Examining the possibility of undetected small terrestrial planets in the Outer Solar System.

Free floating planets (which is to say, planet-sized objects floating free in space, unbound to any star) were first observed more than two decades ago, and this population is now known to contain terrestrial mass objects as well as large Jupiter-type planets.

In a paper published in The Astrophysical Journal Letters on 18 December 2023, Amir Siraj of the Department of Astrophysical Sciences at Princeton University, discusses the possibility that one or more terrestrial sized planets might have been captured by the Sun's gravity early in the history of the Solar System, and be as yet undiscovered components of the Outer Solar System.

Siraj notes that this is a different topic to the search for Planet Nine, a hypothetical body with a mass six times that of Earth and a semi major axis (average distance from the Sun) of about 400 AU (i.e. 400 times as far from the Sun as the Earth), which has been proposed due to observed clustering extreme trans-Neptunian objects in the Outer Solar System.

Siraj instead debates the possibility of sub-Earth-mass planets in the Outer Solar System, motivated by the fact that such bodies have been observed free floating in space, and could potentially be captured by the Sun's gravity.

An artist's impression of a free floating planet. NASA/JPL/CalTech/Wikimedia Commons.

In theory, any stellar system is most likely to capture drifting planets when it is still very young, and within its birth cluster, that is to say a cluster of stars forming within a single molecular cloud, which acts as a stellar nursery. This is the stage at which young stellar systems are most likely to eject planets, and the time when they are close to the largest number of other systems, making it most likely that such planets will be captured.

Stellar nurseries are variable in nature, with planets more likely to be captured in clusters where the molecular cloud is expanding rapidly. To give a conservative estimate of the probability of planet-capture, Siraj assumed a gently collapsing cluster, which is thought to be the environment in which planetary capture is least likely.

Surprisingly, despite applying the most conservative conditions, Suraj's simulation predicts that there wit be approximately 1.2 captured planets with a mass at least equivalent to that of Mars in the Outer Solar System, and 2.4 planets with a mass equivalent to Mercury or larger. If less conservative assumptions are made, this increases to roughly 2.7 planets with a mass equal to that of Mars or larger, and about 5.2 Mercury sized or larger planets. The average distance from the Sun of these planets would be 1400 AU, with half of all such bodies orbiting at between 600 AU and 3500 AU.

Detecting such planets would be another problem, as they would be very faint objects, and we do not actually know where to look for them. The Legacy Survey of Space and Time project at the Vera C. Rubin Observatory is due to Survey the entire Southern Hemisphere sky every three nights in six optical bands ranging from 320 to 1050 nm for a ten year period. 

The largest high-performance optical lens ever fabricated (1.55 m feet in diameter) in a clean room at the SLAC National Accelerator Laboratory, where the lab assembles the 3,200-megapixel digital camera of the Legacy Survey of Space and Time instrument. Farrin Abbott/SLAC.

This survey should be capable of detecting such planets in the Outer Solar System, if they are present and visible from the Southern Hemisphere (bodies in the Outer Solar System will orbit extremely slowly, taking hundreds or even thousands of years to complete a single orbit of the Sun, and are also likely to have more eccentric orbits than the planets of the Inner Solar System, making it possible that bodies could spend an entire ten year period in the northern sky). Suraj estimates that this survey could detect between about 1.0 and 1.4 Mercury sized planets, and 0.7-0.9 Mars sized planets, although only planets in the innermost part of the Outer Solar System, between about 400 AU and about 700 AU from the Sun. 

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Small Bodies Tell the Story of the Solar System: A Scientific Rationale for a Multi-Target Small Body Sample Return Program.

Small bodies are time-capsules of different eras of Solar System history from the most primitive materials within the solar system to evolved pieces of larger bodies. While small bodies include asteroids, comets, dwarf planets, Trojans, Centaurs, trans-Neptunian objects, small satellites, and interplanetary dust, a diverse selection of small bodies either reside near Earth or have been scattered by the gravity of the planets onto near-Earth orbits, ultimately making all parts of the solar system accessible to exploration. Furthermore, the surfaces of many small bodies can be affordably accessed with spacecraft due to their low gravity and non-existent or sparse atmospheres. While our meteorite collections are an important source of knowledge about solar system history, they are a biased and incomplete representation of small body astromaterials.

In a paper published on the arXiv database at Cornell University on 16 July 2020, Seth Jacobson of Michigan State University, Maitrayee Bose of Arizona State University, Dennis Bodewits of Auburn University, Marc Fries of NASA's Johnson Space Center, Devanshu Jha of MVJ College of Engineering, Prajkta Mane of the Lunar and Planetary Institute, Larry Nittler of the Carnegie Institution of Washington, Scott Sandford of NASA's Ames Research Center, and Michelle Thompson of Purdue University, present a scientific rationale for a multi-target small body sample return program including the Earth-based laboratory analysis of returned samples.

Small bodies record the radial compositional gradients of material that were present in the protosolar disk, and they represent all stages of the formation and early evolution of the Solar System. Primitive small bodies are the debris left over from planet formation and contain examples of the primordial ingredients from which the planets and life arose. Small bodies record internal processing such as aqueous alteration, thermal metamorphism, melting and differentiation. These evolved small bodies record processes that occurred during the formation and evolution of planets. Thus, small bodies trace growth from primordial condensates and presolar/interstellar grains to pebbles, planetesimals, and planetary embryos to planets. Since catastrophic collisions can completely shatter protoplanets, small bodies can be samples of core, mantle, or crustal material of once much larger bodies. Indeed, this is the only known reservoir of accessible core and, potentially, deep mantle material. Small bodies also record the history of the Solar System such as the dynamical evolution of the Solar System, the evolution of surface materials through time and as they approach the Sun, and the primordial cosmochemical gradients established within the solar nebula. Samples from small bodies in the solar system represent a diverse set of materials, and study of these materials in terrestrial laboratories can provide information about the history and evolution of the Solar System in ways that cannot be determined by remote and in situ observations.

