Paul Gilster's Blog, page 49

Are Planets with Continuous Surface Habitability Rare?

Put a rocky, Earth-sized planet in the habitable zone of a Sun-like star, and good things should happen. At least, that seems to be the consensus, and since there are evidently billions of such planets in the galaxy, the chances for complex life seem overwhelmingly favorable. But in today’s essay, Centauri Dreams associate editor Alex Tolley looks at a new paper that questions the notion, examining the numerous issues that can affect planetary outcomes. Just how long does a planetary surface remain habitable? Alex not only weighs the paper’s arguments but runs the code that author Toby Tyrrel used as he examined temperature feedbacks in his work. Read on for what may be a gut-check for astrobiological optimists.

by Alex Tolley

The usual course of the discussion about planet habitability assumes that the planet is in the habitable zone (HZ), probably in the continuously habitable zone (CHZ). The determination if the planet is inhabitable concerns the necessary composition and pressure of the atmosphere to maintain a surface temperature able to support liquid water. As stars usually increase their luminosity over time, we see charts like the one below for Earth showing the calculated range of average surface temperature. Atmospheric pressure and composition can be modified to determine the inner and outer edges of the HZ or CHZ.

Image: Life on an exoplanet in a globular cluster. Credit: David Hardy (Astroart.org).

However, these charts say nothing about the various factors that may upset the stability of the climate, especially the geological carbon cycle, where volcanic outgassing of carbon dioxide (CO2) is approximately balanced by weathering of rocks to ultimately sequester carbon as carbonate rocks such as limestone. Imbalances can create significant changes in greenhouse gas (GHG) composition of the atmosphere with temperature impacts [4].

Figure 1. Possible bounds of Earth surface temperature over 4.5 By of changing solar output and the impact of atmosphere. The early Earth would have required a different energy trapping atmosphere to maintain an inhabitable temperature during its early history to prevent freezing. Continued increase in solar luminosity will render the surface too hot for life even with no atmospheric trapping of the sun’s heat. Source: Kasting 1988 [3]

We have much evidence of widely fluctuating average surface temperatures for the Earth, from a possibly hot Archaean eon, several local temperature maximums including the end of the “Snowball Earth”, the Permian/Triassic extinction, and the Eocene thermal maximum. Earth has also cooled, most notably during the “Snowball Earth” period that lasted millions of years and several extreme glaciations over its history.

Before even considering the many other possible factors that may preclude an inhabitable planet, there is a question of just how stable are planetary surface temperatures, especially when subjected to shocks due to excess CO2 emissions from very active volcanism, or conversely from excess weathering depleting the atmospheric CO2 pressure.

The Journal Paper

A new paper [1] by Prof. Toby Tyrrell looks at this question in a very different way. He posits that for the many planetary types and conditions in the galaxy, we should assume a wide variety of possible temperature feedbacks and simulate the average surface temperature of a planet over a long period of geologic time (3.0 By) to simulate how frequently planets could continuously maintain an inhabitable surface temperature throughout this period.

Tyrrell on randomly configured feedbacks:

“It is assumed that there is no inherent bias in the climate systems of planets as a whole towards either negative (stabilising) or positive (destabilising) feedbacks. In other words, it is assumed here that the feedback systems of planets are the end result of a set of processes which do not in aggregate contain any overall inherent predisposition either towards or against habitability.”

His model is very simple. He assumes a randomly chosen number of feedback values (change in temperature over time for a specified temperature) within an inhabitable temperature range. Figure 2 below shows one example of the model showing random feedbacks, calculated temperature attractors, runaway temperature zones, and a time course of temperature impacted by random temperature perturbations. The values for temperatures between each feedback node are interpolated from the 2 surrounding nodes. If the feedback slope is negative and if the 2 points straddle a 0 feedback, the model calculates a temperature attractor at that point, so that temperatures between the 2 feedback nodes tend to stabilize the temperature at the attractor position.

If, however, the slope is positive, the temperature will be destabilized and driven towards the upper node if above the 0 feedback level, and conversely to the lower node if below the 0 level. He sets a minimum (-10C) and maximum(60C) surface temperature range that if the calculated temperature extends beyond those boundary temperatures or is in feedback that will lead to runaway feedback to either a very low or very high temperature, the model assumes the planet is no longer habitable on the surface. The model adds 2 other important elements. Firstly there is a long-term forcing (e.g. increased solar output), which for the Earth is a positive one as the sun continues to increase its output over time. The second is to introduce small, medium, and large temperature perturbations (i.e. shocks) that introduce noise into the model and can flip the climate between attractor temperatures and also into runaway temperature conditions where the feedbacks positively reinforce the temperature change. Figure 2 below is extracted from the paper to indicate an example. [Annotations added for clarity.]

Figure 2. Extracted from the Tyrrell paper and annotated. The left chart shows 9 randomly created feedbacks. Where 2 adjacent feedbacks are connected by a negative slope and cross the zero feedback line, a temperature attractor is created, in this example there are 2 attractors. At either end are zones where the temperature would cause a runaway increase or decrease in temperature and these are indicated by grayed areas. The chart also indicates that over the long term, there is a negative forcing that reduces the average temperature over time. The right-hand chart shows the temperature over time. The 2 attractors are shown, as is the starting temperature [blue square]. The various perturbations are indicated both in time and size by the red triangles below the chart axis. The gray bands show the runaway temperature conditions and the black bands the start of uninhabitable conditions. The 500 My display shows the temperature flipping between the 2 attractors, with each flip due to larger temperature perturbations. A few temperature perturbations approach but do not cross into the runaway temperature zones.

With the parameters he uses, the model demonstrates that with repeated runs, only a few percent of planet runs enjoy a 3 billion year period where surface temperatures stay within the inhabitable temperature range. Once the range is exited, surface life ends and the planet becomes lifeless on the surface.

Figure 3 shows how rarely planets can maintain inhabitable conditions over the entire 3 billion year time period.

Figure 3. The probability of a planet always surviving as inhabitable over 3 billion years over several runs with the same feedback conditions but with no temperature perturbations and with random temperature perturbations. The gray (H1 – chance alone) hypothesis is pure random perturbations withwout feedbacks and the red (H2 – mechanism alone) – feedbacks but without large perturbations – are compared with the simulation results. The most important result is that a planet that can maintain surface inhabitable conditions is quite rare.

Tyrrell:

“The initial prospects for Earth staying habitable could have been poor. If so, this suggests that elsewhere in the Universe there are Earth-like planets which had similar initial prospects but which, due to chance events, at one point became too hot or too cold and consequently lost the life upon them. As techniques to investigate exoplanets improve and what seem at first to be ‘twin Earths’ are discovered and analysed, it seems likely that most will be found to be uninhabitable.”

His conclusion is ominous for astrobiologists. Even if we discover many planets that are in the HZ, and confirm that their atmospheres could support an inhabitable surface, those planets are either frozen or too hot to allow life to exist on the surface. The vast majority of apparently suitable worlds will prove lifeless and appear as if abiogenesis (or even panspermia) has failed to ignite an evolutionary progression to complex life and even possibly technological civilization.

