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‘Dust Traps’ and Planet Formation

Are we homing in on a ‘missing link’ in our theories of planet formation? Perhaps so, judging from the work of researchers at Swinburne University of Technology, Lyon University and St. Andrews University. The work does not challenge a central principle in current thinking, that planets form out of disks of gas and dust grains around young stars. We know that these dust grains grow into centimeter-sized aggregates. We also know that, much later, planetesimals (kilometers in size) grow into planetary cores.

What has been missing is an understanding of how the early ‘pebbles’ are able to aggregate into asteroid-sized objects. One problem is that drag in the disk produced by surrounding gas makes the grains move inward toward the star, a movement that can deplete the disk. The paper describes this as a ‘radial drift barrier,’ in which the grains settle to the midplane of the disk and drift inwards as they lose angular momentum. Taken to its conclusion, the process can lead to accretion into the star, preventing disk grains from ever forming planetesimals.

The second issue: Larger dust grains with higher relative velocities can experience collisions that make aggregation impossible. This is the so-called ‘fragmentation barrier,’ where dust grains shatter instead of sticking after collisions.

How, then, do planets actually form? The researchers have created simulations developing a theory involving ‘dust traps,’ high-pressure locations in the disk where dust grains accumulate as drift motion slows. The accumulation of growing and fragmented grains interacts with circumstellar gas to create these areas, in which trapped particles can grow. At reduced speeds within the traps, the grains avoid fragmentation. The process is depicted in the image below.

dust-trap-formation

Image: The stages of the formation of dust traps. The central (yellow) star, surrounded by the protoplanetary (blue) disk. The dust grains make up the band running through the disk. Credit: © Volker Schubert.

Thus dust grains modify the structure of the surrounding gas. Sarah Maddison (Swinburne University) explains:

“What we have been able to identify is the key role of the drag of dust on the gas. Often in astronomy, the gas tells the dust how to move, but when there is a lot of dust, the dust tells the gas how to move. This effect, known as aerodynamic drag back-reaction, is usually negligible. However, the effect becomes important in dust rich environments, like those found in the planet formation process.”

Back-reaction, in other words, slows the drift of dust grains inward toward the star, giving them time to grow in size to the point where drag from the gas no longer determines their fate. The gas is pushed outwards to form the high pressure region the team calls a dust trap. Concentrated dust grains in the dust traps then spark the subsequent formation of planets.

The process functions in the team’s simulations for a wide range of initial disk structures and dust to gas ratios. From the paper:

We demonstrate that this process is extremely robust and that self-induced dust traps form in different disc structures, with different fragmentation thresholds, and for a variety of initial dust-to-gas ratios. Changing these parameters result in self-induced dust traps at different locations in the disc, and at different evolutionary times.

The process, then, should be widespread despite differences in the stellar environment. The key thing is that the formation of the dust traps makes subsequent planet formation possible. How the fragmentation of dust grains operates determines the result:

While seemingly counter-intuitive, fragmentation is a vital ingredient for planet formation as it helps to form dust traps at large distances from the star. Indeed, fragmentation only allows grains to grow exterior to a certain radial distance and when grains decouple from the gas and start piling up, they do so near that radius. Stronger fragmentation, with a lower fragmentation threshold, implies that this radius lies farther away from the star. This would suggest that most discs thus retain and concentrate their grains at specific locations in time-scales compatible with recent observations of structures in young stellar objects.

The researchers have found a way to overcome the planetary formation bottleneck, allowing micrometer-sized dust grains to grow to centimeter-size and above, forming the structures that will eventually be incorporated into planetesimals. The process of going from planetesimal to planet, the researchers argue, has been considered through various mechanisms, but the missing piece has always been the preservation of the original dust grains in the disk long enough for aggregation to occur before their accretion into the star. Here, at least, we have a theoretical mechanism to explain how the grains are preserved and can grow.

The paper is Gonzalez, Laibe & Maddison, “Self-induced dust traps: overcoming planet formation barriers,” Monthly Notices of the Royal Astronomical Society 467 (2) (2017), pp. 1984-1996 (abstract / preprint).

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Comments on this entry are closed.

  • Ashley Baldwin March 2, 2017, 19:14

    Presumably with ever improving direct imaging technology on the ground and in space it will be possible to visualise protoplanetary disks in the necessary detail , at different stages of development and at different wavelengths ( from visible through increasing I.R all the way up to millimetre and submillimeter ) . With greater optical and spectroscopic resolution allowing both confirmation and analysis of these dust traps which are presumably made up of silicates like olivine , something which has already been discovered in both the interstellar medium and protoplanetary disks . ( often thrown out of aged ASGB stars and planetary nebulae ) .

  • Alex Tolley March 3, 2017, 0:52

    Does this mechanism add anything about the distribution of planet formation in the disk?

  • Paul Gilster March 3, 2017, 8:50

    The models discussed in the paper show how different initial conditions contribute to dust traps forming at varying distances from the star depending on initial dust/gas ratio and fragmentation velocity. Figure 5 in the paper shows this pretty well, adding that larger grain concentrations produce more effective dust trapping. So I’d say that fleshing out this mechanism may be useful in terms of planetary distribution.

    • Robert G March 3, 2017, 16:55

      I guess that then begs the question of how the initial conditions vary between star types and locations within the galaxy. Also, if the process is reasonably deterministic, then perhaps we could use empirical observations of planetary systems to infer back to the conditions present at the system’s formation. Might be interesting to compare against stellar age as well to obtain a picture of how conditions have changed in the galaxy over time.

      Just a thought.

  • jim moore March 3, 2017, 11:43

    Do they model electrical charges on the dust particles?
    I know on earth a lot of types of dust will carry an electrical charge and you can use that to concentrate the dust.

    • Alexander Tolley March 3, 2017, 15:35

      It doesn’t look like it, although that detail may be hidden in the references to the underlying model.

  • Jim Early March 3, 2017, 13:00

    The pressure of a light gas against a denser dust cloud would seem to be an opportunity for Reighley-Taylor instabilities. This might give some preferred size scale for accumulating particles. It could also introduce local vorticity. It should be interesting to see how this idea develops.

  • DJ Kaplan March 3, 2017, 13:44

    We know that larger bodies, such as asteroids, have sufficuent gravity for one to attract another and begin accreting. Do dust particles have the same gravitational attraction, or does their accretion rely on other forces, such as electrostatics?

  • Alexander Tolley March 3, 2017, 15:34

    The model is only for material beyond the snowline and not for the region where silicates are predominant. The model creates planetesimals in the tens of AUs and outwards, i.e. from Neptune outwards.

    Presumably, the model could eventually be extended to deal with material inside the snowline to shed light on the formation of rocky planets and cores.

    This would be needed to understand whether planets must migrate before reaching their current positions or not.

  • Geoffrey Hillend March 3, 2017, 17:42

    According to mainstream astrophysics, the electromagnetic and Van der Waals forces are thought to cause the dust particles to stick together when they are small but once the material becomes boulder sized gravity controls the large scale accretion process through collisions so that they grow through one collision at a time and survival of the fittest or the strongest and largest that survives the collision.

  • Ronald March 7, 2017, 6:35

    Slightly off-topic, but related: what is the present status of ESPRESSO? It was supposed to see first light somewhere late 2016 and to be in full use in 2017. This instrument (spectrograph), in combination with the VLT, will be able to detect earthlike planets around solar type stars through radial velocity measurements with an accuracy of about 10 cm/s.
    I could not find recent news about its present status on the ESO and ESPRESSO sites.