Small bodies from throughout the Solar System have been scattered onto orbits that are accessible by spacecraft from Earth on reasonable mission timescales and budgets. Near-Earth asteroids are sourced from throughout the Main Belt, although parts of the Main Belt itself can be directly accessible for sample return using launch vehicles in development. The moons and Trojans of Mars are also directly within reach. Jupiter-family and long-period comets can have perihelions within 1 Astronomical Unit and are representative examples of the Kuiper Belt and Oort Cloud, respectively. NASA, the European Space Agancy, and the Japan Aerospace Exploration Agency have a long history of successful operations around small bodies. Sample return missions to small bodies have historically been more limited, but past and current missions include NASA’s Stardust and OSIRIS-REx missions and the Japan Aerospace Exploration Agency’s Hayabusa and Hayabusa2 missions.

 

Stardust sample return capsule in a temporary cleanroom at the Michael Army Air Field in Utah. NASA.

Because of their low gravity and lack of atmospheres, many small bodies are accessible using spacecraft missions in the Discovery and New Frontiers classes, as demonstrated by completed missions such as Near-Earth Asteroid Rendezvous, Stardust, Deep Impact, and Dawn, current missions such as New Horizons and OSIRIS-REx, as well as future missions such as Psyche and Lucy. These missions have shown that exploration of small bodies can provide an extremely effective means of learning the story of the origin and evolution of the solar system and demonstrate the potential value provided by future missions. In particular, the science returned by these missions have opened new questions, some of which can only be approached with the in-depth investigation of returned samples.

 

Artist's rendering showing the OSIRIS-REx spacecraft descending towards Asteroid Bennu to collect a sample of the asteroid’s surface. NASA.

The science goals of small body exploration are to establish the early conditions of the solar nebula, understand how materials evolved during the stages of planetary growth, and determine the source of the building blocks of Earth including water and other ingredients necessary for life. These goals are responses to the fundamental questions: ‘What is out there in the universe?’, ‘What is the history of Earth and the solar system?’, and ‘How did life arise on Earth and may it have arisen elsewhere?’ that not only drive the public imagination but have historically driven significant investments in science. While in-situ spacecraft exploration provides detailed information that cannot be obtained from remote telescopic observations, small body sample return missions are the only mechanism to fully connect astrophysical observations, in-situ spacecraft exploration, and meteorite analysis.

 

Artist's impression of the Hayabusa Space Probe arriving at Asteroid Itokawa. Japan Space Agency.

By ‘sample return mission’ Jacobson et al. refer to the acquisition of material (dust particles, surface material, drill cores, captured gases, ices, etc.) by a collecting device on a spacecraft, their return to Earth, and their analysis in a laboratory on Earth. Sample return missions and supporting infrastructure (cameras onboard the spacecraft, ground-based telescopes, etc.) must also characterize the local, regional, and global context of the sampled location.

 

Mosaic of images of Asteroid Eros taken by the Near-Earth Asteroid Rendezvous probe from an orbital altitude of 200 kilometers, providing  an overview of the eastern part of the asteroid's southern hemisphere. In this view, south is to the top and the terminator (the imaginary line dividing day from night) lies near the equator. The conspicuous depression just above the centre of the frame is the saddle-shaped feature Himeros. NASA.

Of all investigative methods, sample return missions provide the deepest insight into a planetary body, because returned samples can be analyzed by the most advanced laboratory techniques in terrestrial laboratories with the highest precision and detection limits and which are not achievable in remotely operated planetary missions, while still in the context of landed and remote sensing observations. This effectively allows the mission’s payload to consist of all the world’s analytical instrumentation. First, many of these analyses cannot be performed in space due to technological or cost limitations. Second, Earth-based laboratories can achieve measurement accuracies and precisions superior to in situ measurements, often with very small sample quantities. Third, returned samples can be and have historically been partially preserved for study in the future by methods that have yet to be developed. Earth-based laboratory analysis of returned samples from small bodies provides detailed information regarding the sample’s geologic characterisation, petrology, and mineralogy. Moreover, laboratories can perform high precision geochemical measurements including molecular, elemental, and isotopic compositions, which can be used for radiometric dating. Using these rich datasets, the history of both the individual parent body as well as the population they represent, can be determined.

 

An artist's impression of the Deep Impact spacecraft delivering a special impactor into the path of Comet Tempel 1. NASA.

Characterisation of returned samples from small bodies can also contextualize laboratory analyses of meteorites and interplanetary dust particles collected on Earth. Laboratory analyses of extraterrestrial materials retrieved on Earth have, and continue to, provide answers to fundamental questions about the early solar system, but because in most cases the sources of the materials cannot be directly determined, their properties cannot be fully put into their parent body context. Returned samples overcome this issue and provide sample-asteroid connections and parent body context without ambiguity. For instance, Stardust connected interplanetary dust particles to cometary dust and Hayabusa connected ordinary chondrite meteorites to S-type asteroids. Thus, by collecting samples with known provenance, sample return missions not only produce unique discoveries of their own, but also enrich the value of laboratory meteorite and dust particle studies by revealing connections between these unique sample sets.

 

View of Dwarf Planet Ceres combining images taken during Dawn's first science orbit in 2015 using the framing camera's red, green and blue spectral filters. NASA/JPL/Caltech/UCLA/MPS/DLR/IDA.

Ground-based telescopic observations of small bodies and astrophysical disks (protoplanetary and debris), spacecraft-based remote and in-situ exploration, and theoretical modeling are enriched by sample return investigations. For instance, competing theoretical models of planet formation can now satisfactorily explain modern-day astrophysical properties such as masses and orbits of planets and small bodies, but have significantly different hypotheses regarding the histories of growth and evolution of those bodies as recorded in their chemical and isotopic compositions. Returned samples provide definitive evidence to test these hypotheses and break existing model degeneracies. Similarly, degeneracies between interpretations of remote observations either from astrophysical telescopes or in-situ spacecraft can only be broken by laboratory investigations that, due to sample preparation and analysis requirements, must occur on Earth. Astrophysical observations, in-situ spacecraft investigations, and Earth-based laboratory analyses are distinct nodes in a continuum of understanding small bodies that become tightly linked by a small body sample return program, enhancing the scientific return of all.

