We now have a better idea of how planets are born

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Mar 3, 2017

Billions of years ago, Earth supposedly started out as a speck of space dust. It then evolved into something immense.

The issue is that science has been unable to fill in the blanks as to how it went from zero to planet because the theory of planet formation has always been incomplete. Now an international team of astronomers believe they have found a crucial missing link that explains one enigma.

Planets are thought to emerge from disc-shaped clouds of dust and gas surrounding young stars ... but theories on how they actually come into being have always been hazy in areas. Astronomers have been able to understand the pre-embryonic phases when microscopic dust particles aggregate into larger particles and the later phases during which the planetesimal that becomes the core of a planet is formed, but the intermediate stage is more nebulous.

"Until now we have struggled to explain how pebbles can come together to form planets," said team lead Dr Jean-Francois Gonzalez of the Centre de Recherche Astrophysique de Lyon, in France. "And yet we've now discovered huge numbers of planets in orbit around other stars. That set us thinking about how to solve this mystery."

Somewhere between particle and planetesimal, the debris that will form a future planet joins into an asteroid-sized object. This phase has eluded scientists because it is also the most perilous. Particles that are actively accumulating more material are prone to crashing into each other and ending up as dust all over again. Gas in the protoplanetary disc surrounding a nascent star is also a destructive force, dragging the particles toward the star until they are annihilated. What simulations run by the team suggest is that fragments about the size of pebbles cluster in high-pressure spontaneous "dust traps."

Gas glows blue and dust glows red in this image of a protoplanetary disc, created using the new dust trap model. 

In their study published in Monthly Notices of the Royal Astronomical Society, Gonzalez and his colleagues state that dust traps happen when "The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped."

While dust is usually pushed around by gas, it can overpower the gas in especially dusty environments and have a strong influence over it — an effect called aerodynamic drag back-reaction. This back-reaction then pushes gas outward and forms a spontaneous high-pressure region. Inside a dust trap, gas drag and high-speed collisions are all but eliminated as drift motion decelerates, lowering the velocity of the particles and giving them time to congregate and stick together with little risk of being smashed or torn apart. Particles drifting inward from the outer reaches of the protoplanetary disc are then gradually added to the growing aggregates that will eventually be the building blocks of a planet.

Astronomers were previously convinced this could only happen in particular settings until the simulations showed otherwise. Gonzalez and his team are optimistic about what the trap model means for the future of planet formation study and look to extending the model to explain later phases of planetary development.

Meanwhile, let your mind be blown by the fact that you are walking on what was probably once a particle of dust.

(via Phys.org)