This conclusion is enough to dampen any astrobiologist’s day and suggests that the search for biosignatures may be as disappointing as the results of SETI.

While the paper shows the results using the values of the published model and code, the supplementary information includes a considerable analysis of the model, for example, extending the inhabitable range, and several other parameters. However, the broad conclusion remains robust. Maintaining an inhabitable temperature over 3 billion years is unlikely.

Tyrrell acknowledges the simplicity of his approach and suggested in a recent SETI Institute webinar that he hopes to apply his approach with a more sophisticated planetary climate model to determine if his findings hold up.

Given the proxy indications of Earth’s paleotemperatures (see figure 4 below) showing wide ranges and some close misses to survival shown by the mass extinctions, why did Earth life survive? Tyrrell argues that the anthropic principle has to be invoked. Just as the universe we live in needs the exact constants for life and we couldn’t be in any universe without those conditions, so we technological humans cannot investigate unless the Earth had maintained a continuous inhabitable surface temperature.

A Critique

Figure 4 below shows the estimated temperature fluctuations in the paleotemperature proxy data. Have we just been lucky that there do not appear to be any clear multiple attractor temperatures?

Figure 4. A chart of paleotemperature of Earth. For 3 billion years Earth’s average surface temperature has fluctuated in a range of less than 30C.

An obvious question is whether his model reflects reality. The random nature of the feedbacks coupled with the temperature perturbations might lead to many situations where even small temperature perturbations will tip the surface temperature beyond the acceptable range. As we can see from Figure 2, the upper-temperature attractor is within 10C of a runaway temperature increase, making habitability susceptible to even relatively small temperature perturbations.

Fortunately, the model code has been placed online and the source code available to experiment with.

Observing several runs it became clear that the model would quickly fail if the current temperature at an attractor temperature was near the boundary range so that even a modest temperature perturbation could push the temperature outside the range. How serious was this effect?

Figure 5. The most benign model. The 2 feedbacks are at the temperature range extremes and result in an attractor at 25C that is maintained across the temperature range. The long-term temperature forcing is set to 0. Only the infrequent large temperature perturbations, average size 32C are operative.

I created an experiment (also suggested at bottom of page 4 of the paper) where the planet would always have the most favorable conditions for a stable surface temperature. Just 2 feedbacks were created at each end of the range, with the calculated attractor in the middle of the range at 25 C, so that any perturbation would have to exceed 35C in either direction to exit the inhabitable conditions. I removed the long-term feedback too. I also removed both the small and medium-sized perturbations, leaving just the rare, large perturbations. The probability of timing and size of the perturbations was left as per the model. By starting the planet’s temperature at the attractor, the inhabitable conditions would be maintained at the attractor temperature unless a random large perturbation exceeded the 35C size.

The results for different perturbation probabilities are shown in figure 6. The average survival time of planetary runs and the %age survival plotted against perturbation probability demonstrate what might be intuitively guessed. Tyrrell’s purely mechanistic run (H2) with optimal feedback and no perturbations had all planet runs complete the 3 By survival. This is consistent with figure 6 where expected large perturbations = 0.

Figure 6. Survival times and %age survival of planets without an attractor and with a single attractor at 25C. With an attractor, survival times are greatly enhanced, especially as the expected number of perturbations increases.

Inspection of the model’s large temperature perturbation distribution indicated that the average size was 32C with a standard deviation of 16C. For the stable model planet I was testing, the temperature would be perturbed beyond either range boundary by a value of just 0.25 standard deviations, i.e. that about 40% of all randomly selected sample perturbations would trigger a surface temperature outside the inhabitable range. When that happened depended on the random timing and would dictate the survival time of inhabitable temperatures. As a control to determine the frequency of perturbations, a model world was created with no attractor temperature so that it sat on a temperature knife edge. Any perturbation would cause runaway heating or cooling. The impact of the lack of the stabilizing temperature attractor on survival time and average % of planets surviving for 3 By is evident.

Given the importance of the large perturbations, just how reasonable are the size of the perturbations and the maximum inhabitable temperature range.?

It is hypothesized that the “Snowball Earth” temperature ranged from deepest glaciation to a temperature maximum could have been as high as 100C (-50C to 50C). The chart of paleotemperature suggests for over the last few billion years an average surface temperature range of 26C (-10C to16C). That the Cryogenian glaciation period encompassing the “Snowball Earth” could have had a surface temperature of -50C, yet life quickly reemerged more vigorous than ever (the Cambrian “explosion”) after the glaciers melted, suggests that the lower temperature bound of -10C may be too conservative. As for the upper bound, it has been suggested that the Archaean eon may have had surface water temperatures of 70-80C. While most complex life has an effective upper limit of 60C, extremophiles have been found at 122C. For complex life, while the resilient tardigrades can withstand extreme temperatures for short periods, the inhabitable surface range is reasonable for complex life. However, we should bear in mind that ecological refugia can provide safety for complex life, for example around undersea vents to resist freezing, and migration to the poles to escape the equatorial temperatures and hence live in regimes that remain below the average surface temperature.

These points were acknowledged in the Tyrrell paper that discussed the limitations and caveats to the model.

Tyrrel:

Geographical variability implies that more extreme average global surface temperatures might be required to force extinction everywhere. Microbial life can potentially survive periods of inhospitable surface conditions within refuges, such as in subsurface rocks or deep in an ice-covered ocean at hydrothermal vents, emerging later to recolonise the surface; evidence from Neoproterozoic Snowball Earth events suggests however that eukaryotic photosynthetic algae persisted through the events and therefore that surface habitability was maintained at some locations. Other environmental conditions can affect habitability, but only temperature (and therefore water availability) are considered here.

Dynamic models are often unstable without tuning. The simplest example is Wolfram’s linear cellular automata with 3 cell states determining the next cell state. With just 8 possible rules for the 3 state combinations, there are 256 combinations of rules, yet just 6 (2.3%) do not converge on static states. The random feedback combinations may reflect a similar outcome, but where the majority of conditions will easily slip out of the inhabitable temperature range, rather than the benign experimental planet conditions I tested.

Conclusion

Dr. Malcolm (Jurassic Park):

“Life Finds a Way”

Given the results from my experiment with the optimal feedbacks for a stable climate, if feedbacks are more stabilizing on average than the hypothesized randomly assigned feedbacks, planets with inhabited surfaces possibly may not be quite as rare as the author’s model indicates. Tyrrel notes that average surface temperatures hide the variability of temperatures and exclude possible refugia, such as undersea hot vents, and lithosphere life warmed by the planet’s core. If we can accept that the Earth was populated by unicellular bacteria and eukaryotes for most of its history, and that the Earth’s complex biota may have even taken a major loss during the Cryogenian period, it seems likely that inhabitable worlds will have some sort of life assuming abiogenesis is easily achievable. While our climate history may be a lucky chance, history does not seem to indicate some attractor temperatures, but rather a single attractor that is subject to GHG source and sink imbalances that last for some time. The hypothesized extreme volcanism that ended the Permian resulted in the greatest extinction event in the fossil record and lasted for 2 million years. Our current fossil fuel burning that is increasing the atmospheric CO2 levels while very much like a temperature shock is not believed to be able to cause a runaway heating as happened on Venus. However, it is suggested that sometime in the next billion years, the Earth’s atmosphere will need to have no CO2 to stay habitable. Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.