 

Image of Kuiper Belt Object Arrokoth made by the New Horizons Space Probe. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko.

Despite the lack of existence of a defined small body sample return program, two successful missions have returned samples from small bodies: NASA’s Stardust Discovery mission, which returned both dust particles from comet 81P/Wild 2 and interstellar dust, and the Japan Aerospace Exploration Agency’s Hayabusa mission, which returned surface samples from asteroid 25143 Itokawa. These two missions clearly illustrate the power of sample return as both have provided unprecedented constraints on specific small bodies and on their relation to extant meteoritic materials. With respect to Stardust , the collection, return, and terrestrial analysis of a few thousand particles from the coma of Wild 2 demonstrated that comets - generally thought to have formed in the outer protosolar nebula - nonetheless contain materials that must have been formed and altered in both the inner and outer parts of the protosolar nebula. This provides clear proof that comets are not, as once thought, pristine collections of interstellar materials and that dust was mixed throughout the entire extent of the disk. Furthermore, laboratory analyses confirmed the long suspected connection of many interplanetary dust particles to cometary sources. Similarly, with respect to the Japan Aerospace Exploration Agency’s Hayabusa mission, laboratory analysis of a few nanograms of Itokawa particles, tens to hundreds of microns in diameter, confirmed a long-suspected connection between S-type asteroids and ordinary chondrite meteorites. This analysis also revealed a rich and complex history of the asteroid’s parent body formation and evolution over billions of years and provided unprecedented information about space weathering processes on asteroids. The measurements that led to all of these discoveries could never have been made by remote or in situ instruments, but instead required the state-of-the-art instrumentation available in terrestrial laboratories, much of which wasn’t even developed at the time the missions were designed and launched.

 

Artist's impression of the planned Psyche Space Probe above Asteroid Psyche. NASA.

Two ongoing sample return missions include the Japan Aerospace Exploration Agency’s Hayabusa2 mission, which is returning with surface samples from 162173 Ryugu, and NASA’s OSIRIS-REx mission, which is in the process of collecting surface samples at 101955 Bennu. These missions targeted C-type asteroids to test the hypothesised connection to carbonaceous chondrite meteorites. The returned samples will have known provenance, rock surfaces unaffected by passage through Earth’s atmosphere, and are predicted to contain examples of the primitive material that delivered water and carbon to Earth. While there are distinct differences in hydration as observed by remote spectroscopy, the overall similarity between the two targets creates an opportunity to compare and contrast the returned samples in the context of individual parent body evolution. Overall, these missions demonstrate the desire for returned samples, and prove, beyond doubt, the technical feasibility of sample return missions from small bodies.

 

Diagram illustrating the  orbital path of the Lucy Probe. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white) and its satellite, (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years. NASA.

Beyond the selection of missions for flight, NASA has also recognized the importance of supporting sample-return science with the development and advancement of laboratory methods for eventual analysis of returned samples. NASA initiated the Sample Return and Laboratory Instrumentation and Data Analysis Program, now the Laboratory Analysis of Returned Samples program, that funded both PI-built advanced instruments such as MegaSIMS, SARISA, and CHILI, as well as the acquisition of commercial instruments. NASA’s Planetary Science Division has further supported acquisition of commercial instrumentation (e.g., electron microscopes, mass spectrometers, FTIR, Raman, etc) for individual PIs and facilities through the Planetary Major Equipment program, in many cases expressly to support analysis of returned samples (as well as meteoritic and/or terrestrial samples). These instruments are available for individual PI use at a variety of institutions, including universities, research institutes, and National Labs or as part of user facilities available to the community at large. Large scale user facilities such as synchrotron X-ray and neutron beam sources are also available and have been extensively used for returned sample analysis. All this unique instrumentation is currently being used but needs continued funding for development work and updating existing instrumentation to satisfy the needs for future returned sample analyses (for an extensive review see the National Academies Report, 'Strategic Investments in Instrumentation and Facilities for Extraterrestrial Sample Curation and Analysis').

Image of comet 81P/Wild 2 made by the Stardust probe on 2 January 2004. Wikimedia Commons/NASA.

While the history of small body sample return and analysis demonstrates high scientific returns, the scope of existing returned samples only begins to fill the domain of possible science. As described above, small body exploration science goals encompass the entire history of the solar system, and samples returned from small bodies, likewise, cover this incredible territory. To understand the early conditions of the solar nebula, sample return mission targets may include comets and undifferentiated asteroids. To understand planetesimal growth and internal processing, targets may include larger asteroids such as Ceres or their fragments that show evidence of aqueous alteration, thermal metamorphism, and other processes at their surfaces. To understand planet formation and evolution, targets may include asteroids that appear similar to expected planetary components such as crustal, mantle, or core material. To understand the delivery to Earth of compounds essential for life such as water and organics, targets may include icy and volatile-rich comets as well as active and/or hydrated asteroids. These are only a few examples of the rich diversity of science objectives and associated potential targets that could be transformed into competitive sample return missions.

  

Asteroid Itokowa, visited by the Japan Space Agency's Hayabusa mission in November 2005. New Technology Telescope/European Sothern Observatory.

Collecting such a rich set of returned samples to achieve these science goals is not without challenges that must be addressed. While some samples may be 'scooped' from a gravelly surface, others will require a more precise and careful collection. For instance, samples from surfaces rich in salts as observed on Ceres would provide key constraints on internal body processes as well as the initial accretion location of the body. However, experience with salt clasts in ordinary chondrite meteorites shows that the handling and analyses of such friable material is extremely challenging. As another example, samples rich in particular volatiles are missing from our meteorite collections because reentry through the Earth’s atmosphere destroys such material. Volatile compounds include the water ice and organics that are essential for life on Earth, and so tracing the origin and evolution of volatile-rich materials is crucial to understanding why life arose here and the potential for life elsewhere. Successfully sampling and transporting such volatile compounds either cryogenically or non-cryogenically, in an intentionally altered state, are major spacecraft instrument design goals. Ambitious sample return missions will deliver exciting results unobtainable through other means, but will require advance investment in instrumentation and mission design.

Hubble Space Telescope image of dust particles being shed by Comet Schwassmann-Wachmann-3. NASA.

