Centauri Dreams
Imagining and Planning Interstellar Exploration
Is ET Lurking in Our Cosmic Backyard?
Jim Benford is continuing his research into the still nascent field known as SETA, the Search for Extraterrestrial Artifacts. A plasma physicist and CEO of Microwave Sciences, as well as a frequent Centauri Dreams contributor, Benford became intrigued with recent discoveries about Earth co-orbital objects — there is even a known Earth Trojan — and their possibilities in a SETI context. If we accept the possibility that an extraterrestrial civilization may at some point in Earth’s 4.5 billion year history have visited the Solar System, where might we find evidence of it? Two papers grew out of this, one in Astrobiology, the other in the Journal of the British Interplanetary Society (citations below). In the first of two posts here, Jim explains where his work has led him and goes through the thinking behind these recent contributions.
by James Benford
Part 1: How Many Alien Probes Could Have Come From Stars Passing By Earth?
1. Searching for Extraterrestrial Artifacts
Alien astronomy at our present technical level may have detected our biosphere many millions of years ago. The Great Oxidation Event occurred around 2.4 billion years ago; it was a rise in oxygen as a waste product due to organisms in the ocean carrying out photosynthesis. Long-lived robotic probes could have been sent to observe Earth long ago. I will call such a probe a “Lurker,” a hidden, unknown and unnoticed observing probe, likely robotic. They could be sent here by civilizations on planets as their stars pass nearby.
Long-lived alien societies may do this to gather science for the larger communicating societies in our Galaxy. The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
Here, in part 1, I estimate how many such probes could have come here. This is explained in detail in [1].
In Part 2, titled ‘A Drake Equation for Alien Artifacts’, I propose a version of the Drake Equation to include searching for alien artifacts that may be located on Moon, Earth Trojans and co-orbital objects [1]. I compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for artifacts to the conventional listening-to-stars SETI strategy.
1.1 Observing Earth
From Figure 1, the time over which our biosphere has been observable from great distances, perhaps thousands of light years, due to oxygen in the atmosphere, is a very long time, measured in the billions of years [7,8]. The first oxidation event occurred about 2 .5 billion years ago and the second, largest oxidation event about 0.65 billion years ago, so 0.65 109 < TL <2.5 109 years.
An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate.
Figure 1. History of Oxygen content of Earth’s atmosphere is observable from great distances. Dashed line is present value. Horizontal axis is in millions of years before present. (Wikipedia Commons)
2. How Often Do Stars Pass By Our Sun?
It is not widely known that stars pass close to our solar system. The most recent encounter was Scholz’s Star, which came 0.82 light-years from the Sun about 70,000 years ago [3]. A star is expected to pass through the Oort Cloud every 100,000 years or so, as Scholz’s Star did, shown in Figure 2.
Bailer-Jones et al. showed that the number of stars passing within a given distance R, NS (R), scales as the square of that distance [4]. This comes about because Earth is in a flow of stars circling the galactic center, so the cross-sectional area is what matters, which gives an R2 scaling, rather than the volume, ~ R3. Figure 3 shows that several stars have approached or will approach our solar system over 105 years.
Figure 2. Our most recent visitor: Scholz’s Star came within 0.82 light-years from the Sun about 70,000 years ago (NASA).
Bailer-Jones et al., using accurate 3D spatial and 3D velocity data for millions of stars from the Second Gaia Data Release has shown that a new passing star comes within one light year of our Sun every half million years, 100 within 10 light years [4].
With the number of stars passing within a given distance, NS (R), and R the distance of the star from the Sun in light years, the rate of passing stars is:
So a new star comes within 10 ly every 5,000 years [3]: during our 10,000-year agricultural civilization, two new stars have come within 10 ly.
Figure 3. Stars come very close to Earth frequently. About 2 stars come within a light year every million years. An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate. (stackexchange.com)
3. How Many Lurkers May Have Come Here?
To calculate the number of Lurkers that could be located at various sites nearby to Earth, such as the Moon, Earth Trojan zone or the co-orbitals, I make the following estimates. The quantities to use in calculating this concept are shown in Table 1.
There are two factors to evaluate: 1) How often do stars get within a given range of Earth? 2) How long would a Lurker reside in a given location near Earth?
Of course, a key factor we do not know is what fraction of the stars have spacefaring civilizations.
