Spontaneous 'dust traps': Astronomers discover a missing link in planet formation

February 27, 2017
An image of a protoplanetary disk, made using results from the new model, after the formation of a spontaneous dust trap, visible as a bright dust ring. Gas is depicted in blue and dust in red. Credit: Jean-Francois Gonzalez

Planets are thought to form in the disks of dust and gas found around young stars. But astronomers have struggled to assemble a complete theory of their origin that explains how the initial dust develops into planetary systems. A French-UK-Australian team now think they have the answer, with their simulations showing the formation of 'dust traps' where pebble-sized fragments collect and stick together, to grow into the building blocks of planets. They publish their results in Monthly Notices of the Royal Astronomical Society.

Our Solar system, and other planetary systems, began life with disks of gas and grains around a young star. The processes that convert these tiny grains, each a few millionths of a metre (a micron) across, into aggregates a few centimetres in size, and the mechanism for making kilometre-sized 'planetesimals' into planetary cores, are both well understood.

The intermediate stage, taking pebbles and joining them together into objects the size of asteroids, is less clear, but with more than 3,500 already found around other stars, the whole process must be ubiquitous.

Dr Jean-Francois Gonzalez, of the Centre de Recherche Astrophysique de Lyon, in France, led the new work. He comments: "Until now we have struggled to explain how pebbles can come together to form planets, 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."

There are two main barriers that need to be overcome for pebbles to become planetesimals. Firstly the drag of gas on dust grains in a disk makes them drift rapidly towards the central star, where they are destroyed, leaving no material to form planets. The second challenge is that growing grains can be broken up in high-speed collisions, breaking them into a large number of smaller pieces and reversing the aggregation process.

This cartoon illustrates the stages of the formation mechanism for dust traps. The central star is depicted as yellow, surrounded by the protoplanetary disk, here shown in blue. The dust grains make up the band running through the disk. In the first stage, the dust grains grown in size, and move inwards towards the central star. The now pebble-sized larger grains (in the second panel) then pile up and slow down, and in the third stage the gas is pushed outwards by the back-reaction, creating regions where dust accumulates, the so-called dust traps. The traps then allow the pebbles to aggregate to form planetesimals, and eventually planet-sized worlds. Credit: © Volker Schurbert

The only locations in planet forming disks where these problems can be overcome are so-called 'dust traps'. In these high-pressure regions, the drift motion slows, allowing to accumulate. With their reduced velocity, the grains can also avoid fragmentation when they collide.

Until now, astronomers thought that dust traps could only exist in very specific environments, but the computer simulations run by the team indicate that they are very common. Their model pays particular attention to the way the dust in a disk drags on the gas component. In most astronomical simulations, gas causes the dust to move, but sometimes, in the dustiest settings, the dust acts more strongly on the gas.

This effect, known as aerodynamic drag back-reaction, is usually negligible, so up to now has been ignored in studies of growing and fragmenting grains. But its effects become important in dust rich environments, like those found where planets are forming.

The effect of the back-reaction is to slow the inward drift of the grains, which gives them time to grow in size. Once large enough, the grains are their own masters, and the gas can no longer govern their motion. The gas, under the influence of this back-reaction, will be pushed outwards and form a high-pressure region: the dust trap. These spontaneous traps then concentrate the coming from the outer disk regions, creating a very dense ring of solids, and giving a helping hand to the formation of planets.

This cartoon illustrates the stages of the formation mechanism for dust traps. The central star is depicted as yellow, surrounded by the protoplanetary disk, here shown in blue. The dust grains make up the band running through the disk. In the first stage, the dust grains grown in size, and move inwards towards the central star. The now pebble-sized larger grains (in the second panel) then pile up and slow down, and in the third stage the gas is pushed outwards by the back-reaction, creating regions where dust accumulates, the so-called dust traps. The traps then allow the pebbles to aggregate to form planetesimals, and eventually planet-sized worlds. Credit: (c) Volker Schurbert

Gonzalez concludes: "We were thrilled to discover that, with the right ingredients in place, dust traps can form spontaneously, in a wide range of environments. This is a simple and robust solution to a long standing problem in planet formation."

Observatories like ALMA in Chile already see bright and dark rings in forming that are thought to be dust traps. Gonzalez and his team, and other research groups around the world, now plan to extend the trap model all the way to the formation of planetesimals.

Explore further: Planetary influences on young stellar disks

More information: J.-F. Gonzalez et al, Self-induced dust traps: overcoming planet formation barriers, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stx016

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13 comments

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Chris_Reeve
1 / 5 (9) Feb 27, 2017
The presented geometry ignores the significance of the Herschel findings, that ...

(1) Stars are commonly observed to form all at once, along the same filament

http://sci.esa.in...lky-way/

"The Herschel data have rekindled the interest of astronomers in studying filaments, emphasizing the crucial role of these structures in the process of star formation ...

filaments must somehow precede the onset of star formation ...

star formation proceeds in two steps: first, turbulent motions of the interstellar gas and dust create an intricate web of filamentary structures; then, gravity takes over ..."

(2) Matter is observed to accrete like branches along the lengths of these filaments.

"... astronomers have detected what appear to be accretion flows, with the most prominent filaments drawing matter from their surroundings through a network of smaller filaments."
Chris_Reeve
1 / 5 (9) Feb 27, 2017
In the light of Herschel's observations that these are oftentimes star SYSTEMS which form all at once along filaments, there is some potential for confusion when observing rings associated with protostars.