The high science value of returned samples is enabled by the extensive and precise capabilities of Earth-based laboratories. For instance, firmly linking asteroids and meteorites requires detailed lab comparisons such as those used by Hayabusa to connect ordinary chondrites and S-type asteroids as well as by Stardust to connect comets and interplanetary dust particles. There are still many well-studied meteorites whose origins are only hypothesized. For instance, identifying the parent body and origin story of iron and stony-iron achondrites is a major science objective that will require laboratory comparison of returned samples. Such knowledge would mark major progress towards the goal of understanding how growing protoplanets differentiate. Relatedly, there are a number of asteroids with rarer spectral appearances, of which it is unclear if they are represented in our meteorite collections at all, such as the A-, O-, R-, E-, M-, and non-Vestoid V-type asteroids. Hypotheses for the origin of these objects include silicate pieces of differentiated bodies, and they may be the key for understanding how the geochemistry of large protoplanets change with growth. Petrologic experiments and meteoritic comparison is only possible with returned samples and laboratory analysis.

 Artist's impression of the New Horizons Space Probe. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Small body investigations began as telescopic observations and meteoritic analyses and, in the past decades, have since included spacecraft-based remote and in-situ analyses. While all of these investigations should continue to be supported by funding agencies including remote and in-situ spacecraft-based investigations, the time is now to create a New Frontiers-like program to support a set of spacecraft missions to bring back samples from a diverse set of small bodies and to increase support for Earth-based laboratories to properly handle and study returned samples. The most effective small body sample return program would have these components: (1) opportunities for small body sample return spacecraft missions, (2) opportunities to develop spacecraft sampling and sample preservation technology, (3) opportunities to create or enhance existing Earth-based laboratory facilities, (4) opportunities to examine existing returned samples and/or their data products.

 Image of haze layers above Pluto taken by the Ralph/Multispectral Visible Imaging Camera on the New Horizons spacecraft. NASA.

The number and diversity of small body targets means that no single mission could accomplish the full set of achievable scientific objectives made possible by small body sample return missions. Ideally, sample return missions should be supported with their own mission program with its own funding line within the planetary sciences division at NASA. Alternatively, the regular calls from the Discovery and New Frontiers programs should always be broad enough to include small body sample return as a potential mission profile and/or commitments should be made for selection of sample return missions on a regular basis. The very strong science cases for small body sample return missions will make them highly competitive against other possible mission profiles. Given the large diversity of possible sample return science goals requiring New Frontiers-level support, it’s important for that program to consider a wide variety of targets. In short, given the diversity of possible small body sample return missions, it is essential that NASA provide regular opportunities for such missions to be flown.

Image of Asteroid Vesta made on 24 July 2011 by the Dawn spacecraft. Wikimedia Commons/NASA/JPL/MPS/DLR/IDA/Björn Jónsson.

Space agencies, such as the Japan Aerospace Exploration Agency and European Space Agency, will continue or start to conduct their own sample return programs; Jacobson et al. believe it is critical that NASA support participating scientist programs for these non-NASA led missions. Relatedly, NASA should encourage the participation of international colleagues in sample return missions as well as the Earth-based study of returned samples. Likewise, NASA should work to maintain access for US-based scientists to returned samples obtained by foreign space agencies.

 

Image of Asteroid Ryugu made by the Optical Navigation Camera on the Hayabusa2 spacecraft on 20 June 2018. Japan Aerospace Exploration Agency/University of Tokyo/Kochi University/Rikkyo University/Nagoya University/Chiba Institute of Technology/Meiji University/University of Aizu/National Institute of Advanced Industrial Science and Technology.

NASA should continue to maintain stable, well-funded research and analysis programs as the backbone for scientific advancement and as an enabler for future missions to small bodies. It is essential that either a new dedicated program be introduced or the PICASSO and MatISSE programs be enhanced to support the development of spacecraft sampling and onboard sample preservation technologies. As the volume of returned samples grows, NASA must increase the funding to LARS as well as preserve/increase funding to related research and analysis programs such as Emerging Worlds and Solar System Workings including support for Planetary Major Equipment requests. The connective nature of returned samples is reflected in the wide number of NASA programs necessary to support their study and return their full science value. Lastly, NASA should maintain and update current facilities at Johnson Space Center and, potentially, elsewhere to store returned samples. As the variety of returned samples increases, NASA will need to develop new storage facilities, new procedures for sample handling and access. NASA should require that sample documentation, measurements, and other data products be archived appropriately, ideally at a Planetary Data System node.

 

A part of the Allende Meteorite, a carbonaceous chondrite. James St. John/Ohio State Univerisity/Flikr/Wikimedia Commons.

Small body sample return investigations are a human endeavor that should be accessible to anyone interested. From large spacecraft mission teams to the smaller research teams supported by research and analysis programs, those selected to conduct the science should represent the diversity of humanity. This can only be achieved if NASA prioritizes accessibility and inclusivity when designing competitive research programs, and if international participation is encouraged. Given historic injustices and existing inequality, NASA will need to consciously build accessibility and inclusivity into a small body sample return program. Proactive measures may include continued support of participating scientist programs to join spacecraft mission teams, maintaining transparent and fair systems to access returned samples with appropriate oversight, and preference to funding facilities that provide for guest access including training and support.

Weekeroo Station Iron Meteorite, etched slice. Exhibit at the Center for Meteorite Studies, Arizona State University, Tempe, Arizona. Wikimedia Commons.

In order to tell the story of the solar system, a multi-target small body sample return program should include: (1) Opportunities for small body sample return spacecraft missions, either as a standalone program or to always include small body sample return as an admissible mission profile in the Discovery and New Frontiers programs. (2) Introduction of a competitive program for developing spacecraft sampling and sample preservation technologies or opportunities to propose these technologies under the PICASSO and MatISSE programs. (3) Opportunities to create or enhance existing Earth-based laboratory facilities via increased support of Laboratory Analysis of Returned Samples and the Planetary Major Equipment program. (4) Regular opportunities to examine existing returned samples and/or their data products via increased support of Laboratory Analysis of Returned Samples and related research and analysis programs such as Solar System Workings and Emerging Worlds.Proactive measures to ensure accessibility, equitableness, and inclusivity, such as participating scientist programs as well as equitable access to returned samples and user facilities.