Once life starts, it is tenacious. A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”. However, there may be many more possible wrinkles to the sustainability of habitability, and eventually, surface life may be unable to survive. For subsurface life, the story may be very different. The lithospheric life might survive all other life until our sun destroys Earth billions of years in the future.

It would be interesting to modify the model so that rather than stopping when the temperature is outside the range, that the instances of these periods are recorded as possible reset conditions for refuge (e.g. lithosphillic) life to restart the evolutionary process rather than assuming the planet is “sterile”.

Time will tell when astrophysicists have cataloged and characterized a statistically useful sample of Earth-like worlds in the HZ that can test the model hypothesis of rare survival of surface inhabitablity over billions of years.

References

1. Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.

2. Ibid Supplementary Information

3. Kasting, James, et al “How Climate Evolved on the Terrestrial Planets”, Scientific American, (1988)

4. Berner, R. A. & Caldeira, K. “The need for mass balance and feedback in the geochemical carbon cycle”. Geology 25, 955–956 (1997).

Seafloor Volcanoes on Europa?

What’s going on on the floor of Europa’s ocean? It’s hard to imagine a place like this, crushed under the pressure of 100 kilometers or more of water, utterly dark, although I have to say that James Cambias does wonders with an ice moon ocean in his novel A Darkling Sea (Tor, 2014). Science fiction aside, Europa Clipper is in queue for a 2024 launch, and we can anticipate a flurry of new studies that feed into plans for the mission’s scientific investigations. The latest of these puts Clipper on volcano watch.

The work deploys computer modeling to show that volcanic activity seems to have occurred recently on Europa’s seafloor. The concept is that there may be enough internal heat to cause melting — at least in spots — of the rocky interior, which would produce the needed results.

How this heating affects the moon is deduced from the 3D modeling of heat production and transfer in the paper, which was recently published in Geophysical Research Letters. The lead author is Marie Běhounková (Charles University, Czech Republic), who describes the astrobiological import of the team’s results:

“Our findings provide additional evidence that Europa’s subsurface ocean may be an environment suitable for the emergence of life. Europa is one of the rare planetary bodies that might have maintained volcanic activity over billions of years, and possibly the only one beyond Earth that has large water reservoirs and a long-lived source of energy.”

Image: Scientists’ findings suggest that the interior of Jupiter’s moon Europa may consist of an iron core, surrounded by a rocky mantle in direct contact with an ocean under the icy crust. New research models how internal heat may fuel volcanoes on the seafloor. Credit: NASA/JPL-Caltech/Michael Carroll.

With massive Jupiter close at hand, it’s no surprise that gravitational interactions should account for heat production in Europa’s mantle, for the rocky interior flexes in the course of the moon’s orbit. The paper drills down into how this flexion operates, where the resulting heat dissipates, and how it results in melting in the mantle.

In Europa Clipper terms, it’s useful to learn that volcanic activity is most likely to occur near the poles, for this is where the most heat is produced from these effects. On top of this, we learn that the volcanic activity likely to be produced here is long-lived, giving life the opportunity to evolve. As an analogue, we can imagine hydrothermal systems like those at the bottom of Earth’s oceans, where seawater and magma interact.

In the absence of sunlight, the resultant chemical energy supports life on the seafloor and could conceivably do so on Europa. Europa Clipper will measure the moon’s gravity and magnetic field, looking for anomalies toward the poles that could confirm the presence of volcanic activity. Long-time Europa specialist and Europa Clipper project scientist Robert Pappalardo (JPL) sees all this as fodder for continuing investigation:

“The prospect for a hot, rocky interior and volcanoes on Europa’s seafloor increases the chance that Europa’s ocean could be a habitable environment. We may be able to test this with Europa Clipper’s planned gravity and compositional measurements, which is an exciting prospect.”

Europa Clipper should reach the Jupiter system in 2030, orbiting the giant planet and performing numerous close flybys of Europa as it surveys the surface, samples any gases that may have been emitted by the exchange of material from below the ocean, and possibly takes advantage of plumes of water vapor. If water is indeed welling up on occasion from below, we may be able to learn a good deal about the interior ocean without having the need to drill down through kilometers of ice.

The paper is Běhounková et al., “Tidally Induced Magmatic Pulses on the Oceanic Floor of Jupiter’s Moon Europa,” Geophysical Research Letters Vol. 48, Issue 3 (22 December 2020). Abstract.

Spiral Galaxies: A Common Path to Formation?

The galaxy UGC 10738 resonates with the galaxy described yesterday — BRI 1335–0417 — in that it raises questions about how spiral galaxies form. In fact, the team working on UGC 10738 thinks it goes a long way toward answering them. That’s because what we see here is a cross-sectional view of a galaxy much like the Milky Way, one that has both ‘thick’ and ‘thin’ disks like ours. The implication is that these structures are not the result of collisions with smaller galaxies but typical formation patterns for all spirals.

Nicholas Scott and Jesse van de Sande (ASTRO 3D/University of Sydney) led the study, which used data from the European Southern Observatory’s Very Large Telescope in Chile. As you can see from the image below, the galaxy, some 320 million light years away, presents itself to us edge on, offering a cross-section of its structure. Key to the work was the team’s assessment of stellar metallicity, as van de Sande explains:

“Using an instrument called the multi-unit spectroscopic explorer, or MUSE, we were able to assess the metal ratios of the stars in its thick and thin discs. They were pretty much the same as those in the Milky Way – ancient stars in the thick disc, younger stars in the thin one. We’re looking at some other galaxies to make sure, but that’s pretty strong evidence that the two galaxies evolved in the same way.”

Image: Galaxy UGC 10738, seen edge-on through the European Southern Observatory’s Very Large Telescope in Chile, revealing distinct thick and thin discs. Credit: Jesse van de Sande/European Southern Observatory.

Published in Astrophysical Journal Letters, the study argues that the thin and thick disks found in the Milky Way — and the similar structures found in UGC 10738 — indicate that rather than being the result of chance collisions with other galaxies, the different disks reveal a standard path of galaxy formation and evolution. They would be unlikely to be repeated elsewhere if the result of a rare and violent merger.

The ‘thick’ and ‘thin’ disks are interesting features, with the former primarily made up of ancient stars that show a low ratio of iron to hydrogen and helium. Tracing the metallicity of the thin disk stars reveals that they are younger and contain more metals. Our Sun is a thin disk star, and as the authors note, it is made up of about 1.5% elements heavier than helium (the definition of ‘metals’ in astronomical parlance). Stars of the thick disk show three to ten times less metal content, a striking difference.

Finding the same differences in the two disk structures in other spiral galaxies points to the likelihood that the Milky Way formed in a way common to such galaxies. Says Scott:

“It was thought that the Milky Way’s thin and thick discs formed after a rare violent merger, and so probably wouldn’t be found in other spiral galaxies. Our research shows that’s probably wrong, and it evolved ‘naturally’ without catastrophic interventions. This means Milky Way-type galaxies are probably very common. It also means we can use existing very detailed observations of the Milky Way as tools to better analyse much more distant galaxies which, for obvious reasons, we can’t see as well.”