Table 1 Passing Stars Parameters
The number of Lurkers that could arrive and now be found, NL, would be fip times TL, the orbital lifetime of the object upon which the Lurker is resident, times the passing star rate, [dNS(R)/dt] from Eq. 1:
We don’t know fip, but we can calculate the ratio
Now we make estimates of NL/fip. Details of these estimates below can be found in [1].
4.0 Locations for Lurkers Near Earth
The time that Lurkers would be in the solar system, TL, will be limited by the lifetime of the orbits they are in. That is determined by the stability of the orbit of the near-Earth object it lands on. This provides an upper bound to how long they could be around. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:
4.1 The Moon
Searching on the Moon has recently been advocated [5, 6]. Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. Then the Earth ecosystem would not attract attention. Later, life became evident in our atmosphere.
We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has photographed about 2 million sites at sub-meter resolutions. We can see where Neil Armstrong walked! The vast majority of these photos have not been inspected by the human eye. Davies and Wagner have proposed searching these millions of photographs for alien artifacts, which would require an AI for initial surveys [5]. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans and the Earth co-orbitals. A recent AI analysis of 2 million images from LRO revealed rockfalls over many regions of the Moon [9]. So we have proof a search for artifacts of ~1-meter scale could be done by AI.
Figure 4 The Apollo 17 site as seen by the Lunar Reconnaissance Orbiter. Note that Moonbuggy tracks can be clearly seen. A study of the >2 million such photos could detect possible artifacts on the Moon (NASA).
4.2 Earth Trojans
Figure 5 shows the many Jupiter Trojans, located at stable Lagrange Points near that planet. There may be many such objects in the Earth Trojan region [11], ~60 degrees ahead of and following Earth. Their lifetime is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable Lagrange Point orbits [11-14].
Figure 6 shows a portion of the orbit of the only Earth Trojan found so far, 2010 TK7. It oscillates about the Sun–Earth L4 Lagrange Point, ~60 degrees ahead of Earth [15]. Its closest approach to Earth is about 70 times the Earth-Moon distance. It is not a primordial Earth Trojan and is estimated to have an orbital lifetime of 250,000 years, when it will go into a horseshoe orbit about the sun. It is clear why there are no other Trojans of the Earth yet found: they are hard to observe from Earth.
There are large stable regions at Lagrange Points, so Trojans may exist for long time scales. It is possible that primordial Earth Trojans exist in the very stable regions around the Lagrange Points. Orbital calculations show that the most stable orbits reside at inclinations <10° to the ecliptic; there they may survive the age of the solar system, so again we use the oxygen time, ~2.5 Gyr. So Trojans’ orbital lifetimes can vary from 2 105 years to 2.5 109 years.
Figure 5. The many Jupiter Trojans, which lead and follow the planet at ~ 60°. (Wikipedia Commons)
Figure 6. Portion of the orbit of the one Earth Trojan found so far, 2010 TK7. (NASA)
4.3 Earth Co-orbitals
See [16] for a discussion of the co-orbitals of Earth. A large number of tadpole, horseshoe and quasi-satellites that approach near to Earth appear to be long-term stable. Figure 7 shows to orbit of the nearest one, 2016 HO3. Morais and Morbidelli, using models of main asteroid belt sources providing the co-orbitals and their subsequent motions, estimate lifetimes to run between 1 thousand and 1 million years. They conclude that the mean lifetime for them to maintain resonance with Earth is 0.33 million years (17).
Figure 7. Orbits around the Sun of Earth and the nearby quasi-satellite 2016 HO3. It comes within 5 million km of Earth (NASA).
5. Conclusions
In [1] the above remarks are quantified. Here I summarize the calculations in the Table, for probes traveling from 10 ly and 100 ly. (Note that, since co-orbitals have a finite lifetime on their orbits near Earth, Table 2 refers to this is the number of probes that may have landed on what was at the time a co-orbital but will now have wandered off somewhere.)
Table 2: NL/fip: The number of Lurkers, from stars that pass by our Solar System that could have arrived and now could be found, for several nearby astronomical bodies, divided by fip, the fraction of stars that have civilizations that develop interstellar probe technology and launch them.
- Clearly, the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
- Of course, fip is the factor we don’t know: how many civilizations develop interstellar probe technology and launch them.
- The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
- Close inspection of bodies in these regions, which may hold primordial remnants of our early solar system, yields concrete astronomical research. It will yield new astronomy and astrophysics, quite apart from finding Lurkers.
