A Long Period Super-Earth around a Nearby Star

Nu2 Lupi is a G-class star not all that far away in astronomical terms (48 light years) in the constellation Lupus, its proximity verified by parallax measurements and firmed up by the Hipparcos satellite. This is one of the closest G-class stars to our own, and it’s a fast mover in other ways, with a high radial velocity. Its age is estimated at roughly 12 billion years, making it one of the oldest stars near our system. HARPS spectrograph data pulled up three planets here in 2019, two of them later found to transit.

And now we have, unexpectedly, a third transit. The surprising nature of the third relates to the distance of the third planet from the star. The two inner worlds, with masses between Earth’s and Neptune’s, take 12 and 28 days to orbit Nu2 Lupi. The third takes 107 days, far enough out that a transit seemed unlikely. The ratio of the diameter of the star to the diameter of the orbit comes into play in determining the probability of a transit.

We have the European Space Agency’s CHEOPS mission (CHaracterising ExOPlanet Satellite) to thank for the surprise discovery, allowing a tight look at the third world. For in combination with radial velocity data, the transit information allows scientists to determine the planet’s density and composition. Nu2 Lupi d turns out to be 2.5 times the radius of Earth and about 9 times its mass. While the inner world at Nu2 Lupi is evidently rocky, the outer two are enveloped by hydrogen and helium atmospheres, with a quarter of the mass of each being composed of water in the form of either high-pressure ice or high-temperature steam.

Image: This infographic reveals the details of the Nu2 Lupi planetary system. This bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf), as shown to the left of the frame, and is known to host three planets (named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’). The relative sizes, orbital periods, and possible compositions of these three planets are depicted to the centre and lower right of the frame, while planet d’s comparative position within our Solar System is shown to the upper right (as defined by the amount of incident light it receives from its star, Nu2 Lupi). Credit: ESA.

Nu2 Lupi d turns out to be a useful find, as co-author David Ehrenreich (University of Geneva) explains:

“Due to its relatively long period, the amount of stellar radiation reaching the planet is mild in comparison to many other discovered exoplanets. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its origin. Since its bright host star is quite close to us, it is easier to study. This makes it a golden target for future study, with no known equivalent.”

I think what Ehrenreich is getting at is the fact that Nu2 Lupi is a G-class star, making its proximity to Earth and its transiting worlds of great interest — the authors point out that this is the first time a planet with an orbital period of over 100 days has been discovered in transit around a star bright enough to be a naked eye object. We can imagine future missions using transmission spectroscopy to analyze its atmosphere.

But let’s look at the paper’s further explanation, for planet formation models are also in play:

With regard to the derived amounts of gas, planetary atmospheric evolution calculations indicate that the innermost planet b was subject to significant atmospheric loss, while planets c and d did not suffer strong evaporation… The two outer planets are indeed sufficiently massive and far away from the host star to be only little affected by mass loss throughout their evolution. Therefore, the current low gas content returned by our internal structure modelling for these two planets is likely of primordial origin.

Core accretion models of planet formation, oft discussed in these pages, see planets accreting large amounts of gas when they reach a critical mass, generally cited as being around 10 times the mass of Earth. As this relates to Nu2 Lupi:

The structure of the two outer planets, as observed today, being likely primordial, these two objects provide a very important anchor for planet formation models, as they indicate that neither of them reached the critical mass during their formation. These two planets, by giving access to both the core mass and gas-to-core ratio for two objects in the same system will provide strong constraints on the understanding of the formation of sub-critical planets. Since the presence of large gas envelopes hinders habitability, the ?2 Lupi system, with its two sub-critical outer planets, also provides an interesting case study for numerical models targeting the emergence of habitable worlds.

The paper is Delrez et al., “Transit detection of the long-period volatile-rich super-Earth ?2 Lupi d with CHEOPS,” Nature Astronomy 28 June 2021 (abstract) / preprint.


New Constraints on Exoplanet Photosynthesis

Most autotrophic organisms on Earth use photosynthesis to work their magic. Indeed, photosynthesis accounts for about 99 percent of Earth’s entire biomass (a figure likely to change as we learn more about what lies beneath the surface). The process allows organic matter to be synthesized from inorganic elements, drawing on solar radiation as the energy source, and providing the oxygen levels needed to drive complex, multicellular life.