So we have what the authors describe as a ‘tension’ between two formation scenarios, one stochastic, one common to spiral galaxies. But let’s get into the weeds a little. The paper goes on to widen the sample into other subtypes of galaxy:

This tension is further enhanced once early-type disk galaxies with [α/Fe]-enhanced thick disks are included in the population of present day galaxies with similar formation histories (Pinna et al. 2019b; Poci et al. 2019, 2021). That early-type disk galaxies are found to contain similar abundance patterns as found in the Milky Way (i.e. increased mean [α/Fe] off the plane of the disk) is unsurprising. Such galaxies represent one plausible end point for the Milky Way’s evolutionary history, suggesting a shared and generic evolutionary pathway for disk galaxies.

All of this harks back to Edwin Hubble’s classification of galaxies (the Hubble Sequence). The phrase “early-type” galaxies” refers to elliptical and lenticular galaxies, as opposed to spiral and irregular galaxies, without implying an evolutionary path from one type to another. So the authors are speculating when they talk about an ‘evolutionary end point’ for the Milky Way, but it’s an interesting speculation:

The Milky Way is often identified as a so-called ‘green valley’ galaxy (Mutch et al. 2011), a galaxy already undergoing a transition to the red sequence. When fully quenched (and assuming no dramatic structural changes in the mean time) the Milky Way will likely resemble a lenticular galaxy similar to FCC 170 (Pinna et al. 2019a).

The ‘green valley’ refers to galaxies where star formation has been slowed, either because they have exhausted their reservoirs of gas, or (in younger galaxies) have undergone mergers with other galaxies. Both the Milky Way and Andromeda are assumed to be green valley galaxies of the first kind.

FCC 170 (NGC 1381) is a lenticular galaxy found in the constellation Fornax, about 60 million light years from Earth, a member of the Fornax Cluster (hence the FCC designation, standing for Fornax Cluster Catalogue). As a lenticular, it is in a middle stage between a spiral and an elliptical galaxy, with a large scale disk but lacking spiral arms on the scale of a true spiral galaxy. Is this the Milky Way’s future?

And then, of course, there is the merger with Andromeda to look forward to. How much we have to learn about how galaxies change over time!

Image: This is NGC 1387, a lenticular galaxy likewise in the Fornax Cluster, and better angled to show the lack of spiral structure in such galaxies than FCC 170. Credit: Fabian RRRR. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). CC BY-SA 3.0.

The paper is Scott et al., “Identification of an [α/Fe]—Enhanced Thick Disk Component in an Edge-on Milky Way Analog,” Astrophysical Journal Letters Vol. 913, No. 1 (24 May 2021), L11 (abstract / preprint).

The Most Ancient Spiral Galaxy Yet Found

My fascination with the earliest era of star and galaxy formation leads me to a new paper on an intriguing find. The authors describe the distant object BRI 1335–0417 as “an intensely star-forming galaxy,” and its image as captured by the Atacama Large Millimeter/submillimeter Array (ALMA) is striking. This is a galaxy that formed a mere 1.4 billion years after the Big Bang, making it the most ancient galaxy with spiral structure ever observed.

Spirals make up perhaps 70 percent of the galaxies in our catalogs, but how they form is an open question. Indeed, the proportion of spiral galaxies declines the further back in the evolution of the universe we observe. The spiral structure observed here extends 15,000 light years from the center of the galaxy (about a third the size of the Milky Way), while the total mass of stars and interstellar matter roughly equals our own galaxy.

Image: ALMA image of a galaxy BRI1335-0417 in the Universe 12.4 billion years ago. ALMA detected emissions from carbon ions in the galaxy. There is a compact and bright area in the center of the galaxy, and spiral arms are visible on both sides of it. Credit: ALMA (ESO/NAOJ/NRAO), T. Tsukui & S. Iguchi.

The BRI1335-0417 observations were performed by Satoru Iguchi (SOKENDAI/National Astronomical Observatory of Japan), working with graduate student Takafumi Tsukui, who is lead author of the paper reporting the work. Huge amounts of dust exist here, obscuring the object in visible light, but ALMA’s prowess at detecting radio emissions from carbon ions makes the investigation possible.

Even so, Tsukui points out that getting the size of the galaxy right is a difficult matter:

“As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early Universe, BRI 1335-0417 was a giant.”

The biggest issue is how such a large structure with obvious spiral structure emerged so soon after the Big Bang. This is a ‘starburst’ galaxy: There are areas of active star formation here and gas instabilities in the outer parts of the disk which point to possible interactions with smaller galaxies or other interstellar matter. The paper points out that dusty galaxies with high rates of star formation, like BRI 1335–0417, are the progenitors of elliptical galaxies, with the spiral structure possibly redistributing angular momentum so as to trigger gas inflows into the center of the galaxy.

The authors are quick to note, however, that gas inflows like this are beyond the detection threshold of these observations. What we do see is the possibility of tidal interactions. The authors describe the likelihood of such events:

The high star formation rates of z > 4 [the measure of redshift] galaxies like BRI 1335–0417 are commonly explained as the result of major mergers, which could produce distorted galactic kinematics. We find that BRI 1335–0417 has only slightly disturbed, rotation dominated kinematics, which can be well described by a rotating disk model. This suggests that the high star formation rate must have been maintained long enough for the disk to form after any major merger event. The Q parameter [a measure of stability in a rotating, gaseous accretion disc] shows the outer disk of BRI 1335–0417 is unstable, which could be caused by gas accretion along large-scale filaments of the cosmic web, and/or minor mergers with accreting satellites.

So we have spiral structure, a rotating galactic disk, and a centralized mass. Is BRI 1335–0417 the ancestor of a giant elliptical galaxy? Answering that question of galactic survival and change involves issues of galaxy evolution that remain wide open.

The paper is Tsukui & Iguchi “Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago,” published online in Science 20 May 2021 (abstract).

Lights of the Nightside City

On the matter of city lights as technosignatures, which we looked at on Friday, I want to follow up with Thomas Beatty’s work on the issue in the context of an assortment of nearby stars. Beatty (University of Arizona, Tucson) assumes Earth-like planets examined via direct-imaging by LUVOIR, a future space telescope in planning, or HabEx, a different architecture for a likewise powerful instrument. What he’s done is to take data from the Soumi National Polar-orbiting Partnership satellite to find the flux from city lights and the spectra of currently available lighting. He goes on to model the spectral energy distribution from such emissions as applied to exoplanet settings at various distances.

Why look at city lights in the first place? Because they’re another form of technosignature that may be within the realm of detection, and we’d like to find out what’s possible and what any results would imply. In particular, Beatty reminds us, the National Academies’ Exoplanet Science Strategy and Astrobiology Strategy reports are on record as recommending space-based, direct imaging that is capable of directly detecting emissions from habitable zone planets. This would obviously support biosignature searches but would also open up a hunt for technosignatures.