- A suggestion for SETI observers: Look at the specific stars that have passed our way in the last 10 million years and ask how many of them are ‘sunlike’ and/or are known to have habitable planets. Observe those stars closely for possible emissions to Earth [16].
For discussion of approaches to study these objects, starting with passive observations, and going on to missions to them, see Reference 14, section 4, “SETI Searches of Co-orbitals”. The actions and observations are:
1. Launch robotic probes and manned missions to conduct inspections, take samples.
2. Conduct passive SETI observations.
3. Use active planetary radar to investigate the properties of these objects
4. Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.
5. Launch robotic probes and manned missions to conduct inspections, take samples.
This argues for a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for alien artifacts [2].
References
1. J. Benford, “How Many Alien Probes Could Have Come From Stars Passing By Earth?”, JBIS 74 76-80, 2021.
2. J. Benford, “A Drake Equation for Alien Artifacts“, Astrobiology 21, 2021.
3. E. Mamajek et al, “The Closest Known Flyby Of A Star To The Solar System” ApJ Lett., 8003 L17, 2015.
4. C. A. L. Bailer-Jones et al, “New Stellar Encounters Discovered in the Second Gaia Data Release”, Astronomy & Astrophysics 616 A37, 2018.
5. P.C.W. Davies, R.V. Wagner, “Searching for Alien Artifacts on the Moon”, Acta Astronautica, doi:10.1016/j.actaastro.2011.10.022, 2011.
6. A. Lesnikowski, L. Bickel and D. Angerhausen, “Unsupervised Distribution Learning for Lunar Surface Anomaly Detection”, arXiv:2001.04634. 2020.
7. X. L. Kaltenegger, Z. Lin and J. Madden, ““High-resolution Transmission Spectra of Earth Through Geological Time”, Astroph. Lett., 2041, 2020.
8. Y. V. S. Meadows et al., “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment“, Astrobiology 18, 620, 2018.
9. V. Bickel V. et al., 2020 Impacts drive lunar rockfalls over billions of years, Nature Communications, 11:2862 | https://doi.org/10.1038/s41467-020-16653-3
10. R. Malhotra, “Case for a Deep Search for Earth’s Trojan Asteroids”, Nature Astronomy 3, 193, 2019.
11. M, ?uk, D. Hamilton and M. Holman, “Long-term stability of horseshoe orbits”, Monthly Notices Royal Astronomical Society, 426, 3051, 2012.
12. F. Marzari, H. Scholl, “Long term stability of Earth Trojans”, Celestial Mechanics and Dynamical Astronomy, 117, 91, 2013.
13. Zhou, Lei; Xu, Yang-Bo; Zhou, Li-Yong; Dvorak, Rudolf; Li, Jian, “Orbital Stability of Earth Trojans”, Astronomy & Astrophysics, 622, 14, 2019.
14. R. Dvorak, C. Lhotka, L. Zhou, “The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids”, Astronomy & Astrophysics, 541, 2012.
15. P. Wiegert, K. A. Innanen and S. Mikkola, “An Asteroidal Companion to the Earth”, Nature, 387, 685, 1997.
16. J. Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables”, AsJ, 158:150, 2019.
17. M. Morais and A. Morbidelli, ‘The Population-of Near-Earth Asteroids in Co-orbital Motion with the Earth”, Icarus 160, 1, 2002.
Biosignatures: The Oxygen Question
Just how useful is oxygen as a biosignature? It’s a question we’ve examined before, always with the cautionary note that there are non-biological mechanisms for producing oxygen which could make any detected biosignature ambiguous. But let’s go deeper into this, thanks to a new paper on ‘oxygen false positives’ out of the University of California at Santa Cruz. The paper, produced by lead author Joshua Krissansen-Totton and team, offers scenarios that can place an oxygen detection in the broader context that would distinguish any such find as biological.
Let’s begin with the fact that in addition to its obvious interest because of Earth’s history, photosynthesis involving oxygen requires the likely ubiquitous carbon dioxide and water we would expect on habitable zone planets. Helpfully, oxygen should be readily detectable on exoplanets because of its absorption features, which are prominent not only in visible light but in the near infrared and thermal infrared, if we include ozone. Space-based missions as well as ground-based Extremely Large Telescopes should be able to find oxygen signatures.
