A new planet begins its life in a rotating circle of gas and dust, a cradle called a protostellar disk. My colleagues and I used computer simulations to show that these disks are likely to have surprising flattened shapes on newborn gas planets. This finding, published in Astronomy and Astrophysics Letters, could add to our picture of how planets form.
It is extremely difficult to observe newly formed protoplanets still within their protostellar disks. So far only three such young protoplanets have been observed, two of them in the same system, PDS 70.
We need to find systems that are young, and close enough for our telescopes to be able to detect faint light from the planet itself and distinguish it from the light of the disc. The entire process of planetary formation lasts only a few million years, which is just a blink of an eye in astrophysical scales. This means that we have to be lucky to catch them while they are forming.
Our research group performed computer simulations to determine the properties of gaseous protoplanets under a variety of thermal conditions in the planetary cores.
The simulations have sufficient resolution to be able to follow the evolution of a protoplanet in the disc from an early stage, when there is only condensation within the disc. Such simulations are computationally demanding and were run on DiRAC, the UK’s astrophysics supercomputing facility.
Usually, multiple planets form within a disk. The study found that protoplanets called oblate spheroids are shaped like Smarties or M&M’s, rather than spherical. They grow by drawing gas mainly through their poles rather than their equator.
Technically, the planets in our Solar System are also oblate spheroids but they expand slightly. Saturn balances 10%, Jupiter 6%, but Earth only 0.3%.
In comparison, the typical flattening of a protoplanet 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 the “core accretion”. According to this model, tiny dust particles smaller than sand collide with each other, group together and gradually grow into larger and larger bodies. This is effectively what happens to the dust under your bed when it is not cleaned.
Once a dust core with many giant forms, it pulls gas from the disk to form a gas giant planet. This bottom-up approach would take several million years.
The other approach, from top to bottom, is the disc instability theory. In this model, the protostellar disks hosting young stars are gravitationally unstable. In other words, they are too heavy to maintain and so they split into pieces, which evolve into planets.
The primordial theory has been around for a long time and can explain many aspects of how our Solar System formed. However, disc instability can better explain some of the exoplanetary systems we have discovered in recent years, such as those orbiting a gas giant 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 that suggest that planets exist in very young disks.
Our study focused on gas giant planets formed through a disk instability model. They are flattened because they form from the compression of an already flat structure, the protostellar disk, but also because of the way they rotate.
No fair world
Although these protoplanets are very flat overall, their cores, which will evolve into the gas giant planets as we know them, are not as flattened – only about 20%. This is only twice the tilt of Saturn. Over time they are expected to become more spherical.
Rocky planets, like Earth and Mars, cannot form through disc instability. They are thought to form by the slow assembly of dust particles into pebbles, rocks, kilometer objects and eventually planets. They are too compact to be noticeably flat even when newborn. There is no way the Earth was flattened that high when it was young.
But our study supports a role for disk instabilities for certain worlds in some planetary systems.
We are now moving from the era of exoplanet discovery to the era of exoplanet characterization. Many new observatories are to be operational. These will help discover more protoplanets embedded in their discs. Predictions from computer models are also becoming more sophisticated.
The comparison between these theoretical models and observations is bringing us closer and closer to understanding the origin of our Solar System.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Dimitris Stamatellos receives funding from the Science and Technology Facilities Council (STFC).