Technosignature searches can take place within the context of ongoing biosignature investigations on the same planets. Both LUVOIR and HabEx should be capable of this, generating data sets that can be scanned for both biological and technological returns. One area of investigation has been satellites — could we detect satellite constellations in orbit around an Earth-like planet? Large-scale photovoltaic arrays would show a different signature than vegetation on the surface. Various forms of pollution in atmospheres are within LUVOIR’s range, so the field is wide.

A lack of a specific technosignature is itself interesting, as it helps us begin to constrain the field. Just as we started searching for planets around Proxima Centauri by first ruling out gas giants of a certain mass, then Jupiter-class objects, then ice giants of Neptune size, we first learn what is not there and then can specify what remains to be searched for. The lack of SETI detections at radio or optical frequencies, for example, makes it less likely that technological civilizations are broadcasting powerful beacons aimed at us from stars near the Sun, thus paring down earlier possibilities.

But back to city lights, which Jean Schneider and colleagues first studied in 2010 (citation below). We’ve learned through the work of Avi Loeb and Elisa Tabor that artificial illumination from the nightside of Proxima b could be detected, though with great difficulty, by the James Webb Space Telescope. LUVOIR will up the ante and widen the range. Beatty points out that city lights are compelling because they would presumably be long-lived artifacts of a technological culture and would offer a unique spectroscopic signature that is unlike anything produced by natural processes.

Image: This is Figure 1 from the paper. Caption: Figure 1. The nightside of Earth shows significant emission from city lights in the optical. This is a composite, cloud-free, image of Earth’s city lights compiled using Day/Night Band observations taken using the Visible Infrared Imaging Radiometer Suite instrument on the Soumi National Polar-orbiting Partnership satellite (Roman et al. 2018). Searching for the emission from city lights is a compelling technosignature because it requires very little extrapolation from current conditions on Earth, should be relatively long-lived presuming an urbanized civilization, and offers a very distinct spectroscopic signature that is difficult to cause via natural processes. This places the emission from nightside city lights high on the list of potential technosignatures to consider (Sheikh 2020).

Beatty considers the detectability of city lights first as a function of stellar distance and the amount of a planet’s surface covered by urbanization on Earth-like planets around G-, K- and M-dwarf stars. He then calculates their detectability on planets orbiting stars within 10 parsecs of the Sun, and finally estimates detectability on two dozen known, potentially habitable planets around stars close to the Sun. The tables within this paper are worth scanning, but here are some of the highlights:

We learn that LUVOIR should be able to detect city lights on Proxima b at an urbanization level of 0.5% (10 times Earth’s). Lalande 21185 b, Luyten’s Star b and Tau Ceti e and f would show detectable emission from city lights at urbanization levels of 3% to 10% in LUVOIR imaging.

Detection of city lights should be easiest on M-dwarf planets, and Beatty notes in particular planets around Proxima Centauri, Barnard’s Star, and Lalande 21185, but he points out how quickly the habitable zone around this kind of star moves within the inner working angle (IWA) of LUVOIR with distance, making it beyond the capacity of the instrument.

Earth-analog planets around Sun-like stars can be imaged at greater distances, but the distance drives the minimum detectable urbanization fraction higher. Here Beatty suggests Alpha Centauri A and B, Epsilon Eridani, Tau Ceti and Epsilon Indi A as potential targets.

And this brings up memories of Isaac Asimov’s global city Trantor: What Beatty calls an ‘ecumenopolis’ — a planet-wide city — would be detectable at much larger distances. The author surveys 80 nearby stars on which such a city would be at least marginally detectable.

Thus the work moves from the study of Earth’s urbanization fraction (0.05%) up to an ecumenopolis, showing how detectability scales with the amount of planetary surface covered. The paper assumes 100 hours of observing time for generic Earth-class planets around stars within 10 parsecs. Earth itself would not be detectable by LUVOIR in this range, but planets around M-dwarfs near the Sun would show detection for urbanization levels of 0.4% to 3%. City lights on planets orbiting nearby Sun-like stars would be detectable at urbanization levels in the range of 10 percent.

From the paper:

The possibility of directly detecting technosignatures on the surfaces of potentially habitable exoplanets is thus starting to be in the realm of practicality. Perhaps unsurprisingly, the 15m LUVOIR A architecture would be the most capable observatory for detecting city lights on the nightsides of nearby exoplanets, though LUVOIR B [smaller than LUVOIR A) or HabEx with a starshade would also have significantly sized detection spaces. Much of this proposed capability has been spurred by the goals of characterizing the atmospheres of and detecting biosignatures on potentially habitable exoplanets, but it also would afford us the opportunity to search for other, technological, signs of life.

In short, we’re going to be looking hard at many of these planets within a few decades as we search for biosignatures. The same data may show technosignatures, the strength of which we need to examine to see what’s possible. We are simply defining the limits of the search.

The paper is Beatty, “The Detectability of Nightside City Lights on Exoplanets,” in process at Monthly Notices of the Royal Astronomical Society (abstract). The Schneider et al. paper is “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, No. 1 (22 March 2010). Abstract. Thanks to my friend Antonio Tavani for the pointer to this paper.

Proxima Centauri b: Artificial Illumination as a Technosignature

Our recent look at the possibility of technosignatures at Alpha Centauri is now supplemented with a new study on the detectability of artificial lights on Proxima Centauri b. The planet is in the habitable zone, roughly similar in mass to the Earth, and of course, it orbits the nearest star, making it a world we can hope to learn a great deal more about as new instruments come online. The James Webb Space Telescope is certainly one of these, but the new work also points to LUVOIR (Large UV/Optical/IR Surveyor), a multi-wavelength space-based observatory with possible launch in 2035.

Authors Elisa Tabor (Stanford University) and Avi Loeb (Harvard) point out that a (presumably) tidally locked planet with a permanent nightside would need artificial lighting to support a technological culture. As we saw in Brian Lacki’s presentation at Breakthrough Discuss (see Alpha Centauri and the Search for Technosignatures), coincident epochs for civilizations developing around neighboring stars are highly unlikely, making this the longest of longshots. On the other hand, a civilization arising elsewhere could be detectable through its artifacts on worlds it has chosen to study.

We learn by asking questions and looking at data. In this case, asking how we would detect artificial light on Proxima b involves factoring in the planet’s radius, which is on the order of 1.3 Earth radii (1.3 R⊕) as well as that of Proxima Centauri itself, which is 0.14 that of the Sun (0.14 R⊙). We also know the planet is in an 11 day orbit at 0.05 AU. Other factors influencing its lightcurve would be its albedo and orbital inclination. Tabor and Loeb use recent work on Proxima Centauri c’s inclination (citation below) to ballpark an orbital inclination for the inner world.

Image: Northern Italy at night. City lights are an obvious technosignature, but can we detect them at interstellar distances? Credit: NASA/ESA.

The question then becomes whether soon to be flown technology like the James Webb Space Telescope could detect artificial lights if they were present at Proxima b. The authors detail in this paper their calculation of the lightcurves that would be involved, using two scenarios: Artificial lighting with the same spectrum found in LEDs on Earth, and a narrower spectrum leading to the “same proportion of light as the total artificial illumination on Earth.” The calculations draw on open source software source code called Exoplanet Analytic Reflected Lightcurve (EARL), and likewise deploy the JWST Exposure Time Calculator (ETC) to estimate the feasibility of detection.

