ALMA, located in the Chilean Atacama desert, is the most powerful telescope for observing the cool … [+]
The conventional view of solar system formation has long been that planets are simply byproducts of star formation. The idea is that cold, dense molecular clouds collapse into rotating disks of gas and dust from which stars and planets form in relative isolation.
The triggers for such star-forming collapsing clouds run the gamut —- everything from the cloud’s own gravity; shockwaves generated by nearby supernovae; or even collisions with other molecular clouds. It’s a process almost as old as time itself and is a scenario that our own G-2, yellow dwarf star likely followed some 4.56 billion years ago.
But a new paper to be published in The Astrophysical Journal note that the continual infall of both gas and dust from the interstellar medium (ISM) play a much bigger role in the formation of planetary systems than previously thought. The paper is based on computer simulations and calculations, but is motivated by new observations from ALMA, the European Southern Observatory’s Atacama Large Millimeter/submillimeter Array based in northern Chile.
ALMA has uncovered evidence of in-falling material onto protoplanetary disks in several nearby star-forming regions, Andrew Winter, the paper’s lead author and an astronomer at France’s Observatoire de la Cote d’Azur, told me via email. This suggests the process is common, he says.
Conventional models for planet formation assume that a protoplanetary disk forms during proto stellar collapse, and then planets grow from material in the isolated star-disk system, the authors write. But we show that accretion of the ISM is an important process in driving proto planetary disk evolution, the authors note.
Material in the disk is constantly replenished over its lifetime, they write. In fact, the authors estimate that 20 to 70 per cent of disks are mostly composed of material captured in the most recent half of their lifetime, the authors write.
A key point of the paper is that these young planetary systems are inextricably connected to the interstellar medium in ways that was heretofore not appreciated.
People usually assume that these disks form very early on, at the same time as the young star, and then evolve in isolation, says Winter. In this paper, we’re showing that in contrast to this usual picture, disk material is constantly replenished by capturing new gas and dust from the ISM, he says.
The material captured is replenished by the turbulence of the ISM across a wide range of scales, all the way from the galactic (many tens of thousands of light years across) down to the scale of individual disks on the order of several hundred earth-sun distances across.
Feeding The Disk
The ISM material continually feeds the disk and can also ‘stir up’ the disk (by adding kinetic energy), and the resultant turbulence in the disk can change how dust grows to form planets, says Winter. It means that the type of planets that form probably depends on the density of the ISM that surrounds the star, he says.
Thus, the picture of planet formation itself has now changed because the surroundings of the star continue to be important even after the collapse of this molecular cloud, says Winter. All of the processes that help the planet grow from dust can be influenced by the in-falling gas, and this directly links the planet formation process to the external environment, he says.
Infalling Material
In-fall of material onto the disk has been inferred in several young planetary systems, and if it is rapid enough could potentially cause a gravitational instability, which means the rapid collapse of material to form planets or even small stars, says Winter.
This influence becomes less important as a star gets older; mostly because the star ‘decouples’ from the region of enhanced density in which it forms, he says. This is why mature stars like our sun aren’t constantly capturing new material, says Winter.
Interstellar gas and dust could make a big difference.
Perhaps more or less in-falling gas could lead to the big differences we see between planetary systems, says Winter.
The Bottom Line?
If the ISM drives disk evolution, this represents a considerable change of direction for protoplanetary disk theory, which has largely focused on processes operating on an isolated star-disk system, says Winter. The consequences of this new picture of disk evolution are wide-reaching and relate to every stage of planet formation, he says.
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