I found the authors’ discussion of M-dwarfs fascinating. We have to weigh our strategies with these small stars in mind because they are the ones for which atmospheric spectroscopy will first become available for habitable zone rocky planets. Already we’re in deep water, because the oxygen we might find on such worlds could have complex origins. From the paper:
…several features of M?dwarfs make them susceptible to non?biological oxygen accumulation. In particular, the extended pre?main sequence of late M?dwarfs could yield habitable zone terrestrial planets with hundreds or thousands of bar O2 from XUV?driven hydrogen loss (Luger & Barnes, 2015). At least some of this oxygen will likely dissolve in a surface magma ocean and be sequestered in the mantle, but retaining oxygen?rich atmospheres is still possible, especially for highly irradiated terrestrial planets…
Not only does this throw a spanner into the works for biosignature detection, but it could act as an active deterrent to the emergence of life by making prebiotic chemistry impossible. All this is under active study, as the paper’s numerous citations make clear, with the authors adding that “photochemical runaways yielding O2?CO rich atmospheres remain a strong possibility for late M dwarfs.” These older red dwarfs tend to be the ones of higher astrobiological interest given that younger stars in this category are given to higher amounts of flare activity.
All of this points to the problems of oxygen as a biosignature and the need to examine how non-biological oxygen can accumulate on the planets we’re interested in, and this extends to planets around F-, G- and K-class stars as well, although the problem here seems highly dependent on the initial inventory of volatile elements, as the authors make clear. It’s also clear we have a great deal to learn about oxygen production via non-biological methods like hydrogen escape and water photodissociation, all reviewed in this crisp and clearly written paper. The interplay between atmosphere and geochemistry is the study’s central point:
The robustness of oxygen biosignatures rests on the assumption that for temperate planets with effective cold traps, small abiotic oxygen source fluxes from H escape will be overwhelmed by geological sinks. To test this assumption, it is necessary to model the redox [oxidation-reduction] evolution of terrestrial planet from formation onwards. This is because planetary redox evolution depends on both the initial state of the atmosphere and mantle after the magma ocean has solidified, and on the subsequent internal evolution and atmospheric state. Interior evolution dictates crustal production rates and outgassing fluxes, which determine the efficiency of geologic sinks of oxygen.
The authors use a model of planetary development that includes a wide range of initial volatile elements in varying abundance, taking rocky worlds all the way from their original formation up through eras of geochemical cycling lasting billions of years. The goal is to produce scenarios in which a lifeless planet around various stellar types could evolve with atmospheric oxygen. Context is all, meaning we have to know what other molecules beyond oxygen are available, and the range of outcomes is wide indeed. For a given scenario, distinguishing between false positives and genuine biosignatures is the key, and the paper explores the various options.
Image: By varying the initial inventory of volatile elements in a model of the geochemical evolution of rocky planets, researchers obtained a wide range of outcomes, including several scenarios in which a lifeless rocky planet around a sun-like star could evolve to have oxygen in its atmosphere. Credit: J. Krissansen-Totton).
The photodissociation referred to above occurs as ultraviolet light from the star breaks water molecules into hydrogen and oxygen in the upper atmosphere, with the lighter hydrogen escaping into space and the oxygen remaining as a potentially deceptive biosignature. But the paper also examines how oxygen can be removed from an atmosphere, through outgassing of carbon dioxide and hydrogen, which will react with oxygen. The weathering of rock also affects oxygen levels, all factors that need to be included in this model of geochemical evolution.
The model is given weight when we see that it can reproduce the evolution of the atmosphere both on the Earth and on Venus. Using it, then, we can explore possibilities. We can imagine a planet with more water than Earth, one whose deep oceans preclude weathering of rock that would remove oxygen. Conversely, on still molten young worlds with only a small inventory of water, the magma surface can solidify quickly, with water remaining in the atmosphere. Oxygen remains behind as hydrogen in the upper atmosphere escapes. Says Krissansen-Totton:
“The typical sequence is that the magma surface solidifies simultaneously with water condensing out into oceans on the surface. On Earth, once water condensed on the surface, escape rates were low. But if you retain a steam atmosphere after the molten surface has solidified, there’s a window of about a million years when oxygen can build up because there are high water concentrations in the upper atmosphere and no molten surface to consume the oxygen produced by hydrogen escape.”
