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(MENAFN- The Conversation) A new planet begins its life in a rotating circle of gas and dust, a cradle known as a protostellar disk. My colleagues and I have used computer simulations to show that the newborn gaseous planets in these disks probably have surprisingly flattened shapes. This finding, published in Astronomy and Astrophysics Letters, could contribute to our idea of how exactly planets form.
Observing newly formed protoplanets that are still within their protostellar disks is extremely difficult. Only three of these young protoplanets have been observed so far, two of them in the same system, PDS 70.
We need to find systems that are young and close enough that our telescopes can detect the planet’s faint light and distinguish it from that of the disk. The entire process of planetary formation lasts only a few million years, which is nothing more than the blink of an eye on astrophysical scales. This means we have to be lucky to catch them in the act of forming.
Our research group performed computer simulations to determine the properties of gaseous protoplanets under a variety of thermal conditions in planet cradles.
The simulations have sufficient resolution to be able to follow the evolution of a protoplanet in the disk from an early phase, when it is nothing more than a mere condensation within the disk. These simulations are computationally demanding and were run at DiRAC, the UK’s astrophysical supercomputing facility.
Typically, several planets form within a disk. The study found that protoplanets have a shape known as oblate spheroids, like Smarties or M&M’s, rather than being spherical. They grow by extracting gas predominantly through their poles rather than their equators.
Technically, the planets in our Solar System are also oblate spheroids, but their flattening is small. Saturn has an oblateness of 10%, Jupiter 6%, while the Earth barely 0.3%.
In comparison, the typical flattening of protoplanets is 90%. Such flattening will affect the observed properties of protoplanets and must be taken into account when interpreting observations.
How planets begin
The most widely accepted theory of planet formation is “core accretion.” According to this model, tiny dust particles smaller than sand collide with each other, clump together and progressively grow to form increasingly larger bodies. This is effectively what happens to the dust under the bed when it is not cleaned.
Once a dust core with enough mass forms, it pulls gas out of the disk to form a gas giant planet. This bottom-up approach would take a few million years.
Close-up image of a simulated protoplanet seen from the side. The arrows correspond to the speed of the gas as it flows toward the protoplanet. UCLAN, author provided (no reuse)
The opposite, top-down approach is the disk instability theory. In this model, the protostellar disks surrounding young stars are gravitationally unstable. In other words, they are too heavy to be maintained and therefore fragment into pieces that evolve into planets.
The theory of core accretion has been around for a long time and can explain many aspects of how our Solar System formed. However, disk instability may better explain some of the exoplanetary systems we have discovered in recent decades, such as those in which a gas giant planet orbits very, very far from its host star.
The appeal of this theory is that planet formation occurs very quickly, within a few thousand years, which is consistent with observations suggesting that planets exist in very young disks.
Our study focused on gas giant planets formed using the disk instability model. They are flattened because they are formed from the compression of an already flat structure, the protostellar disk, but also because of the way they rotate.
No flat Earth
Although these protoplanets are generally very flattened, their cores, which will eventually evolve into the gas giant planets as we know them, are less flattened: only about 20%. This is only twice the flattening of Saturn. They are expected to become more spherical over time.
Rocky planets, like Earth and Mars, cannot form through disk instability. They are believed to form by slowly assembling dust particles into pebbles, rocks, kilometer-sized objects, and eventually planets. They are too dense to flatten significantly even as newborns. There is no chance that the Earth was flattened to such a high degree when it was young.
But our study supports the role of disk instability for some worlds in some planetary systems.
We are now moving from the era of exoplanet discoveries to the era of exoplanet characterization. Many new observatories are about to come into operation. These will help discover more protoplanets embedded in their disks. Predictions from computer models are also becoming more sophisticated.
The comparison between these theoretical models and observations brings us closer to understanding the origins of our Solar System.
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