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Determining the ages of presolar silicon carbide grains from the Murchison Meteorite.

Interstellar dust is an important component of our galaxy. It influences star formation as well as the thermal and chemical evolution of the galaxy. Although dust only presents about 1% of the mass in the interstellar medium, it carries a large fraction of the elements heavier than helium, including the elements that form terrestrial planets and are essential for life. Thus, interstellar dust is a key ingredient of stars and habitable planetary systems, making increased knowledge about its composition and lifecycle desirable. Compositional, structural, and size information of interstellar dust can be obtained through astronomical spectroscopic observations, but dust lifetime estimates mainly rely on sophisticated theoretical models. These models, however, focus on the more common small dust grains and are based on assumptions with large uncertainties. These uncertainties mainly pertain to the residence time of the dust in various regions of the interstellar medium, which exhibit different rates of dust destruction through sputtering and collisions in supernova shock waves. Most of these models currently predict an average lifetime of interstellar grains on the order of 100 million years. However, more recent models and a few models for larger grains predict much longer survival times in the interstellar medium of up to billions of years.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 13 January 2020, Philipp Heck, Jennika Greer, and Levke Kööp of the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History, and the Chicago Center for Cosmochemistry and Department of the Geophysical Sciences at the University of Chicago, Reto Trappitsch of the Nuclear and Chemical Sciences Division at Lawrence Livermore National Laboratory, Frank Gyngard of the Physics Department at Washington University, and the Center for NanoImaging at Harvard Medical School, Henner Busemann and Colin Maden of the Institute of Geochemistry and Petrology at ETH Zürich, Janaína Ávila of the Research School of Earth Sciences at the Australian National University, Andrew Davis, also of the obert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History, the Chicago Center for Cosmochemistry and Department of the Geophysical Sciences, and of the Enrico Fermi Institute at the University of Chicago, and Rainer Wieler, also of the Institute of Geochemistry and Petrology at ETH Zürich, present a laboratory-based approach of determining the interstellar lifetimes of individual large presolar silicon carbide stardust grains.

Presolar silicon carbide grain morphology. Scanning electron microscope images (secondary electrons) of representative samples of the two morphological types of presolar silicon carbide grains studied here. Grain L3_01 has a euhedral shape indicating it evaded shattering; (A) before and (B) after pressing into gold and after nanoscale secondary ion mass spectrometry and Sensitive High Resolution Ion Micro Probe analysis but before laser extraction of noble gases. Grain L3_20 has a shard-like appearance with fractures (C) before pressing and (D) got fractured further upon pressing into the gold substrate. Heck et al. (2020).

The presolar grains analyzed in Henk et al.'s study were isolated by chemical methods from the Murchison CM2 Meteorite, where they had remained unaltered since their incorporation into the meteorite parent body in the early Solar System 4.6 billion years ago. These grains are identified as presolar by their large isotopic anomalies that exclude an origin in the Solar System. Presolar stardust grains are the oldest known solid samples available for study in the laboratory, represent the small fraction of material that formed in circumstellar environments, and survived processing in the interstellar medium and Solar System. The presolar stardust grain abundance in our parent interstellar cloud was a few percent of all interstellar dust present in this cloud, with the other dust having condensed in the interstellar medium. In the solar nebula, more dust condensed from the cooling gas and presolar stardust became an even more minor component. Most presolar grains were subsequently destroyed after accretion in their parent bodies during thermal metamorphism and aqueous alteration. Thus, their abundance in the most primitive Solar System materials that evaded destructive parent body processing is a few parts per million to about 200 parts per million, except for interplanetary dust presumably from comet Grigg-Skjellerup dust, which contains up to 1% presolar materials. Henk et al. used mass spectrometry to analyse the abundance of nuclides produced in the grains by spallation reactions with galactic cosmic rays, which comprise mostly high-energy protons and α-particles, during their residence in the interstellar medium. When these high-energy particles hit a grain, small fractions of the target nuclides break up. The resulting atomic fragments accumulate in the grain, and their concentrations are proportional to the timespan the grains were irradiated. Suitable daughter elements to study are those with a very low initial abundance in the grains such that the cosmic rayproduced ('cosmogenic') fraction becomes detectable. This is the case for helium, neon, and lithium. Silicon carbide is the best-suited interstellar phase for cosmogenic nuclide dating, due to its relatively large grain size, high retentivity of cosmogenic nuclides, and durability. Even though silicon carbide is only a small fraction of the total amount of interstellar dust, due to its durability, we consider it a useful tracer. In the most common silicon carbide grains, the ones that originate from low-to-intermediate-mass asymptotic giant branch stars, the initial helium and neon isotopic compositions, incorporated from their parent stars, are well known, so the cosmogenic fraction can be readily identified. Improved knowledge of production and retention of such cosmogenic nuclides enabled Henk et al. to obtain ages with improved reliability. While radiometric dating based on the uranium-lead decay system can provide ages with high accuracy and is often the method of choice for samples of Solar System materials, it has not yet been successfully applied to presolar grains. These grains have masses that are orders of magnitudes smaller than samples dated so far. Furthermore, presolar grains have large isotopic anomalies in essentially every element, so each grain may have a distinct initial lead isotopic composition, uranium isotope ratio, and age, robbing the uranium-lead system of some of its most desirable characteristics for geochronology. Until these obstacles are overcome, exposure age dating is the preferred method for determining presolar ages of individual stardust grains.