Another scenario involves high amounts of carbon dioxide in relation to water, resulting in a runaway greenhouse. Here again, as the paper notes, we’ve got an oxygen problem:
The lack of liquid surface water precludes CO2?drawdown via silicate weathering… Reactions between supercritical water and silicates will be severely kinetically limited by sluggish solid state diffusion, and are therefore assumed to be negligible (Zolotov et al., 1997). Consequently, a dense CO2 atmosphere and supercritical surface temperature persist indefinitely… despite the planet residing in the habitable zone. Moreover, there is sufficient steam in the atmosphere to ensure diffusion?limited hydrogen escape provides an appreciable source flux of oxygen…
Its focus on the geochemical and thermal evolution of a planet in the habitable zone, emphasizing interactions between crust and atmosphere, make this a noteworthy addition to the ongoing attempt to understand biosignatures. We may well get a biosignature detection involving oxygen relatively quickly once we have the tools in place to delve into rocky worlds in the habitable zone. The effort to sort out its meaning will take considerable time.
I’l stand by a previous prediction: Initial euphoria will quickly wear off as we consider how deeply ambiguous any biosignature detection is going to be. I think we’ll be seeing plenty of interesting hints, but it will be many years before we can say with certainty that we have found life around another star.
The paper is Krissansen-Totton et al., “Oxygen False Positives on Habitable Zone Planets Around Sun?Like Stars,” Vol. 2, Issue 2 (June 2021). Full text.
Breakthrough Discuss Concluding Today
Breakthrough Discuss 2021 wraps up today, with presentations on mission concepts to Alpha Centauri, lightsail technologies and fusion propulsion. Of particular interest to me, in light of the magnitude of the problem as it affects the Breakthrough Starshot idea, is a session on the current state of deep space optical communications. This has been a lively and robust meeting — James Cameron’s appearance was particularly engaging, as was the Yuri’s Night panel discussion — and the public is invited to watch again today at https://www.youtube.com/breakthroughprize. Gathering my notes is going to be time-consuming, but many of these presentations will make their way into upcoming discussions on Centauri Dreams.
In light of the wide-ranging discussion on the Centauri stars and the challenge they present, it seems appropriate to introduce a quote I just ran into from Alan Lightman’s new book Probable Impossibilities (Pantheon, 2021). This is from a section talking about quantum cosmology, but it resonates with the energies that drive all human discovery:
In the 1940s, the American psychologist Abraham Maslow developed the concept of a hierarchy of human needs, starting with the most primitive and urgent, and ending with the most lofty and advanced for those fortunates who had satisfied the baser needs. At the bottom of the pyramid are physical needs for survival, like food and water. Next up is safety. Higher up is love and belonging, then self-esteem, and finally self-actualization. This highest of Maslow’s proposed needs is the desire to get the most out of ourselves, to be the best we can be. I would suggest adding one more category at the very top of the pyramid, even above self-actualization: imagination and exploration. The need to imagine new possibilities, the need to reach out beyond ourselves and understand the world around us. Wasn’t that need part of what propelled Marco Polo and Vasco da Gama and Einstein? Not only to help ourselves with physical survival or personal relationships or self-discovery, but to know and comprehend this strange cosmos we find ourselves in. The need to explore the really big questions asked by the quantum cosmologists. How did it all begin? Far beyond our own lives, far beyond our community or our nation or planet Earth or even our solar system. How did the universe begin? It is a luxury to be able to ask such questions. It is also a human necessity.
Breakthrough Discuss Ongoing
There is a public YouTube channel for watching the Breakthrough Discuss meetings, which began today and extend through tomorrow. Click here to go to sessions on “The Alpha Centauri System: A Beckoning Neighbor.” I’ll have thoughts on some of these presentations in coming weeks.
Dustfall: Earth’s Encounter with Micrometeorites
Interesting news out of CNRS (the French National Center for Scientific Research) renews our attention to the mechanisms for supplying the early Earth with water and carbonaceous molecules. We’ve looked at comets as possible water sources for a world forming well inside the snow line, and asteroids as well. What the CNRS work reminds us is that micrometeorites also play a role. In fact, according to the paper just out in Earth and Planetary Science Letters, 5,200 tons of extraterrestrial materials — dust particles from space — reach the ground yearly.
Image: From the paper’s Figure 1, although not the complete figure. The relevant part of the caption: Fig. 1. Left: Location of the CONCORDIA station (Dome C, Antarctica). Centre: View of a trench at Dome C. Credit: Rojas et al.