What Loeb and Tabor find is that JWST could detect LED lighting “making up 5% of stellar power” with 85 percent confidence — in other words, 5% of the power the planet would receive at its orbital distance from Proxima Centauri. That would mean our space telescope could find a level of illumination from LEDs that is 500 times more powerful than found on Earth.

To detect the current level of artificial illumination (including but not limited to LEDs) on Earth, the spectral band would have to be 103 times narrower. “ In either case,” the authors add, “JWST will thus allow us to narrow down the type of artificial illumination being used.”

All of this demands maximum performance from JWST’s Near InfraRed Spectrograph (NIRSpec). Much depends upon what methods a civilization at Proxima b might use. From the paper:

Proxima b is tidally locked if its orbit has an eccentricity below 0.06, where for reference, the eccentricity of the Earth’s orbit is 0.017 (Ribas et al. 2016). If Proxima b has a permanent day and nightside, the civilization might illuminate the nightside using mirrors launched into orbit or placed at strategic points (Korpela et al. 2015). In that case, the lights shining onto the permanent nightside should be extremely powerful, and thus more likely to be detected with JWST.

That last comment calls to mind Karl Schroeder’s orbital mirrors lighting up brown dwarf planets in his novel Permanence (2002). A snip from the book, referring to a brown dwarf planet named Treya as seen by the protagonist, Rue:

A pinprick of light appeared on the limb of Treya and quickly grew into a brilliant white star. This seemed to move out and away from Treya, which was an illusion caused by Rue’s own motion. Treya’s artificial sun did not move, but stayed at the Lagrange point, bathing an area of the planet eighty kilometers in diameter with daylight. The sun was a sphere of tungsten a kilometer across. It glowed with incandescence from concentrated infrared light, harvested from Erythrion [the brown dwarf] by hundreds of orbiting mirrors. If it were turned into laser power, this energy could reshape Treya’s continents— or launch interstellar cargoes.

A flat line of light appeared on Treya’s horizon. It quickly grew into a disk almost too bright to look at. When Rue squinted at it she could make out white clouds, blue lakes, and the mottled ochre and green of grassland and forests. The light was bright enough to wash away the aurora and even make the stars vanish. Down there, she knew, the skies would be blue.

Back to Proxima b: The LUVOIR instrument should be able to confirm the presence or lack of artificial illumination with greater precision, serving as a follow-up to JWST observations with significantly higher performance. Loeb has previously worked with Manasvi Lingam to show the likelihood of detecting a spectral edge in the reflectance of photovoltaic cells on the planet’s dayside, so in terms of technosignatures, we’re learning what we will be able to identify based on a growing set of scenarios for any civilization there.

The paper is Tabor & Loeb, “Detectability of Artificial Lights from Proxima b,” (preprint). The paper on photovoltaic cells is Loeb & Lingam, “Natural and Artificial Spectral Edges in Exoplanets,” Monthly Notices of the Royal Astronomical Society Vol. 470, Issue 1 (September 2017), L82-L86 (abstract). The work on Proxima c’s orbital inclination is Kervella, Arenou & Schneider, “Orbital inclination and mass of the exoplanet candidate Proxima c,” Astronomy & Astrophysics Vol. 635, L14 (March 2020). Abstract / Full Text.

Exploring Ice Giant Oceans

Laboratory work on Earth is, as we saw yesterday, leading to hypotheses about how planets form and the effect of these processes on subsequent life. Whether in our own outer Solar System or orbiting other stars, planets in the ‘ice giant’ category, like Uranus and Neptune, remain mysterious, with Voyager 2’s flybys of the latter the only missions that have gone near them. We also know that sub-Neptune planets are common, many of these doubtless sharing the characteristics of their larger namesake.

Thus recent experiments probing ice giant interiors catch my eye this morning. Involving an international team of collaborators, the work looks at the interactions between water and rock that we would expect to find in the extreme conditions inside an ice giant. Planets like Uranus and Neptune are thought to house most of their mass in a deep water layer, a dense fluid overlaying a rocky core, a sharp departure from terrestrial worlds. What happens at that interface is ripe for examination.

The experiments were performed at Arizona State University’s DanShimLab, which is dedicated to the study of planetary materials at a wide range of pressures and temperatures using diamond-anvil and shockwave techniques. To probe this environment, the scientists immersed the rock-forming minerals olivine and ferropericlase in water and then compressed them to high pressures using a diamond-anvil. Heating the sample with a laser, the team could then track the water/mineral reaction under these conditions by way of X-ray measurements.

The result: High concentrations of magnesium, with implications for the composition of oceans much different from Earth’s, as study co-author Sang-Heon Dan Shim (Arizona State University) explains:

“We found that magnesium becomes much more soluble in water at high pressures. In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth’s ocean. If an early dynamic process enabled a rock–water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet.”

Image: A diamond-anvil (top right) and laser were used in the lab on a sample of olivine to reach the pressure-temperature conditions expected at the top of the water layer beneath the hydrogen atmosphere of Uranus (left). In this experiment, the magnesium in olivine dissolved in the water. Credit: Shim/ASU.

Laser-heated diamond anvil cells can create pressures in the range of 1-5,000,000 bars by compressing materials between diamond ‘anvils’ that are transparent to X-rays, infrared and visible light. The lab’s laser heating systems can take samples to 1,000-5,000 K, all by way of exploring how materials behave under conditions thought to exist in planetary interiors. Thus the range of such laboratory work extends through rocky worlds and into the realm of not just the ice giants but gas giants like Jupiter.

Given the lab’s finding of high concentrations of magnesium under conditions of high pressure and temperature, Shim argues that the mineral may become as soluble in ice giant interiors as salt is in Earth’s oceans. Conceivably, this finding could explain why the atmosphere of Uranus is considerably colder than Neptune’s, for magnesium in larger amounts could block heat from escaping the interior. “This magnesium-rich water may act like a thermal blanket for the interior of the planet,” says Shim.

Image: An electron microscopy image of the olivine sample shows a large empty dome structure where magnesium under high-pressure water precipitated as magnesium oxide. Credit: Kim et al.

Oceans rich in magnesium may thus be common in ice giants, with a thick layer of water covering a rocky interior. Moreover, the idea that the interior of water worlds is sharply differentiated between rock and water has been challenged in recent work, with implications for the thermal evolution of these planets. To probe deeper into these matters, the study calls for an examination of other icy materials like CO2 and NH3, but note this cautionary remark in its conclusion:

Extrapolation of our results beyond the pressure range covered in our experiments should [be] treated with caution because of possible changes in the properties of H2O at very high pressures. Nevertheless, the models based on our experiments demonstrate that geochemical cycles and thermal history of water-rich planets could be sensitive to the size of the planet because of pressure-dependent chemical processes.

We have a useful methodology here to extend the study of the geochemical cycle on a range of planetary interiors. We have much to learn, for as the paper points out, the interactions of major rock-forming minerals at the interface between ocean and rock in ice giants have rarely been explored at the high pressures used in these experiments.