The first such studies were made on assemblages of thousands of silicon carbide grains from chemical separates. Ages of about 10 to 100 million years were derived from neon²¹, but it was suggested that individual grains might show much higher presolar ages of up to 2 billion years. However, it has been shown that much of the measured neon²¹ was not cosmogenic but was implanted neon from the helium shell of the parent asymptotic giant branch star, while, at the same time, they concluded that losses of cosmogenic neon²¹ upon production due to recoil out of the grains were much larger than assumed. It has been deduced much lower presolar ages for bulk silicon carbide assemblages of a few times 100 million years only, based on cosmogenic xenon, for which recoil losses are smaller. The first interstellar exposure ages on individual exceptionally large (about 5 to 60 μm) silicon carbide grains have been reported based on lithium isotopes and with helium and neon. The large grains contain greater amounts of cosmogenic nuclides, and, more importantly, require a smaller recoil correction. These studies both reported ages of between a few megayears to about 1 billion years, but the average of the lithium-based ages was considerably higher than the average noble gas age. It has been suggested that the many ages of less than 200 Ma may be explained by increased dust production after a galactic starburst 1 to 2 billion years prior to the birth of the Sun.

The Murchison meteorite at the National Museum of Natural History. Wikimedia Commons.

Henk et al. provide presolar ages based on cosmogenic neon isotopes, significantly increasing the total number of presolar grain ages. They also present re-evaluated ages from previously published data. This will enable us to further advance our understanding of the lifetimes of interstellar dust. Previous interstellar production rates of cosmogenic nuclides were based on fluxes deep within the heliosphere that were extrapolated to interstellar space. Henk et al. use, instead, improved interstellar production rates that were determined with a purely physical model that uses a state-of-the-art nuclear cross-section database and an interstellar galactic cosmic ray spectrum based on data collected by NASA’s Voyager 1 space probe at the edge of the heliosphere. Voyager 1 recorded  the low-energy part of the galactic cosmic ray spectrum, something that is not possible deeper within the heliosphere. To correct for recoil losses of cosmogenic nuclides from silicon carbide grains grains, Henk et al. use a physical recoil model that considers the energies of galactic cosmic ray protons and α-particles from the new cosmic ray spectrum.

Another aspect that was not considered in previous studies is the potential exposure of presolar grains to the enhanced particle flux of the early active sun. Large excesses of cosmogenic noble gases in single olivine grains in some primitive meteorites have been attributed by some workers to a high flux of energetic particles from the early Sun, although others contested this conclusion. Recently, however, unambiguous evidence for an enhanced exposure of hibonite (an aluminum−calcium oxide), possibly the earliest solar nebula condensate, to energetic particles from the early active sun has been reported. This implies that some of the presolar grains Henk et al. studied might have been exposed to the same enhanced solar particle flux. They were, therefore, also required to estimate the upper limit of cosmogenic nuclides concentrations produced in the early Solar System rather than in the interstellar medium.

Henk et al. processed their noble gas data from 27 silicon carbide grains and reprocessed data from published results from 22 silicon carbide grains to calculate an internally consistent set of presolar cosmic ray exposure ages for nearly 50 grains with the improved cosmogenic nuclide production rates and nuclear recoil corrections. The cosmogenic neon component can be clearly resolved from the two other main components, nucleosynthetic neon and adsorbed atmospheric neon based on distinct isotopic neon compositions. Nucleosynthetic neon is implanted into circumstellar grains from the hot post-asymptotic giant branch star wind emanating from the exposed helium shell, and its concentrations decrease with increasing grain size. Using carbon, nitrogen, and silicon isotopes, all but three grains have been classified as mainstream silicon carbide grains, originating in the outflows of low- to intermediate-mass (post) asymptotic giant branch stars. The three other grains are of AB type, based on their low carbon¹²/carbon¹³ ratios. All newly analysed grains are mainstream silicon carbide.

Henk et al. determined helium³ and neon²¹ exposures ages of 30 and 24 grains, respectively, and obtained upper age limits for 12 (helium³) and 16 (neon²¹) grains. For 18 grains, Henk et al. obtained both helium³ and neon²¹ exposures ages. Nominal recoil-corrected helium³ exposures ages for 16 out of these 18 grains are higher than recoil-corrected neon²¹ exposures ages, whereas uncorrected ages show an opposite trend. Helium is more easily lost through heating and through recoil than neon, so both effects would result in lower nominal helium³ than neon²¹ exposures ages before a recoil correction. Hence, a recoil correction will be larger for helium³ than for neon²¹. For grains of less than 10 μm, nominal cosmogenic helium³ recoil losses are over 94% for the smallest grains analysed by Henk et al., whereas corresponding losses for neon²¹ are over 40%. Hence, any uncertainties in recoil corrections will result in a larger uncertainty of helium³ exposures ages. Heating of grains to high temperatures (at leasr 900 K) would result in near-complete helium loss. Helium loss works in the opposite direction of the trend seen in the data. This implies that, while some helium loss cannot be excluded, no significant loss occurred; otherwise, much more helium than neon would have been lost, and even overcorrected helium³ exposures ages would be smaller than neon²¹ exposures ages. The helium³ exposures ages are less reliable than neon²¹ exposures ages, mainly because of larger uncertainties in the helium³ recoil correction. The 16 recoil-corrected helium³ exposures ages exceeding recoil-corrected neon²¹ exposures ages, consequently, indicate an overestimation of the recoil loss for helium³. The reason for this may be that these grains were actually irradiated in the interstellar medium as parts of larger grains or as grain aggregates, or the grains were coated with large mantles of ices and organics while in the interstellar medium. Henk et al. estimate the original sizes of the irradiated objects in the interstellar medium by varying the grain size and modeling the resulting recoil correction until the recoil-corrected helium³ and neon²¹ exposures ages match. The estimated object diameters during irradiation are factors of about 3 × up to about 30 × higher than those of the analysed grains. This results in ages of 44 to 85% of the original recoil-corrected ages. In principle, it would be possible to test this result with cosmogenic xenon that has a much smaller recoil loss. Unfortunately, the amounts of cosmogenic xenon produced are below current detection limits for single-grain analyses, due to the low amounts of suitable target elements for xenon production in silicon carbide. Bulk analyses of silicon carbide give mixed signals and are not useful in this regard, as these do not resolve cosmogenic gas contributions from grains with different lifetimes. Seventy-five percent of the 16 analysed grains that were part of much larger objects have euhedral shapes, which indicates they are not fragments of larger grains and were more likely parts of aggregates. The remainder look like they are shattered fragments of larger silicon carbide grains grains, but, given the large object sizes estimated during interstellar medium irradiation, larger than any known presolar silicon carbide grain, they were likely also part of aggregates. Aggregates of minerals, suspected by some to be presolar, in an organic material matrix were recently observed in interplanetary dust particles. A previous study observed organic coatings on about 60% of pristine presolar silicon carbide that were physically separated from their host meteorite without the use of chemical reagents. However, no aggregates or clustering of larger presolar grains have yet been observed during the in situ ion imaging searches of polished sections of meteorites. The lack of such clustering of larger grains could be due to preferred breakup of larger clusters of several dozen to hundred micrometers during accretion onto planetesimals in the early Solar System, while smaller clusters composed of smaller grains which have lower inertia, stayed intact. Henk et al. propose that grains in the size range we analyzed formed in the outflows of (post) asymptotic giant branch parent stars and coagulated there with organic matter to form larger aggregates. While large silicon carbide dust grains are rare in the interstellar medium, they are consistent with observations of circumstellar dust around asymptotic giant branch and post-asymptotic giant branch stars. Far-infrared excess associated with such dust may indicate the presence of up to millimeter-sized grains. Up to 5-mm-large dust grains were proposed to explain radio observations of dust around the Egg Nebula, a post-asymptotic giant branch star. Another possibility is that the high-density winds from post-asymptotic giant branch stars are the sources of the large presolar silicon carbide grains, such as the size fraction studied by Henk et al.