This conclusion comes from a study spanning almost twenty years, conducted by scientists in an international collaboration involving laboratories in France, the United States and the United Kingdom. CNRS researcher Jean Duprat has led six expeditions during this time span to the Franco-Italian Concordia station (Dome C), 1100 kilometers from the coast in the Antarctic territory Terre Adélie, which is claimed by France. France’s permanent outpost in Terre Adélie is Dumont d’Urville Station. The Dome C area is considered ideal for the study of micrometeorites because of the near absence of terrestrial dust and low rates of accumulation of snow.
Image: Electron micrograph of a Concordia micrometeorite extracted from Antarctic snow at Dome. Credit: © Cécile Engrand/Jean Duprat.
The dust particles collected by these expeditions have ranged from 30 to 200 micrometers (µm) in size, and enough have been gathered to calculate the mass accreted to Earth per square meter per year, the aforementioned 5,200 tons. This would make micrometeorites the primary source of extraterrestrial matter on our planet — the flux from meteorites is measured as less than 10 tons per year.
The paper explains what happens as micrometeorites encounter the Earth’s atmosphere:
The degree of heating experienced by the particles during their atmospheric entry depends on various factors including the initial mass of the particles, their entry angle and velocity. The ablated metallic vapours oxidize and the resulting metal oxides, hydroxides and carbonates condense into nm-sized particles termed meteoric smoke (Plane et al., 2015). These particles are transported by the general atmospheric circulation until eventually deposited at the surface, where their flux can be evaluated by elemental or isotopic measurements (Gabrielli et al. (2004).
The size distribution of extraterrestrial particles before entry into the atmosphere was studied by measuring the craters that high velocity sub-millimetre grain collisions produced on the Long Duration Exposure Facility satellite (LDEF) panel. This was an interesting mission. The LDEF spent over five years orbiting the Earth to measure the effects of micrometeoroids, space debris, radiation particles, atomic oxygen, and solar radiation on spacecraft materials, components, and systems before being retrieved in 1990 by the Space Shuttle STS-32 mission.
Image: Collecting micrometeorites in the central Antarctic regions, at Dome C in 2002. Snow sampling. Credit: © Jean Duprat/ Cécile Engrand/ CNRS Photothèque.
The paper is Rojas et al., “The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth,” Earth and Planetary Science Letters Volume 560 (15 April 2021) 116794 (full text).
How Planetesimals Are Born
What governs the size of a newly forming star as it emerges from the molecular cloud around it? The answer depends upon the ability of gravity to overcome internal pressure within the cloud, and that in turn depends upon exceeding what is known as the Jeans Mass, whose value will vary with the density of the gas and its temperature. Exceed the Jeans mass and runaway contraction begins, forming a star whose own processes of fusion will arrest the contraction.
At the Max Planck Institute for Astronomy (Heidelberg), Hubert Klahr and colleagues have been working on a different kind of contraction, the processes within the protoplanetary disk around such young stars. Along with postdoc Andreas Schreiber, Klahr has come up with a type of Jeans Mass that can be applied to the formation of planetesimals. While stars in formation must overcome the pressure of their gas cloud, planetesimals work against turbulence within the gas and dust of a disk — a critical mass is needed for the clump to overcome turbulent diffusion. The new paper on this work extends the findings reported in a 2020 paper by the same authors.
The problem this work is trying to solve has to do with the size distribution of asteroids; specifically, why do primordial asteroids — planetesimals that have survived intact since the formation of the Solar System, without the collisions that have fragmented so many such objects — tend to be found with a diameter in the range of 100 kilometers? To answer the question, the researchers look at the behavior of pebbles in the disk, small clumps in the range of a few millimeters to a few centimeters in size. Here the plot thickens.
Pebbles tend to drift inward toward the star, meaning that if these pebbles accrete slowly, they would fall into the star before reaching the needed size. Klahr and Schreiber argue that this drift can be overcome thanks to turbulence within the gas of the protoplanetary disk, creating traps where the pebbles can accumulate enough mass for them to become bound together by gravity. Turbulence is a local effect, to be sure, but it can also vary as the disk changes with increased distance from the star. A fast enough drop in gas pressure with distance can create a streaming instability within the disk, so that pebbles move chaotically and churn the gas around them.