Does photosynthesis occur in other star systems? We know that it emerged early on Earth, and can trace its development back to the Great Oxidation Event in the range of 2.4 billion years ago, although its origins are still under scrutiny. In a new paper, lead author Giovanni Covone (University of Naples) and colleagues examine the conditions needed for oxygen-based photosynthesis to develop on an Earth-like planet not just at Earth’s level of stellar flux but throughout the classical habitable zone.

The key to the study is stellar radiation as received by the planet from the host star, with the authors examining the efficiency with which living organisms could produce nutrients and molecular oxygen using oxygenic photosynthesis. Here we are considering what the paper describes as photosynthetically active radiation (PAR). Key to the analysis is the idea of exegy, which the authors explain as follows:

…we estimate the efficiency of the PAR radiation driving OP [oxygenic photosynthesis] as a function of the host-star temperature by means of the notion of exergy. Exergy can be defined as the maximum useful work obtainable from the considered system in given environmental conditions (see e.g. Petela 2008; Ptasinski 2016). In other words, exergy is a measure of the quality of energy (Austbø, Løvseth & Gundersen 2014). Living organisms are dissipative structures away from thermodynamic equilibrium with the environment thanks to the constant input of exergy stellar radiation.

This idea of the quality of energy has been the subject of several exoplanet investigations, most recently that of Caleb Scharf (Columbia University), who studied photosynthetic efficiency as a function of a star’s effective temperature over the entire radiation spectrum. Covone and team keep their focus on photosynthetically active radiation, constructing a table showing the parameters of Earth-analog planets and their host stars, including worlds at Proxima Centauri, Kepler 186 and Trappist-1.

The question is whether living organisms can efficiently produce the nutrients and molecular oxygen they need in these conditions via normal photosynthesis.

Table 1. Parameters of the known Earth analogue planets in the HZ and their host stars. Equilibrium temperature values with * have been derived in this work. For Proxima Centauri b the estimate of the mass is given since the planet is probably not a transiting one. Credit: Covone et al.

Considered in terms of the exergetic efficiency of a star’s radiation within this range, the authors find that only Kepler=442b receives a photon flux sufficient to sustain a biosphere something like the Earth’s. This is an interesting world, a confirmed super-Earth orbiting a K-class star in Lyra about 1200 light years out. It does not appear to be tidally locked and offers what the scientists consider to be a good target for a search for biosignatures. But the other worlds lack the energy in the correct wavelength range to sustain a rich biosphere. The figure below is striking:

Image: This is Figure 1 from the paper. Caption: Photons flux in two differently defined PAR ranges at the surface of planets at the two edges of the HZ (dark blue lines for an upper limit of 800?nm and light blue for an upper limit of 750?nm), as a function of the star effective temperature, in units of 1020 photons s?1 m?2 (HZ inner edge: continuous line; HZ outer edge: dotted line). The green dot and circle show the photon flux in PAR range on the Earth surface, yellow dots and circles the estimated photon flux on the surface of known Earth analogues (see Table 1), respectively, with an upper limit for the PAR range of 800?nm (dots) and 750?nm (circles). The red dotted line shows the average photon flux which is necessary to sustain the Earth biosphere. The green dotted line shows the typical lower threshold for OP on Earth. Credit: Covone et al.

Of the planets cited in Table 1, then, only Kepler-442b comes close to receiving the stellar radiation needed. Indeed, given these findings, many stars in the K-class would be unlikely to supply the radiation needed to support a complex biosphere. Nor would red dwarf stars, which would not deliver enough energy to their planets to activate photosynthesis in the first place. Giovanni Covone comments:

“Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope.”

And he adds:

“This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the “sweet spot” for hosting a rich Earth-like biosphere is not so wide.”

A much narrower than expected range for the habitable zone? Perhaps in terms of that exact ‘sweet spot’ that mirrors Earth. But the authors are quick to add that caution is in order in terms of biomass production, which softens the message considerably. This passage receives prominence in the paper’s conclusion (italics mine):

…we should bear in mind that biomass production on Earth is not limited by the quantity neither [sic] the quality of the incoming radiation, but rather by the availability of nutrients. For instance, Lin et al. (2016) found that in ocean phytoplankton populations about 60 per?cent of the absorbed PAR solar energy is dissipated as heat. Generally, phytoplankton operate at a much lower photosynthetic efficiency than they are potentially capable of achieving, just because in most situations light is a very abundant resource on Earth. Moreover, OP does not respond linearly to the input photon flux (see Ritchie et al. 2018). For these reasons, it is not immediate to draw consequences on the amount of biomass produced from the estimated PAR photon flux and its exergy content. Exoplanets with lower values of these quantities could host a biosphere comparable with the one on our planet.

