Centauri Dreams

Imagining and Planning Interstellar Exploration

A Shifting, Seething Solar Wind

In search of ever-higher velocities leaving the Solar System, we need to keep in mind the options offered by the solar wind. This stream of charged plasma particles flowing outward from the Sun carves out the protective bubble of the heliosphere, and in doing so can generate ‘winds’ of more than 500 kilometers per second. Not bad if we’re thinking in terms of harnessing the effect, perhaps by a magnetic sail that can create the field needed to interact with the wind, or an electric sail whose myriad tethers, held taut by rotation, create an electric field that repels protons and produces thrust.

But like the winds that drove the great age of sail on Earth, the solar version is treacherous, as likely to becalm the ship as to cause its sails to billow. It’s a gusty, turbulent medium, one where those velocities of 500 kilometers and more per second can as likely fall well below that figure. Exactly how it produces squalls in the form of coronal mass ejections or calmer flows is a topic under active study, which is where missions like Solar Orbiter come into play. Studying the solar surface to pin down the origin of the wind and the mechanism that drives it is at the heart of the mission.

Launched in 2020, Solar Orbiter carries a panoply of instruments, ten in all, for the analysis, including an Electron Analyzer System (EAS), a Proton-Alpha Sensor (PAS) for measuring the speed of the wind, and a Heavy Ion Sensor (HIS) designed to measure the heavy ion flow. Critical to the analysis of this paper is the Spectral Imaging of the Coronal Environment (SPICE) instrument, as we’ll see below. Steph Yardley (Northumbria University) is lead author of the paper on this work, which has just appeared on Nature Astronomy:

“The variability of solar wind streams measured in situ at a spacecraft close to the Sun provide us with a lot of information on their sources, and although past studies have traced the origins of the solar wind, this was done much closer to Earth, by which time this variability is lost. Because Solar Orbiter travels so close to the Sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origins and how this complexity is driven by the changes in different source regions.”

What the work is analyzing is a theory that the process of magnetic field lines breaking and reconnecting is critical to producing the slower solar wind. Different areas of the Sun’s corona are implicated in the origin of both the fast and slow winds, with the ‘open corona’ being those regions where magnetic field lines extend from the Sun into space, tethered to it at one end only and creating the pathway for solar material to flow out in the form of the fast solar wind. Closed coronal regions, on the other hand, are those where the magnetic field lines connect to the surface at both ends, forming loops.

As you would imagine, the process is wildly turbulent and marked by the frequent breakage of these closed magnetic loops and their subsequent reconnection. The researchers have probed the theory that the slow solar wind originates in the closed corona during these periods of breakage and reconnection by studying the composition of solar wind streams, for the heavy ions emitted vary depending on their origins in either the closed or open corona. Solar Orbiter’s Heavy Ion Sensor (HIS) is able to take the needed measurements to relate the effects of this activity on the surrounding plasma.

The image below is from the Solar Dynamics Observatory spacecraft rather than Solar Orbiter, reminding us of the different views we are gaining by our various missions to our star. The comparison of key datasets tells the story.

Image: This is part of Figure 1 from the paper. The caption reads: SDO/AIA [Solar Dynamics Observatory data using its Atmospheric Imaging Assembly] 193 Å image showing the source region from the perspective of an Earth observer. Open magnetic field lines that are constructed from the coronal potential field model are overplotted, coloured by their associated expansion factor F. The large equatorial CH [Coronal Hole] and AR [Active Region] complex are labeled in white. The FOVs [fields of view] of SO EUI/HRI and PHI/HRT [references to instruments aboard Solar Orbiter] are shown in cyan and pink, respectively. The back-projected trajectory of SO [Solar Orbiter] from 1 March 2022 until 9 March 2022 is shown by the olive dotted line (from right to left).

So because we have Solar Orbiter, we can now combine observations of the Sun from various sources including other space missions, like the Solar Dynamics Observatory, with the measurements of the solar wind actually flowing past the spacecraft. Susan Lepri (University of Michigan) is deputy principal investigator on the HIS system:

“Each region of the Sun can have a unique combination of heavy ions, which determines the chemical composition of a stream of solar wind. Because the chemical composition of the solar wind remains constant as it travels out into the solar system, we can use these ions as a fingerprint to determine the origin of a specific stream of the solar wind in the lower part of the Sun’s atmosphere.”

The results have been productive. The analysis gives us a precise breakdown of just what Solar Orbiter has encountered during the period studied. This is a thorny quotation but it includes a key finding. From the paper:

Combining the SO [Solar Orbiter] trajectory, coronal field model, magnetic connectivity tool, the SPICE composition analysis of the AR [Active Region] complex, and the in situ plasma and magnetic field parameters, we suggest that SO was immersed in three fast wind streams… originating from the three linked sections of the large equatorial CH [Coronal Hole]… These were followed by two slower streams associated with the negative polarities of the AR complex… The decrease of the solar wind speed can be explained by the expansion of the open magnetic field associated with the CH-AR complex, as the connectivity of SO transitioned across these regions. Credit: Yardley et al.

The findings described here are significant. We learn from this work just how complex the solar wind flow is, in this case involving three fast streams and two slower ones, all involving changes in magnetic field connectivity. Matching the composition of the solar wind streams to different areas on the corona gives us new insights into the turbulent mix found where the open and closed corona meet. The slow solar wind’s ‘breakout’ from closed magnetic field lines is demonstrated. The phenomenon of magnetic reconnection proves critical to the wind’s variability.