What we sometimes see is a ring or a set of rings (seemingly enhanced in this image, at other times not).

If the Herschel data is correct, we should be asking if these rings we are seeing are in fact cross-sections of the filamentary network said to connect all of these protostars together at the time of their formation.
RealityCheck
2.8 / 5 (6) Feb 27, 2017
Hi Chris_Reeve.:)

Careful, mate. This article is ONLY about the proto-planetary formation dynamics (in a pre-existing star's accretion disc AFTER that pro-STAR has formed; ie, already Nova'd).

Try to curb your enthusiam and take extra care to separate the two significantly different dynamical situations when discussing different cases from your 'overall perspective'. OK? Cheers. :)
Chris_Reeve
1 / 5 (7) Feb 27, 2017
I appreciate the clarification.

However, if they have completely misunderstood the process by which stars form, it's quite possible that they do not actually know what they are looking at here.

The filaments wouldn't necessarily completely disappear.
RealityCheck
1 / 5 (4) Feb 27, 2017
Hi Chris_Reeve. :)

I appreciate the clarification.
Glad to be of help re clarity, anytime. :)

However, if they have completely misunderstood the process by which stars form, it's quite possible that they do not actually know what they are looking at here.

The filaments wouldn't necessarily completely disappear.
That's as may be, but there is a new force/dynamics/complication in such situations because of the extremely strong newly-established high spin, magnetic fields, radiation blast and particle/ion 'winds' and polar-jets. Therefore it will be a real cauldron of a mess early on, and that will set the dynamical evolutionary trajectories/possibilities for many a long time/process affecting whatever the initial/aggregating constituents in the accretion disc may be.

Naturally, such high energy cases will also involve complex inter-activity/feedback between local gravitational field formed by the new star AND the plasmic material around such new stars. :)
jonesdave
4.9 / 5 (12) Feb 27, 2017
Stop beating about the bush, RC. There are so many protoplanetary discs that have been viewed now, with planets identified as clearing out the disc, that it is idiotic to claim otherwise. Stop faffing around with the idiot Reeve and just say what you believe. Indeed, what everybody else believes. That is, it is bloody obvious that it has nothing to do with the Velikovskian woo that the idiot Reeve believes in.
Stop humouring cretins.
Nik_2213
5 / 5 (8) Feb 27, 2017
Nice to see a viable bridge between the dust and boulder stages...
cantdrive85
1 / 5 (7) Feb 27, 2017
The intermediate stage, taking pebbles and joining them together into objects the size of asteroids, is less clear, but with more than 3,500 planets already found around other stars, the whole process must be ubiquitous.

Nice to see circular reasoning as the viable bridge between boulders and planets...
cantdrive85
1.4 / 5 (9) Feb 27, 2017
This effect, known as aerodynamic drag back-reaction, is usually negligible, so up to now has been ignored in studies of growing and fragmenting grains.

This statement highlights the ignorance of these plasma ignoramuses. Plasmas and the dust in them behave electrodynamically. The fanciful ideal gases astrophysicists pontificate about are just as real as the rest of the unicorns of the standard theory, DM and BHs.
Whydening Gyre
5 / 5 (1) Feb 28, 2017
This effect, known as aerodynamic drag back-reaction, is usually negligible, so up to now has been ignored in studies of growing and fragmenting grains.

OOPs, accidentally fived you for a stupid statement.
This statement highlights the ignorance of these plasma ignoramuses.

No, it only highlights the ignorance of your unwillingness to accept anything other than your own view.
Plasmas and the dust in them behave electrodynamically.

More or less, yes. Due to larger gravitational effect.
The fanciful ideal gases astrophysicists pontificate about are just as real as the rest of the unicorns of the standard theory, DM and BHs.

They don't play in the same ball park. Please elucidate what you see as the connection.
cantdrive85
1 / 5 (3) Feb 28, 2017
More or less, yes. Due to larger gravitational effect.

Right, it's why all the outlets in your house are in the ceiling, so your electricity can flow downhill.
They don't play in the same ball park. Please elucidate what you see as the connection.

They are all figments of an astrophysicists fanciful imaginings.
Whydening Gyre
5 / 5 (1) Feb 28, 2017
More or less, yes. Due to larger gravitational effect.

Right, it's why all the outlets in your house are in the ceiling, so your electricity can flow downhill.

Did your Mom or Dad drop you on your head when you were little or something?
In space, gravity doesn't work that way. Try a little "cross discipline" study into interacting "sets"...
They don't play in the same ball park. Please elucidate what you see as the connection.

They are all figments of an astrophysicists fanciful imaginings.

As opposed to YOUR fanciful meanderings?
You need to be more inclusive of varying scale factor ratios of ALL contributing members of a particular "set"...
wduckss
1 / 5 (2) Mar 01, 2017
Article is only paraphrase of my article (bad plagiarism):
"Growth doesn't stop with atoms; on the contrary, joining goes on. Through joining, chemical reactions and combined, gas, dust, sand, the rocks named asteroids and comets, etc., are all created. Even further, planets are created the same way. Then, when planets grow to the 10% of Sun's mass, they become stars, which can be really gigantic (super-giants).

Millions of craters scattered around the objects of our Solar system are the evidence of objects' growth. Constant impacts of asteroids into our atmosphere and soil are the evidence of these processes being uninterrupted today, just the same as it used to be in any earlier period of the past. It is estimated that 4 000 – 100 000 tons of extraterrestrial material falls yearly to Earth..." from http://www.svemir...#1growth

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