We should not, in other words, read this as a definitive statement on habitable zone width but rather a pointer to further work that will need to broaden the investigation. The authors themselves mention “exergy destruction that occurs as consequence of biological conversion taking place after the light harvesting, in the leaf transpiration and metabolism” and also atmospheric absorption that changes the radiation spectrum. But solutions beyond oxygenic photosynthesis are also possible, a direction of study that could point to near-infrared light harvesting on red dwarf planets.

The paper is Covone et al., “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone,” Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 3 (August 2021), pp. 3329–3335 (full text).


How Far Can Civilization Go?

Robert H. Gray, author of The Elusive Wow: Searching for Extraterrestrial Intelligence, has searched for radio signals from other worlds using the Very Large Array and other radio telescopes. You’ll find numerous links to his work in the archives here. In today’s essay, Gray takes a look at a classic benchmark for assessing the energy use of civilizations, introducing his own take on Earth’s position in the hierarchy and how these calculations affect the ongoing SETI effort. His article on the extended Kardashev scale appeared in The Astronomical Journal https://iopscience.iop.org/article/10.3847/1538-3881/ab792b. Photograph by Sharon Hoogstraten.

by Robert H. Gray

Human civilization has come an amazingly long way in a short time. Not long ago, our major source of energy was muscle power, often doing hard work, while today much more energy is available from fuels, fission, hydro, solar, and other sources without breaking a sweat. How far can civilization go?

It’s probably impossible to say how far civilizations can go in areas like art or government, because such things can’t be measured or forecast, but energy use is measurable and has trended upward for centuries.

The astrophysicist Nikolai Kardashev outlined a scheme for classifying civilizations according to the amount of energy they command, in order to assess the type of civilization needed to transmit information between stars. He defined Type I as commanding the energy available to humanity in 1964 when he was writing, Type II could harness the energy of a star like our Sun, and Type III would possess the energy of all of the stars in a galaxy like our Milky Way.

Harnessing the energy of stars might sound like science fiction, but solar panels are already turning sunlight into electricity at a modest scale, on the ground and in space. Gerald O’Neill and others have envisioned orbiting space settlements soaking up sunshine, and Freeman Dyson envisioned something like a sphere or swarm of objects capturing all or much of a star’s light.

Carl Sagan suggested using Arabic numerals instead of Kardashev’s Roman numerals, to allow decimal subdivisions, and he suggested more uniform power levels. He re-defined Type 1 as 1016 watts—very roughly the Sun’s power falling on the Earth—and he rounded off Type 2 and 3 levels to 1026 and 1036 watts respectively, so planetary, stellar, and galactic categories increase in steps of 1010 or ten billion. A simple formula converts power values into decimal Types (the common logarithm of the power in megawatts, divided by ten). In the recent year 2015, human power consumption was 1.7×1013 watts, or Type 0.72—we’re short of being a Type 1.0 planetary civilization by a factor of roughly 600. In 1800 we were Type 0.58, and in 1900 we were Type 0.61.

The 2015 total power consumption works out to an average of 2,300 watts per person, which is 23 times the 100 watts human metabolism at rest, but it’s not many times more than the 500-1,000 watts a human can produce working hard. Maybe we haven’t gone all that far, yet.

I recently extended the scale. Type 0 is 106 watts or one megawatt, which is in the realm of biology rather than astronomy—the muscle power of a few frisky blue whales or several thousand humans. That seems like a sensible zero point, because a civilization commanding so little power would not have enough to transmit signals to other stars. Type 4 is 1046 watts, roughly the power of all of the stars in the observable Universe.

One use for the scale is to help envision the future of our civilization, at least from the perspective of energy. If power consumption increases at a modest one percent annual rate, we will reach planetary Type 1 in roughly 600 years and stellar Type 2 in 3,000 years—roughly as far in the future as the Renaissance and ancient Greece are in the past. That simplistic growth rate would put us at galactic scale Type 3 in 5,000 years which is almost certainly wrong, because some parts of our galaxy are tens of thousands of light years away and we would need to travel faster than light to get there.

There are, of course, many limits to growth—population, land, food, materials, energy, and so on. But humans have a history of working around such limits, for example producing more food with mechanization of agriculture, more living space with high rise buildings, and more energy from various sources. It’s hard to know if our civilization will ever go much beyond our current scale, but finding evidence of other civilizations might give us some insight.