Henk et al. also obtained lithium isotope data for 19 silicon carbide grains. Many of these grains have a lithium⁷/lithium⁶ ratio below the chondritic ('solar') value of 12.06, indicating the contribution of a cosmogenic lithium component (the end-member cosmogenic lithium⁷/lithium⁶ ratio is about 1.2). These observations could be due to a combination of contamination with terrestrial or Solar System lithium, matrix effects, or additional, unidentified lithium components that would have contributed to the measured lithium concentration. Because of low concentrations of cosmogenic lithiun and high abundance of normal lithiun, a reliable determination of cosmogenic lithiun is very difficult. Currently, lithium does not allow the obtaining of a reliable age.

Evidently, the neon ages are more reliable than the lithium and helium ages, and Henk et al. base their discussion mainly on neon²¹ ages. They range from 4 million years to 3200 million years, and upper limits range from 3 to 3300 million years. Henk et al. obtained neon²¹ ages for two out of three AB grains; the calculated ages, 65 million years and 260 million years, fit into the age range of the mainstream grains. No age was determined for the third AB grain, due to an insufficient gas amount; the 2-μm-sized grain was the smallest one analysed in Henk et al.'s study.

Overall, the neon²¹ age distribution trend is similar to what was previously reported for a smaller sample set, with most exposure ages below 300 million years (60%) and 50% below 200 million years. This is consistent with most theoretical lifetime estimates for much smaller, under 1 μm interstellar dust of 100 to 300 million years, but in contrast to the longer lifetimes expected for large grains. Assuming constant dust production rates from asymptotic giant branch stars and constant dust destruction rates, Henk et al. would expect to encounter younger grains more frequently than older grains simply because older ones have a higher probability of encountering a destructive process. However, their age distribution does not fit any of the assumed steady-state models for different average lifetimes. Having many large grains in a relatively narrow age range seems to require an explanation other than simply a lifetime effect, which would apply to small grains. Henk et al. propose that this age distribution can be explained by these large grains being late-stage products of asymptotic giant branch stars with initial masses of two times that of the Sun, that formed together. While less massive stars were more abundant, their evolutionary lifetimes were too long to reach the dust-producing asymptotic giant branch phase before the formation of the Solar System, and, hence, their dust has not been incorporated into meteorite parent bodies. The rarer, more massive asymptotic giant branch stars (with initial masses of over three times that of the Sun) are not likely to be a source of large silicon carbide grains, as their higher radiation pressures would have ejected circumstellar grains before they grew to the large grain sizes observed here. It was previously suggested that the grains’ parent stars originated in a presolar starburst that could have been triggered by a galactic merger, which has been proposed to explain the silicon isotopic compositions of presolar mainstream silicon carbide. Most observational and theoretical work on the history of the star formation rate of our galaxy does not see evidence of a large starburst event in presolar times as hypothesised previously, nor a flat star formation rate, but most studies conclude that the star formation rate only mildly fluctuated. These studies find a moderately enhanced star formation rate around 7 to 9 billion years ago. Several of the observational studies show that this broad peak consists of two peaks, with the more recent one close to 7 billion years ago. Recent modeling work based on observations of the chemical compositions of stars in the solar neighborhood reveals a moderately enhanced star formation rate that peaked around 7 billion years ago. In this model, this enhancement was caused by streams of cold matter that accreted onto the galactic disk from the halo. Based on stellar main-sequence lifetime calculations, Henk et al. estimate that stars with about 1.6 to 1.9 times the mass of the Sun, that formed together during this enhanced star formation rate episode about 7 billion years ago, reached their dust-producing asymptotic giant branch phase between about 4.9 and about 4.6 billon years ago. These dust grains would then have been exposed to interstellar galactic cosmic rays for up to 300 million years before being shielded in the forming Solar System. What we are seeing in the silicon carbide age peak are the first arrivers of dust formed in the late stages of stars originating in the presolar enhanced star formation rate peak. The rest of the peak must be more recent than the start of the Solar System and was not sampled in the presolar grain population. Although speculative, this scenario is consistent with Henk et al.'s data and, barring another explanation, may be a plausible reason for the observed presolar silicon carbide age distribution for large grains with presolar ages of up to 300 million years. While Henk et al. see older grains, they do not see older peaks (other than from individual grains) in their age distribution. They explain this by two effects. First, grain destruction reduced the number of surviving old grains, and, second, the signal to noise ratios farther back in time are currently too low to show peaks within Henk et al.'s dataset. Older interstellar neon²¹ exposure ages obtained for at least 7 grains are over 300 million years and, for a few grains (3 out of 24, excluding those with upper limits; 5 out of 40 including upper limits), are consistent with what is expected for large grains. In particular, if these grains were over 100-μm aggregates in the interstellar medium, long lifetimes are expected. Erosion by sputtering is slower than the time the grain is exposed to shock-heated gas, but large grains can erode significantly when they get slowed down in the cooled postshock gas and experience rare collisions with other large grains. Gradual erosion by collisions with smaller grains would leave cratered surfaces, something that has not been observed with silicon carbide grains to date. Possible evidence of a microimpact crater was so far only found in a large presolar aluminum oxide grain. Some of the old grains could have been shielded from destructive processes in clumps. Such protective density inhomogeneities have been observed astronomically in shocked regions of the interstellar medium.