The paper is Kim et al., “Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets,” Nature Astronomy 17 May 2021 (abstract).

Planet Formation Modes as a Key to Habitability

While a planet’s position in the habitable zone is thought critical for the development of life like ourselves, new work out of Rice University suggests an equally significant factor in planetary growth. Working at a high-pressure laboratory at the university, Damanveer Grewal and Rajdeep Dasgupta have explored how planets capture and retain key volatiles like nitrogen, carbon and water as they form The team used nitrogen as a proxy for volatile distribution in a range of simulated protoplanets.

Two processes are under study here, the first being the accretion of material in the circumstellar disk into a protoplanet, and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope. The interplay between these processes is found to determine which volatiles the subsequent planet retains.

Most of the nitrogen is found to escape into the atmosphere during differentiation and is then lost to space as the protoplanet cools or, perhaps, collides with other protoplanets during the turbulent era of planet formation. The data, however, demonstrate the likelihood of nitrogen remaining in the metallic core. Says Grewal:

“We simulated high pressure-temperature conditions by subjecting a mixture of nitrogen-bearing metal and silicate powders to nearly 30,000 times the atmospheric pressure and heating them beyond their melting points. Small metallic blobs embedded in the silicate glasses of the recovered samples were the respective analogs of protoplanetary cores and mantles.”

Nitrogen, the researchers learned, is distributed in different ways between the core, the molten silicate shell and the atmosphere, with the extent of this fractionation being governed by the size of the body. The takeaway: If the rate of differentiation is faster than the rate of accretion for planetary embryos of Moon or Mars-size, then the planets that form from them will not have accreted enough volatiles to support later life.

Earth’s path would have been different. The scientists believe that the building blocks of Earth grew quickly into planetary embryos before they finished differentiating, forming within one to two million years at the beginning of the Solar System. The slower rate of differentiation allowed nitrogen, and other volatiles, to be accreted. Adds Dasgupta:

“Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth’s nitrogen budget. This work shows there’s much greater affinity of nitrogen toward core-forming metallic liquid than previously thought.”

Image: Nitrogen-bearing, Earth-like planets can be formed if their feedstock material grows quickly to around moon- and Mars-sized planetary embryos before separating into core-mantle-crust-atmosphere, according to Rice University scientists. If metal-silicate differentiation is faster than the growth of planetary embryo-sized bodies, then solid reservoirs fail to retain much nitrogen and planets growing from such feedstock become extremely nitrogen-poor. Credit: Illustration by Amrita P. Vyas/Rice University.

This work takes the emphasis off the stellar nebula and places volatile depletion in the context of processes within the rocky body in formation, especially the affinity of nitrogen toward metallic cores. Here’s how the paper sums it up:

…we show that protoplanetary differentiation can explain the widespread depletion of N in the bulk silicate reservoirs of rocky bodies ranging from asteroids to planetary embryos. Parent body processes rather than nebular processes were responsible for N (and possibly C) depleted character of the bulk silicate reservoirs of rocky bodies in the inner Solar System. A competition between rates of accretion versus rates of differentiation defines the N inventory of bulk planetary embryos, and consequently, larger planets. N budget of larger planets with protracted growth history can be satisfied if they accreted planetary embryos that grew via instantaneous accretion.

And the nebular conclusion:

Because most of the N in those planetary embryos resides in their metallic portions, the cores were the predominant delivery reservoirs for N and other siderophile volatiles like C. Establishing the N budget of the BSE [bulk silicate Earth] chiefly via the cores of differentiated planetary embryos from inner and outer Solar System reservoirs obviates the need of late accretion of chondritic materials as the mode of N delivery to Earth.

Rajdeep Dasgupta, by the way, is principal investigator for the NASA-funded CLEVER Planets project (one of the teams in the Nexus of Exoplanetary Systems Science — NExSS — research network). CLEVER Planets, according to its website, is “working to unravel the conditions of planetary habitability in the Solar System and other exoplanetary systems. The overarching theme of our research is to investigate the origin and cycles of life-essential elements (carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus – COHNSP) in young rocky planets.”

All of which reminds us that the essential elements for life must be present no matter where a given planet exists in its star’s habitable zone.

The paper is Grewal et al., “Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets,” Nature Geoscience 10 May 2021 (abstract / preprint).

Alpha Centauri and the Search for Technosignatures

Is there any chance we may one day find technosignatures around the nearest stars? If we were to detect such, on a planet, say, orbiting Alpha Centauri B, that would seem to indicate that civilizations are to be found around a high percentage of G- and K-class stars. Brian Lacki (UC-Berkeley) examined the question from all angles at the recent Breakthrough Discuss, raising some interesting issues about the implications of technosignatures, and the assumptions we bring to the search for them.

We’re starting to consider a wide range of technosignatures rather than just focusing on Dysonian shells around entire stars. Other kinds of megastructure are possible, some perhaps so exotic we wouldn’t be sure how they operated or what they were for. Atmospheres could throw technosignatures by revealing industrial activity along with their potential biosignatures. We could conceivably detect power beaming directed at interstellar spacecraft or even an infrastructure within a particular stellar system. One conceivable technosignature, rarely mentioned, is a world that has been terraformed.

All this takes us well beyond conventional radio and optical SETI. But let’s take the idea, as Lacki does, to Alpha Centauri, which we can begin by noting that in the past several years, Proxima Centauri b, that promising world in the habitable zone of the nearest red dwarf, has found its share of critics as a possible home to advanced life, if not life itself.

Michael Hippke noted in a 2019 paper that rocket launch to orbit from a super-Earth would be difficult, possibly inhibiting a civilization there from building a local space infrastructure. Milan Ćirković and Branislav Vukotić asked in 2020 whether the frequent flare activity of Proxima Centauri would inhibit radio technologies altogether. Whether abiogenesis could occur under these kinds of flare conditions remains unknown.

Alpha Centauri A and B, the central binary, offer a much more benign environment, both stars being exceedingly quiet at radio wavelengths. Moreover, we have known since the 1990s that habitable orbits are possible around each of them. We have no knowledge about whether the evolution of life would necessarily lead to intelligence; as Lacki pointed out, this is an empirical question — we need data to answer it.

The Drake Equation points to further unresolved issues. In what portion of a star’s lifespan would we expect technological cultures to emerge? Our own civilization has used radio for about a century — one part in 100,000,000 of the lifespan of the Sun. Hence the slide below, which is telling in several ways. Lacki refers here to temporal coincidence, meaning that we might expect societies around other stars to be separated in time, not just in space. Hence this simple graph of a very deep subject.

Deep time always takes getting used to, no matter how many times we think we’ve gotten a handle on figures like 4.6 billion years or, indeed, 13.8 billion years, the lifetimes of the Sun and cosmos respectively. I stared at this figure for some time. Lacki has arbitrarily placed a civilization at Centauri B, as shown by the vertical line, and another at Centauri A and C. Our own is shown in its known place in the Sun’s lifetime, except for the striking fact that the thickness of humanity’s timeline on the chart corresponds to a lifetime of 10 million years. If we wanted a line showing our 100 years of technological use — i.e., radio — the line would have to be 100,000 times thinner.