Another use for the scale is to help envision extraterrestrial civilizations that might be transmitting interstellar signals, or whose large-scale energy use we might detect in other ways.

If we envision ET broadcasting in all directions all of the time, they would need something like 1015 watts or 100,000 big power plants to generate a signal that our searches could detect from one thousand light years away using the 100-meter Green Bank Telescope. That means we need to assume at least a Type 0.9 nearly planetary-scale civilization—and considerably higher if they do anything more than broadcast—a civilization hundreds or thousands of times more advanced than ours. That seems awfully optimistic, although worth looking for. If we envision civilizations soaking up much of a star’s light with structures like Dyson spheres or swarms, then unintentional technosignatures like waste heat re-radiated in the infrared spectrum conceivably could be detected. Some infrared observations have been analyzed along those lines, for example by Jason Wright and associates at Penn State.

If, on the other hand, we envision ET transmitting toward one star at a time using a big antenna like the 500 meter FAST in China, then we need to assume only something like 108 watts or one-tenth of one big power plant, although the signal would be detectable only when the antenna’s needle beam is pointed at a target star. To catch intermittent signals like that, we will probably need receiver systems that monitor large areas of sky for long periods of time—ideally, all-sky and full-time—and we can’t do that yet at the microwave frequencies where many people think ET might transmit. A modest prototype microwave receiver system called Argus has been monitoring much of the sky over Ohio State University in Columbus for a decade with very low sensitivity, and an optical system called PANOSETI (Panoramic SETI) is planned by Shelly Wright of UCSD and Paul Horowitz of Harvard to potentially detect lasers illuminating us.

Detecting some signature of technology near another star would be a historic event, because it would prove that intelligence exists elsewhere. But the U.S. government has not funded any searches for signals since Sen. Richard Bryan (D-NV) killed NASA’s program in 1993, even though thousands of planets have been discovered around other stars.

Both Kardashev and Sagan thought civilizations could be characterized by the amount of information they possess, as well as by energy. An information scale much like the energy scale can be made using 106 bits or one megabit as a zero point—roughly the information content of one book. Sagan thought that 1014 or 1015 bits might characterize human civilization in 1973 when he was writing on the topic, which would be Type 0.8 or 0.9 using the power formula (he used the letters A, B, C… for 106, 107, 108… bits, but letters don’t allow decimal subdivisions). More recent estimates of humanity’s information store range from 1018 to 1025 bits or Types 1.2 to 1.5, depending on whether only text is counted, or video and computer storage are included.

Nobody knows what information interstellar signals might contain. Signals could encode entire libraries of text, images, videos, and more, with imagery bypassing some translation problems. What might motivate sending information between stars is an open question; trade is one possible answer. Each world would have its own unique history, physical environment, and biology to trade—and conceivably art and other cultural stuff as well. Kardashev thought that the information to characterize a civilization could be transmitted across the Galaxy in one day given sufficient power.

Whether any interstellar signals exist is unknown, and the question of how far civilization can go is critical in deciding what sort of signals to look for. If we think that civilizations can’t go hundreds or thousands of times further than our energy resources, then searches for broadcasts in all directions all of the time like many in progress might not succeed. But civilizations of roughly our level have plenty of power to signal by pointing a big antenna or telescope our way, although they might not revisit us very often, so we might need to find ways to listen to more of the sky more of the time.

Additional Resources

N. S. Kardashev, Transmission of Information by Extraterrestrial Civilizations, SvA 8, 217 (1964).

C. Sagan, The Cosmic Connection: An Extraterrestrial Perspective, Doubleday, New York (1973).

V. Smil, Energy Transitions: Global and National Perspectives, 2nd edition, Praeger (2017).

R. H. Gray, The Extended Kardashev Scale, AJ 159, 228-232 (2020). https://iopscience.iop.org/article/10.3847/1538-3881/ab792b

R. H. Gray, Intermittent Signals and Planetary Days in SETI, IJAsB 19, 299-307 (2020). https://doi.org/10.1017/S1473550420000038


A Chronological Look at a Transiting Earth

Call it the Earth Transit Zone, that region of space from which putative astronomers on an exoplanet could see the Earth transit the Sun. Lisa Kaltenegger (Cornell University) is director of the Carl Sagan Institute and the author of a 2020 paper with Joshua Pepper (LeHigh University) that examined the stars within the ETZ (see Seeing Earth as a Transiting World).