The oldest grains based on both helium³ and neon²¹ ages are the smallest, and an inverse trend between age and grain size is apparent onsistent with the preliminary trend observed in xenon bulk silicon carbide analyses. The trend persists in the recoil-corrected data and in the size-corrected subset but gets less prominent in the latter. Smaller grains are more abundant than larger grains in the interstellar medium, resulting in a higher number of smaller grains that are old compared to larger ones. We can exclude a sampling bias, as we have not disproportionally analyzed small grains; on the contrary, only 12 of the 49 grains are under 4 μm. 

It has been proposed that grains with presolar ages older than the sun’s galactic year (about 230 million years) might have had the time to radially migrate from the inner parts of the galaxy toward the galactocentric distance of the forming Solar System. Because of the compositional gradient within our galaxy, we would expect these grains to reflect the metallicity of their parent stars. However, we do not observe a correlation between age and silicon isotopic composition, which is a proxy for metallicity of stellar sources. Either our dataset is too small to reveal such a trend, the grains did not migrate as suggested, or there is no galactic gradient for silicon isotopic composition, in contrast to oxygen isotopic composition and (iron/hydrogen). Recent astronomical observations did not find a galactocentric silicon²⁹ proportion trend within about 200‰, a range that was less than expected from galactocentric variations in other isotope ratios but similar to the one measured in presolar silicon carbide mainstream grains.

Henk et al. highlight that, at the end of their interstellar journey, the presolar grains could have been exposed to enhanced particle radiation from the young Sun. Based on cosmogenic helium and neon concentrations in hibonite, an aluminum−calcium oxide, from the Murchison meteorite, the solar cosmic ray flux these grains might have been exposed to was orders of magnitudes higher than today, consistent with what is expected during the T Tauri phase of the Sun. Hibonites were among the first condensates in the protoplanetary disk and were transported to the disk surface far enough from the sun to evade significant heating, where they were irradiated by an enhanced solar cosmic ray flux. If the presolar silicon carbide grains had a similar exposure history to solar energetic particles in the protoplanetary disk as the hibonites, they would also have acquired a similar concentration of solar cosmic ray produced noble gases. The difference in irradiation time on the disk surface between presolar solar cosmic ray and hibonites is not known. Given that the high-temperature condensate hibonite was present very early in the disk, the short disk lifetime of a few megayears, and the exposure required to explain the cosmogenic hibonite data, we consider that the time difference between the hibonite and silicon carbide exposure duration was probably small. Henk et al.'s results show that the majority of the cosmogenic neon²¹ was acquired during presolar galactic cosmic ray exposure. Specifically, at least 80% of the cosmogenic neon²¹ for grains with neon²¹ ages greater than 100 million years was acquired by presolar galactic cosmic ray exposure. For these grains, the amount that might have been acquired during early Solar System formation is smaller than the uncertainty of the presolar exposure ages and, hence, not detectable. These findings only apply if the presolar grains were exposed to the early active sun at all. At most, the five grains with the lowest ages might have acquired all their cosmogenic neon²¹ in the early Solar System. Early Solar System exposure does not significantly affect our interpretation of presolar ages, except, possibly, for these five grains.

However, our observation has implications for the origin of hibonites that formed in the solar nebula: The cosmogenic nuclide concentrations in the hibonites are typically much lower than that observed in presolar silicon carbide grains, indicating that the irradiated hibonites are indeed early Solar System products and not of presolar origin.

Henk et al. note that a presolar exposure age of a silicon carbide grain is a nominal age and that the actual residence time in the interstellar medium might have been shorter if the grains were exposed to a high energetic particle flux from other nearby stars in addition to background galactic cosmic ray exposure. Henk et al. estimate that the chances of such a close encounter for the average interstellar silicon carbide are low and that such exposure could have also led to destruction of the grain. Modeling of this probability is difficult due to many unknowns and beyond the scope of this work.

With this study, Henk et al. have increased the number of presolar silicon carbide grain neon exposure ages, calculated with improved recoil corrections and cosmogenic nuclide production rates. Based on neon isotopes, they conclude that a majority (about 60%) of the large presolar silicon carbide grains analysed have interstellar cosmic ray exposure ages below 300 million years before the formation of the Solar System. This is compatible with most theoretical estimates of interstellar dust lifetimes of 100 to 200 million years. This age distribution is also consistent with the hypothesis that these grains originate from stars that initially formed during an enhanced stellar formation rate about 7 billion years ago and became dust-producing asymptotic giant branch stars between about 4.9 an 4.6 billion years ago. Furthermore, a significant fraction has presolar ages above 300 million years ago, with at least about 8% above 1 billion years, making them the oldest dated samples so far. These old ages require that these grains evaded destruction in supernova shockwaves, possibly in dense clumps that formed in such shockwaves. Based on a comparison of cosmogenic hellium and neon, it is clear that some grains were part of larger particles or aggregates and might have had large mantles of ices and organics during cosmic ray exposure in the interstellar medium.

The studied presolar grains might have acquired a small but, in most cases, undetectable fraction of their cosmogenic neon during exposure of energetic particles from the early active sun. However, only particularly young grains with very low interstellar residence times might have received a significant fraction of their cosmogenic nuclides in the early Solar System, before accretion onto planetesimals. The specifics of this exposure, such as the solar particle flux and exposure, are currently unknown.

We conclude that neon exposure age dating is currently the only viable method to date presolar grains. While the method provides ages relative to the start of the Solar System and suffers from relatively large uncertainties, it can provide unique information about the interstellar dust cycle and star-forming events in the Galaxy before the birth of the Sun.

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