Image: The solar primordial nebula of gas and dust surrounds the sun in the form of a flat disk. In it are vortices in which the small pebbles accumulate. These pebbles can grow up to asteroid size by gravitational collapse, if this is not hindered by turbulent diffusion. Hubert Klahr, Andreas Schreiber, MPIA / MPIA Graphics Department, Judith Neidel.
So on the one hand, turbulence helps create the pebble clusters that can aggregate into larger objects, but to grow larger still, such clouds need to reach a certain mass or they will be dispersed by these instabilities. Klahr and Schreiber’s calculations of the necessary mass show that objects would need to reach 100 kilometers in size to overcome the turbulence pressure for most regions within our own Solar System, matching observation. Thus we have a limiting mass for planetesimal formation.
All of this grows out of simulations and the comparison of their results with telescopic observations of protoplanetary disks around young stars. In the numerical models the authors used, planetesimals grow quickly from the projected pebble traps. Low mass pebble clouds below this turbulence-based Jeans Mass are less likely to grow through accretion, while larger clouds would collapse as they exceeded the critical mass.
Image: In this schematic the sequence of planetesimal formation starts with particles growing to pebble size. These pebbles then accumulate at the midplane and drift toward the star to get temporarily trapped in a zonal flow or vortex. Here the density of pebbles in the midplane is eventually sufficient to trigger the streaming instability. This instability concentrates and diffuses pebbles likewise, leading to clumps that reach the Hill density at which tidal forces from the star can no longer shear the clumps away. If now also the turbulent diffusion is weak enough to let the pebble cloud collapse, then planetesimal formation will occur. Hubert Klahr, Andreas Schreiber, MPIA / MPIA Graphics Department, Judith Neidel.
Planetesimal size does vary at larger distances, such that objects forming in the outer regions of the disk cluster around 10 kilometers at 100 AU. The authors suggest that the characteristic size of Kuiper Belt Objects will be found to decrease with increasing distance from the Sun, something that future missions into that region can explore. The 100 kilometer diameter seems to hold out to about 60 AU, beyond which smaller objects form from a depleted disk.
New Horizons comes to mind as the only spacecraft to study such an object, the intriguing 486958 Arrokoth, which at 45 AU is the most distant primordial object visited by a space probe. Future missions will give us more information about the size distribution of Kuiper Belt Objects. We can also think about the Lucy mission, whose solar panels we looked at yesterday. The target asteroids orbit at the Jupiter Lagrange points 4 and 5, one group ahead of the planet in its orbit, one group behind. Lucy is to visit six Trojans, objects which evidently migrated from different regions in the Solar System and may include a population of primordial planetesimals.
Klahr and Schreiber have come up with a model that can make predictions about how planetesimals form within the protoplanetary disk and their likely sizes within the disk. Whether these predictions are born out by subsequent observation will help us choose between this new Jeans Mass model for planetesimals and other formation processes, including the possibility that ice is the mechanism allowing pebbles to stick together, or whether aggregates of silicate flakes are the answer. Klahr is not convinced that either of these models can withstand scrutiny:
“Even if collisions were to lead to growth up to 100 km without eventually switching to a gravitational collapse, this method would predict too large a number of asteroids smaller than 100 km. It would also fail to describe the high frequency of binary objects in the Edgeworth-Kuiper belt. Both properties of our Solar System are easily reconcilable with the gravitational pebble cloud collapse.”
The effects of turbulence in other stellar systems will likewise rely upon the concentration of mass within the disk, with the ability for larger planetesimals to form decreasing as the gas within the disk is depleted, either by planet formation or by absorption by the host star. If Klahr and Schreiber’s model of planetesimal formation as a function of gas pressure is applicable to what we are seeing in protoplanetary disks around other stars, then it should prove useful for scientists building population synthesis models to incorporate planetesimals into the mix.
The previously published paper is Klahr & Schreiber, “Turbulence Sets the Length Scale for Planetesimal Formation: Local 2D Simulations of Streaming Instability and Planetesimal Formation,” Astrophysical Journal Volume 901, Issue 1, id.54, (September, 2020). Abstract. The upcoming paper is “Testing the Jeans, Toomre and Bonnor-Ebert concepts for planetesimal formation: 3D streaming instability simulations of diffusion regulated formation of planetesimals,” in process at the Astrophysical Journal (preprint).
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