crucially, this model is consistent with some of the latest observations.
Planets form in unstable gas disks around stars. The researchers show how small, millimeter-sized dust particles accumulate in the disk and become trapped in the ring-shaped disturbances. The authors call these phenomena migration traps. Because they are trapped, the particles cannot be pulled towards the star by gravity. A lot of material from which planets form accumulates in these compact regions of the disk, creating the conditions for rapid planet formation.
“We observe a rapid formation of several gas giants from the initial disk substructure,” the researchers write in their article. “The migration trap near the substructure enables the formation of cold gas giants.”
This is an image of the HL Tau planet-forming disk taken by the Atacama Large Millimeter Array (ALMA). ALMA has imaged many of these protoplanetary disks with gaps. The gaps have been interpreted as rings cut out of the disk by planet formation, but this new model has a different explanation. Image 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 like those we see in our solar system. The process is efficient because the first gas giants to form prevent the dust needed to form the next planet from drifting inward toward the star.
“When a planet becomes large enough to influence the gas disk, this leads to a renewed accumulation of dust further out in the disk,” explains Til Birnstiel, co-author and professor of theoretical astrophysics at LMU and member of the ORIGINS Cluster of Excellence. “The planet drives the dust – like a sheepdog drives its flock – into the area outside its own orbit.”
These images are snapshots from five different points in time in one of the simulations showing sequential planet formation. The solid line represents gas density, and the dashed line represents dust density. Each dot is a planet formed. Over time, the dust density peak moves farther away from the star, accompanied by newly formed planets. Image credit: Lau et al. 2024.
The process then repeats itself. “This is the first time that a simulation has tracked the process by which giant planets form from fine dust,” said Tommy Chi Ho Lau, the lead author of the study.
The Atacama Large Millimeter-submillimeter Array (ALMA) specializes in observing protoplanetary disks. It can see through the dust that obscures planet formation around young stars. It has found gas giants in young disks at distances of over 200 AU. In our solar system, Jupiter is at about 5 AU and Neptune is at about 30 AU. The authors say their model can explain all these different architectures. It also shows how in our solar system, no more planets formed after Neptune because the material ran out.
“This work demonstrates a scenario of sequential formation of giant planets triggered by an initial disk substructure,” the authors write in their conclusion. “Planetary cores form rapidly from the initial disk substructure, which can then be trapped in the migration trap and trigger gas accretion.” The results show that “… in each generation, up to three cores can form and grow into giant planets.”
How the substructures form is beyond the scope of this work. Further investigations are needed to investigate this.
This work can explain how gas giants form, but it cannot explain how the timing works in our solar system. This requires further research into how gas accretion works, which the astronomical community is actively pursuing.
“To model the formation time of the giant planets in our solar system, further studies specifically on gas accretion are needed,” the authors conclude.
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