The odds that the lines of any two stars would coincide seem infinitely small, unless we are talking about societies that can persist over many millions of years. But here we can begin to turn the question around. We might want to rethink nearby technosignatures if we remind ourselves that what they represent is not the civilization itself, but the works it created, which might greatly outlive their builders. Objects like Dyson spheres would seem to fit this category and would exist at planetary scale.

Small artifacts can also last for vast periods — our own Voyagers will be intact for millions of years — though finding them would be an obvious challenge (here it’s worth thinking about the controversy over ‘Oumuamua. If a piece of dead technology were to pass near the Sun, would we be able to recognize its artificial nature? I simply raise the question — I remain agnostic on the question of ‘Oumuamua itself).

But can we be sure we are the first intelligent technological society on Earth? It’s worth considering whether we would know it if an advanced culture had existed hundreds of millions of years ago, perhaps on Earth or a different planet in the Solar System. Looking forward, if we go extinct, will another intelligent species evolve? We don’t know the answer to these questions, and the depth of our ignorance is shown by the fact that we can’t say for sure that intelligence might not evolve over and over again.

If interstellar flight is possible, and it seems to be, we can consider the possibility of intelligence spreading throughout the galaxy, perhaps via self-reproducing von Neumann probes. Even with very slow interstellar velocities, the Milky Way could be settled in relatively short order, astronomically speaking. Michael Hart pointed this out in the 1970s, and Frank Tipler argued in 1980 that at current spacecraft speeds alone, self-replicating probes could colonize the galaxy in less than 300 million years.

Sending physical craft to other stars may be difficult, but there are advantages. So-called Bracewell probes could be deployed that would wait for evidence of intelligence and report back to the home system, as well as carrying inscribed messages intended for the target system. This is Jim Benford’s ‘lurker’ scenario, one which he proposes to explore by searching nearby objects in our own system including the Moon. After all, if they might exist elsewhere, there may be a lurker here.

We don’t know how long a probe like this might remain active, but as a physical artifact, it remains a conceivably detectable object for millions, perhaps billions of years. Finding such a probe in our own system would imply similar probes around other stars and, indeed, the likelihood of a civilization that has spread widely in the galaxy. We might well wonder whether a kind of galactic Internet might exist in which information relays around stars are common, perhaps using gravitational lensing.

In other words, the question of technosignatures at Alpha Centauri doesn’t necessarily imply anything found there would have come from a civilization that originated in that system. The same civilization might have seeded stars widely in the Orion Arm and beyond.

All of these ‘ifs’ define the limits of our knowledge. They also point to a case for looking for technosignatures no matter what our intuitions are about their existence at Alpha Centauri. A lack of obvious technosignatures in our system would imply a similar lack around the nearest stars, but we haven’t run the kind of fine-grained search for artifacts that would find them on our own Moon, much less on nearby smaller objects.

Image: How quickly would a single civilization using self-replicating probes spread through a galaxy like this one (M 74)? Moreover, what sort of factors might govern this ‘percolation’ of intelligence through the spiral? We’ll be looking further at these questions in coming days. Image credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

Intuition says we’re unlikely to find a technosignature at Alpha Centauri. But I return to the chart on civilizational overlap reproduced above. To me, the greatest take-away here is the placement of our own civilization within the realm of deep time. Given how short the lifetime of technology has been on Earth, I’m reminded viscerally of how precious — and perhaps rare — intelligence is as an emerging facet of a universe becoming aware of itself. That’s true no matter what we find around the nearest stars.

Voyager: A Persistent Clue to the Density of the Interstellar Medium

What are the long-lasting waves detected by Voyager 1? Our first working interstellar probe — admittedly never designed for that task — is operating beyond the heliosphere, which it exited back in 2012. A paper just published in Nature Astronomy explores what’s going in interstellar space just beyond, but still affected by, the heliosphere’s passage through the Local Interstellar Medium (LISM).

We have a lot to learn out here, for even as we exit the heliosphere, the picture is complex. The so-called Local Bubble is a low-density region of hot plasma in the interstellar medium, the environment of radiation and matter — gas and dust — that exists between the stars. Within this ‘bubble’ exists the Local Interstellar Cloud (LIC), about 30 light years across, with a slightly higher hydrogen density flowing from the direction of Scorpius and Centaurus. The Sun seems to be within the LIC near its boundary with the G-cloud complex, where the Alpha Centauri stars reside.

Image: Map of the local galactic neighborhood showing the Sun located near the edge of our local interstellar cloud (LIC). Alpha-Centauri is located just over 4 light-years away in the neighboring G-cloud complex. Outside these clouds, the density may be lower than 0.001 atoms/cc. Our Sun and the LIC have a relative velocity of 26 km/sec. Credit: JPL.

But if the interstellar medium is a sparse collection of widely spaced particles and radiation, it proves to be anything but quiet. We learn this from Voyager 1’s Plasma Wave Subsystem, which involves two antennae extending 30 meters from the spacecraft (see image below). What the PWS can pick up are clues to the density of the medium that show up in the form of waves. Some are produced by the rotation of the galaxy; others by supernova explosions, with smaller effects from the Sun’s own activity.

Vibrations of the ionized gas — plasma — in the interstellar medium have been detectable since late 2012 by Voyager 1 in the form of ‘whistles’ that show up only occasionally, but offer ways to study the density of the medium. The new work in Nature Astronomy, led by Stella Koch Ocker (Cornell University), sets about finding a more consistent measure of interstellar medium density in the Voyager data.

Image: An illustration of NASA’s Voyager spacecraft showing the antennas used by the Plasma Wave Subsystem and other instruments. Credit: NASA/JPL-Caltech.

A weak signal appearing at the same time as a ‘whistle’ in the 2017 Voyager data seems to have been the key finding. Ocker describes it as “very weak but persistent plasma waves in the very local interstellar medium.” When whistles appear in the data, the tone of this plasma wave emission rises and falls with them. Adds Ocker:

“It’s virtually a single tone. And over time, we do hear it change – but the way the frequency moves around tells us how the density is changing. This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far. This provides us with the most complete map of the density and the interstellar medium as seen by Voyager.”

So we have an extremely useful instrument, Voyager 1’s Plasma Wave Subsystem, continuing to return data with increasing distance from the Sun. Analyzing the data over time, we learn that the electron density around the spacecraft began rising in 2013, just after its exit from the heliosphere, and reached current levels in 2015. These levels, which persist to the end of 2020 through the dataset, show a 40-fold increase in electron density. Up next for Ocker and team is the development of a physical model of the plasma wave emission that will offer insights into its proper interpretation.

As we begin to think seriously about interstellar probes in this century, it’s striking how much we have to learn about the medium through which they will pass. Voyager 1 is helping us learn about conditions immediately outside the heliosphere. A probe sent to Alpha Centauri will need to cross the boundary between the Local Interstellar Cloud and the B-cloud, a region we have yet to penetrate. The nature of and variation within the interstellar medium will require continuing work with our admittedly sparse data.

The paper is Ocker et al., “Persistent plasma waves in interstellar space detected by Voyager 1,” Nature Astronomy 10 May 2021. Abstract / Preprint.