One of the reasons radio observatories such as ALMA and others are so useful is that different wavelengths of radio emissions let us tune in on different sizes of dust grains in the universe. That’s because dust grains tend to emit radio signals with wavelengths around their own size. This fact allows us to study the types of dust being formed in early planetary systems.

Map of dust grains around DG Tauri. Credit: J. Greaves, et al.

For example, recent observations from ALMA found millimeter-sized dust grains orbiting a brown dwarf star known as Rho Ophiuchi 102. This is somewhat surprising, since it would imply that clouds around small brown dwarfs are similar to dust clouds around larger stars. It also suggests that brown dwarfs may form rocky planets. Another team using the e-Merlin array found somewhat larger grains around the star DG Tauri. Despite the limited resolution, it is clear that a dust belt has formed around the young star.

What these observations show is that protoplanetary disks are not only common around stars, but that dust seems to be present early on. It’s one more clue pointing to the idea that planets are the norm rather than the exception for stars in our universe.

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We have a pretty good idea of how planets form around stars. We know that dust is formed from the remnants of supernovae, that protoplanetary disks of dust form around young stars, and that dust grains can clump together to form pebbles. We also know how larger planetoids can drive the formation of planets, and how planets can migrate from their point of origin to their stable orbits. But there are still gaps in our understanding.

For example, we aren’t entirely sure how we get from pebble-sized particles to planetoid-sized objects. It all has to do with the amount of drag objects experience while moving through the protoplanetary disk. Initially most of the dust grains are extremely tiny, less than a micrometer in size. Because they are so tiny, the amount of drag they experience from the surrounding gas is minimal. They are so tiny that they are almost gas-like in their behavior. For large clumps, on the order of hundreds of meters in diameter, the drag of surrounding gases is also relatively negligible. Their mass is large enough that over the 10 million year period of planetary formation, the gas doesn’t slow them down much. But in between there is a size that is big enough for drag to matter, but not massive enough to overcome that drag. This is on the scale of about a meter. When clumps reach this size, the drag of the surrounding gas would cause them to slow down rather quickly. As a result they would spiral into the star before they can grow much larger.

If this were true, then planets would be very rare. But we know planets are quite common, so there must be a mechanism that prevents planetary seeds from falling into the star. There have been some ideas, such as pressure waves in the planetary disk causing pebble-sized grains to clump into larger planetoids rather quickly, but this would require gravel-sized grains to form early on, which we aren’t sure could happen.

Now a new paper has found evidence of gravel-sized dust grains in the Orion Molecular Cloud. The team used radio telescope observations to determine the size of dust grains in the cloud. This technique is nothing new, but the team found filaments of dust grains that are a millimeter to a centimeter in size. This is much, much larger than typical dust grains observed. What’s particularly interesting about this discovery is that the filaments are in a region where stars will likely start forming in the next 100,000 to a million years. It is possible, then, that these large grains formed within the general cloud itself. If that’s the case, protoplanetary disks could have a ready-made source of large grains that could kickstart the formation of planetoids early on.

Although this is an exciting result, we should still be a bit cautious. The radio observations could be explained by things other than large dust grains (though that doesn’t seem likely), so we will need further observations to confirm the result. We also aren’t sure whether these grains actually formed in the molecular cloud, or if they are debris remnants from another process.

But it seems we’ve found a gravel road from dust to planets.

Paper: Schnee et al. Evidence for Large Grains in the Star-forming Filament OMC-2/3. Monthly Notices of the Royal Astronomical Society (2014)

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Earth gets bombarded from space all the time. With all the micrometeorites, not to mention the occasional meteor or comet, almost 50 tons of material falls to Earth every day. Since all this material from space reaches Earth, you might think that interstellar material falls to our planet all the time. But it turns out that isn’t the case. The vast majority of material comes from within our solar system. Even cometary material comes from the Oort cloud at best, and that’s still on the outer edge of our solar system. There is plenty of interstellar gas and dust beyond our solar system, but even when some of it heads our way it tends to be pushed away from the inner solar system by the solar wind. So we haven’t had any samples of interstellar dust to study.

But now that may have changed. A new paper published in Science has announced the discovery of dust grains that appear interstellar in origin. The samples were gathered by NASA’s Stardust spacecraft, which made a flyby of the comet 89P/Wild. The spacecraft was equipped with aerogel blocks to collect samples of cometary dust.  By analyzing the tracks made by dust fragments as they are caught by the aerogel, the team could determine their trajectory of origin. Part of the reason for doing this is to distinguish cometary fragments from tiny fragments of material that came from the spacecraft itself.

An interstellar dust grain caught by aerogel.
Credit: Rhonda Stroud, Naval Research Laboratory

When analyzing the tracks, the team found seven dust grains with tracks that seem to be of interstellar origin. Initial analysis of the particles finds that they don’t match any single model for interstellar dust formation. So while these models might be on the right track, they don’t provide the whole picture.

This project is also an example of how citizen science can be successful. To analyze the trajectories of particles in the aerogel, the team used a project called stardust@home, where volunteers can help determine particle trajectories. A significant portion of the trajectories were determined through this project. Which just proves the point that you don’t have to be a scientist to do real science.

Paper: Andrew J. Westphal, et al. Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Vol. 345 no. 6198 pp. 786-791 (2014)

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We are the dust of stars, as Carl Sagan so famously said. The elements in our bodies (with the exception of hydrogen) were formed within stars, and then cast out to the universe when large stars explode as supernovae. Of course simply creating elements by nuclear fusion and sending them flying into the cosmos isn’t quite enough to make stardust.  The elements also have to clump into dust particulates.  Understanding that process has posed a bit of a challenge, but now a new paper in Nature has observed it happening in real time.

The team observed the remnant of a supernova that exploded in 2010. By measuring the spectra of this remnant, they could identify the elements and molecules of the remnant. By observing the infrared emissions of the remnant they could also determine the size of dust particles within the remnant.  What they found was that within a couple years dust grains had not only appeared, but had grown to 4 micrometers in size.  It’s estimated that within 20 years the remnant will have produced about a solar mass of dust.

This is quite a lot of dust produced in what is a cosmic blink of an eye.  If other supermovae produce dust at similar rates, then this would explain how dust could have formed so quickly in the early universe.

Paper: Christa Gall, et al. Rapid formation of large dust grains in the luminous supernova 2010jl. Nature (2014) doi:10.1038/nature13558

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