by Paul Gilster | Mar 2, 2017 | Exoplanetary Science |
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.
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).
by Paul Gilster | Mar 1, 2017 | Exoplanetary Science |
The binary system SDSS 1557, about 1000 light years from Earth, was thought to be a single white dwarf star until detailed measurements revealed that the brighter star was being gravitationally influenced by a hither unseen brown dwarf. And that, in turn, has given us an intriguing look at possible planetary formation around both members of a close binary. We’ve found gas giants in such systems, but researchers led by Jay Farihi (University College London) have found signs of rocky debris here that point to the possibility of planets of a much different composition.
“Building rocky planets around two suns is a challenge,” says Farihi, “because the gravity of both stars can push and pull tremendously, preventing bits of rock and dust from sticking together and growing into full-fledged planets. With the discovery of asteroid debris in the SDSS 1557 system, we see clear signatures of rocky planet assembly via large asteroids that formed, helping us understand how rocky exoplanets are made in double star systems.”
Image: A disc of rocky debris from a disrupted planetesimal surrounds white dwarf plus brown dwarf binary star. The white dwarf is the burned-out core of a star that was probably similar to the Sun, the brown dwarf is only ~60 times heavier than Jupiter, and the two stars go around each other in only a bit over two hours. Credit: Mark Garlick, UCL, University of Warwick and University of Sheffield.
Can planets, or at least their debris, survive the red giant expansion phase that leads to a white dwarf? Yes, says the new paper on this work, noting that we now have more than three dozen planetary system remnants that have been found through study of circumstellar disks of white dwarfs; we also have several hundred white dwarfs that show signs of accretion of planetary debris.
The debris around SDSS 1557 is spread around the two stars, offering a helpful target for the team’s analysis. Working with observations from the Gemini Observatory South instrument and the European Southern Observatory’s Very Large Telescope, the team found material with high metal content including silicon and magnesium, which could be identified as it was drawn onto the surface of the white dwarf. Such atmospheric pollution has become a tool for the analysis of white dwarf systems. In this case, the UCL team found that 1017 grams of matter — the equivalent of a 4 km asteroid — produced the observed result.
The paper on this work points out that planets of Neptune up to Jupiter size are unlikely to form where they have been found in Kepler detections of circumbinary planets, but are likely the result of migration. However, models exist for smaller planet formation within the snowline, which gives us the possibility of planets like the famous ‘Tatooine,’ from George Lucas’ Star Wars. And work on polluted white dwarfs adds weight to the idea.
From the paper:
The current paradigm of disrupted and accreted asteroids has been unequivocally confirmed by numerous studies, including the recent detection of complex and rapidly evolving photometric transits from debris fragments orbiting near the Roche limit of one star. To date, all polluted white dwarfs with detailed analyses indicate the sources are rocky planetesimals comparable in both mass and composition to large Solar System asteroids, and thus objects that formed within a snow line. These findings unambiguously demonstrate that large planetesimal formation in the terrestrial zone of stars is robust and common.
Dr. Farihi is on record (on his homepage) as saying that he believes we will learn more about extrasolar terrestrial planets using white dwarfs than any other method. The reason: The atmospheres of cool white dwarfs feature hydrogen and helium that can easily become polluted by small amounts of heavy elements. Work like the current paper reminds us that we can use this metal pollution to measure the composition of rocky material around the star.
The debris responsible for white dwarf atmospheric pollution is believed to come from tidally destroyed asteroids whose parent bodies were large and differentiated. Farihi notes that we may be looking at the parent bodies of planetesimals or even fragments of major planets, with compositions similar to material found in our own inner Solar System. He estimates that 20% to 30% of all white dwarfs are orbited by the remains of terrestrial planetary systems.
Image: Dr. Jay Farihi, among some exceedingly interesting standing stones. Credit: Jay Farihi/UCL.
The SDSS 1557 discovery calls for continuing follow-ups, for as co-author Boris Gänsicke (University of Warwick) points out, the signature is transient. Says Gänsicke:
“Any metals we see in the white dwarf will disappear within a few weeks, and sink down into the interior, unless the debris is continuously flowing onto the star. We’ll be looking at SDSS 1557 next with Hubble, to conclusively show the dust is made of rock rather than ice.”
So at SDSS 1557 we have all the markers of a parent body that formed within the snowline, which implies that rocky planet formation in circumbinary orbits within a close double system is feasible. The paper concludes: “These observations therefore support a picture where additional mechanisms can promote planetesimal growth in the terrestrial zones of close binary stars, which are predicted to be substantially wider than in planet forming disks around single stars.”
The paper is Farihi, Parsons & Gänsicke, “A circumbinary debris disk in a polluted white dwarf system,” Nature Astronomy 1 March 2017 (abstract / preprint).