While Kaltengger and Pepper identified 1004 main sequence stars within 100 parsecs that would see Earth as a transiting planet, Kaltenegger reminds us that stars are ever in motion. Given the abundant resources available in the European Space Agency’s Gaia eDR3 catalog, why not work out positions and stellar motions to examine the question over time? After all, there are SETI implications here. We study planetary atmospheres using data taken during transits. Are we, in turn, the subject of such study from astronomers elsewhere in the cosmos?

Thus Kaltenegger’s new paper in Nature, written with Jackie Faherty (American Museum of Natural History), which identifies 2,034 nearby star systems (within the same 100 parsecs, or 326 light years) that either could have seen the Earth transiting by observing our Sun within the last 5,000 years or will be able to within the same span of time going forward. Kaltenegger takes note of the dynamic nature of the dataset:

“From the exoplanets’ point-of-view, we are the aliens, we wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light. And because stars move in our dynamic cosmos, this vantage point is gained and lost.”

Image: With the plane of the Milky Way galaxy seen stretching from the top to the bottom of the image, this artistic view of the Earth and Sun from thousands of miles above our planet, shows that stars (with exoplanets in their own system) can enter and exit a position to see Earth transiting the Sun. Credit: Kaltenegger & Faherty/Cornell University.

Looking at the results more closely, we find 117 stars over the 10,000 year window that are within 100 light years of our Solar System, while 75 of these stars have been in the Earth Transit Zone since the advent of commercial radio roughly 100 years ago. From the paper:

Among those sources, 29 were in the ETZ in the past, 42 will enter it in the future, and 46 have been in the ETZ for some time. These 46 objects (2 F, 3 G, 2 K and 34 M stars and 5 WDs [white dwarfs]) would be able to see Earth transit the Sun while also being able to detect radio waves emitted from Earth, which would have reached those stars by now… Seven of the 2,034 stars are known exoplanet host stars… Four of the planet hosts are located within 30 pc of the Sun.

It’s intriguing to look at some well known systems in this context. Trappist-1, for example, with its seven transiting worlds, will not enter the Earth Transit Zone for 1,642 years, but once within it, will have the ability to see a transit for 2,371 years.

In fact, even the closest stars tend to spend a millennium or more in the ETZ once there, plenty of time for extraterrestrial astronomers, if such exist, to take note of biological and/or technological activity on our world. Ross 128 is another interesting system. Here we have the second-closest known possibly temperate exoplanet after Proxima b, orbiting a red dwarf in Virgo about 11 light years out. Denizens of this world had a 2,158 year window they entered about 3,057 years ago but moved out of 900 years ago.

As I’m curious about unusual venues for potential life, I found this interesting:

109 of the objects in our catalogue are WDs, dead stellar remnants. Whereas most searches for life on other planets concentrate on main sequence stars, the recent discovery of a giant planet around a WD opened the intriguing possibility that we might also find rocky planets orbiting evolved stars. Characterizing rocky planets in the HZ of a WD would answer intriguing questions on lifespans of biota or a second ‘genesis’ after a star’s death.

It should come as no surprise that Kaltenegger and Faherty find M dwarfs dominating the spectral types, given their wide distribution n the galaxy; 1,520 of these stars are M-dwarfs. They also find 194 G-class stars like the Sun and a range of other stellar types, of which 102 are K-class stars like Alpha Centauri B. At the present time, 1,402 stars within the 100 parsec bubble can see Earth as a transiting world. Early observations of stars in the ETZ have begun via Breakthrough Listen as well as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China.

While technological activity may be difficult to observe, the authors point out that Earth’s biosphere has been at work on its atmosphere for billions of years, meaning observations of Earth transits would have identified it as a living world ever since the Great Oxidation Event. A transiting Earth’s biosignature should be hard to miss.

The paper is Kaltenegger & Faherty, “Past, present and future stars that can see Earth as a transiting exoplanet,” Nature 594 (23 June 2021), 505-507 (abstract).


Mixing and Growth in the Sun’s Protoplanetary Disk

The Allende meteorite is the largest carbonaceous chondrite meteorite ever discovered. Falling over Mexico’s state of Chihuahua in 1969 and breaking up in the atmosphere, the object yielded over two tons of material that have provided fodder for scientists interested in the early days of the Solar System. The meteorite contains numerous calcium-aluminum-rich inclusions (CAIs), which are considered to be the first kind of solids formed in the system 4.5 billion years ago.

Samples of the Allende meteorite are considered ‘primitive,’ which in this parlance means unaffected by significant alteration since formation. Now a team led by Tom Zega (University of Arizona Lunar and Planetary Laboratory) has gone to work on a dust grain from this object, in order to simulate the conditions under which it formed in the Sun’s protoplanetary disk. The grain was drawn from one of several CAIs discovered in the Allende meteorite sample. Analysis of the sample’s chemistry and crystal structure provides a look at the conditions that produced it.

The work appears in The Planetary Science Journal and is, according to Zega, something of a ground-breaker:

“As far as we know, our paper is the first to tell an origin story that offers clues about the likely processes that happened at the scale of astronomical distances with what we see in our sample at the scale of atomic distances.”

Image: A slice through an Allende meteorite reveals various spherical particles, known as chondrules. The irregularly shaped “island” left of the center is a calcium-aluminum rich inclusion, or CAI. The grain in this study was isolated from such a CAI. Credit: Shiny Things/Wikimedia Commons.

To examine the CAI, the researchers used a scanning transmission electron microscope located at the University of Arizona as well as a twin instrument at the Hitachi factory in Hitachinaka, Japan. The microstructure of the sample was examined at varying scales down to the level of individual atoms, in a process co-author Venkat Manga (UA) likens to opening a book recording what happened 4.567 billion years ago in the nebula that gave birth to the Sun.

It’s a book with an intriguing plot. The investigation revealed types of minerals called spinel and perovskite. Examined at the level of atomic-scale crystal structure, the sample creates a puzzle, with Zega noting that the chemical pathways that produced it seem to contradict current theories on the processes at work in protoplanetary disks. I like the way Zega puts this: “Nature is our lab beaker, and that experiment took place billions of years before we existed, in a completely alien environment.”

The paper recounts the team’s effort to create new formation models that could produce the sample CAIs found in the Allende object, simulating their chemistry and cutting across the fields of materials and mineral science as well as microscopy in the kind of multidisciplinary challenge that the study of planetary systems involves. Just how wide-ranging such work can be is made clear in the paper:

Our motivation is to understand the micro- and atomic-scale structure of the materials reported herein, which defy existing thermochemical models of the early solar protoplanetary disk. To this end, we combine state-of-the-art nanoscale to atomic-scale characterization of several CAIs with quantum-mechanical, thermodynamic modeling and dust-transport calculations to explain their origin. We propose that such an integrated approach is an important step toward a more comprehensive paradigm for reverse engineering the microstructures observed within primitive meteorites and extracting from them the thermal and dynamic histories that they have recorded.

The history of the particle at the heart of the model is worked out in detail, showing us a dynamic journey moving from a region of the disk near Earth’s current orbit to close proximity to the Sun and later transport to cooler regions further out. The sample winds up being part of an asteroid which, as it fragmented, became the parent body of meteorites like the Allende object. Crucial here is the history of movement and mixing within the disk, which challenges earlier, more static models.

It is clear from the above observations and computational effort that some assemblages within the oldest solar system solids deviate from long-standing assumptions and, rather than remaining in contact with the same parcel of gas in a monotonically cooling system, are admixed into other regions, possibly warming once or multiple times before cooling down in the disk. Indeed, this mixing and transport are necessary for the preservation of CAIs for millions of years and the delivery of CAI-like grains to outer regions of the solar nebula to be accreted by comets as revealed by the Stardust mission (Brownlee et al. 2006).

Image: Illustration of the dynamic history that the modeled particle could have experienced during the formation of the solar system. Analyzing the particle’s micro- and atomic-scale structures and combining them with new models that simulated complex chemical processes in the disk revealed its possible journey over the course of many orbits around the sun (callout box and diagram on the right). Originating not far from where Earth would form, the grain was transported into the inner, hotter regions, and eventually washed up in cooler regions. Credit: Heather Roper/Zega et al.

The authors say that their work is part of a longer-term effort to “deconstruct CAIs phase by phase,” so we should be hearing more about these methods. It will be interesting to see how asteroid sample return missions provide further insights.

We are digging here into the nature of planet formation as we try to understand how material moves around inside a protoplanetary disk. These are presumably processes common to the emerging systems we also see with the Atacama Large Millimeter/submillimeter Array (ALMA), whose images show the earliest stages of stellar system growth. But examining that commonality with a critical eye will be part of the continuing study of evolving disks and the movement of materials within them.

The paper is Zega et al., “Atomic-scale Evidence for Open-system Thermodynamics in the Early Solar Nebula,” Planetary Science Journal Vol. 2, No. 3 (17 June 2021). Full text.