Demonstrating these linkages means that we can now use the findings to probe further into the origins of the solar wind. But this is a variability that is in no way predictable, making the prospect of riding the solar wind via electric or magnetic sail a daunting one. We’ll continue to learn more, though, as we bring in data from missions like the Parker Solar Probe. It will be fascinating to see one day how we use the solar wind to test out possible spacecraft designs in search of a faster route to the outer Solar System.

Addendum: In an earlier draft, I mistakenly criticized the authors for not initially clarifying some of the acronyms in this paper. I’ve removed that comment because a later reading showed I was mistaken about the two examples I cited.

The paper is Yardley et al., “Multi-source connectivity as the driver of solar wind variability in the heliosphere,” Nature Astronomy 28 May 2024 (full text).

And Then There Were Four (or Maybe Not)

I’m delighted to see the high level of interest in Dysonian SETI shown not only by reader comments here but in the scientific community at large. I wouldn’t normally return to the topic this quickly but for the need to add a quick addendum to our discussions of Project Hephaistos, the effort (based at Uppsala University, Sweden) to do a deep dive into data from different observatories looking for evidence of Dyson spheres in the form of quirks in the infrared data suggesting strong waste heat.

Swiftly after the latest Hephaistos paper comes a significant re-examination of the seven Dyson sphere candidates that made it through that project’s filters. You’ll recall that all seven were M-dwarfs, which struck me at the time as unusual. Only seven candidates emerged from over five million stars sampled, interesting especially because the possibility of a warm debris disk seemed to be ruled out. But Tongtian Ren (Jodrell Bank Centre for Astrophysics), working with Michael Garrett and Andrew Siemion, who share an affiliation with the same institution, has other ideas.

The researchers brought in new data from the Very Large Array Sky Survey, the NRAO VLA Sky Survey and two other sources that would allow a cross-matching of the seven Hephaistos candidates with radio sources. Hephaistos had been working with Gaia data release 3 along with the findings of the Two Micron All-Sky Survey (2MASS) and results from the Wide-field Infrared Survey Explorer, which now operates as NEOWISE. The search for radio counterparts to its Dyson candidates drew hits in three cases.

This looks strongly like data contamination, and the Jodrell Bank scientists think they’ve found the sources of the infrared signatures for these three:

Candidates A and G are associated with radio sources offset approximately ∼ 5 arcseconds from their respective Gaia stellar positions. We suggest that these radio sources are most likely to be DOGs (dust-obscured galaxies) that contaminate the IR (WISE) Spectral-Energy Distributions (SEDs) of the two DS candidates. The offsets for candidate B are smaller, approximately ∼ 0.35 arcsecond. Since M-dwarfs very rarely present persistent radio emission (≤ 0.5% of the sample observed by Callingham et al. (2021)), we suspect that this radio source is also associated with a background DOG lying very close to the line-of-sight. We note that the radio source associated with G has a steep spectral index with a best fit of α = −0.52 ± 0.02 – this value is typical of synchrotron emission from a radio-loud AGN with extended jets.

Let’s untangle this. A dust-obscured galaxy is generally studied at infrared wavelengths, being too difficult a target for visible light observations. There is likely strong star formation going on here, and perhaps an AGN, or active galactic nucleus, emitting energy across the electromagnetic spectrum. Usefully a DOG with an AGN can also be examined at radio wavelengths, which can tease out information about the gas content of the galaxy. So here we have background objects that can contaminate our infrared observations and can be identified by using surveys at different wavelengths.

All seven of the Hephaistos candidates are implicated in possible contamination if we bring in the objects known as hot dust-obscured galaxies, which have inevitably achieved the acronym Hot DOGs. The authors propose that the Spectral-Energy Distributions (SEDs) of each of the Hephaistos objects are “significantly contaminated” by background galaxies of this category. If this is the case, then the oddity of finding seven Dyson sphere candidates around M-dwarfs is resolved, but it will take deeper observations of all seven to confirm this, an effort the authors believe is warranted.

Image: Here is an artist’s impression of the Hot DOG W2246-0526, based on the results of a 2016 paper by Díaz-Santos et al. (2016). In that work (not connected with today’s paper), the authors used ALMA observations to show that the interstellar medium in the Hot DOG is dominated by turbulence, and may be unstable against the energy being injected by the AGN here, potentially producing an isotropic outflow. The WISE mission was essential to finding this galaxy because the galaxy is covered in dust, obscuring its light from visible-wavelength telescopes. But the radio signature of such objects, detected by other methods, raises questions about the recent Hephaistos findings. Image credit: NASA/JPL.

Bear in mind that only 1 out of every 3,000 galaxies that WISE observed fits into this category, so we are dealing with comparatively unusual objects. But given that the Hephaistos survey ran five million objects through its pipeline, the possibility of contamination in the data in the seven proposed candidates seems worth pursuing. The hunt continues, but more and more it appears that if Dyson spheres are achievable by advanced civilizations (and if such civilizations actually exist), they are seldom built.

The paper is Tongtian Ren et al., “Background Contamination of the Project Hephaistos Dyson Spheres Candidates,” available as a preprint.

Tantalizing New Images of Europa

What a pleasure to see new images from JunoCam, the visible-light camera aboard the Juno spacecraft that has now imaged in its peregrinations around Jupiter the surface of its most interesting moon. Our probing of Europa’s secrets has depended heavily upon the imagery returned by the Galileo spacecraft. That mission made its last flyby in 2000, and we have another wait while ESA’s Juice mission and Europa Clipper make the journey, the former enroute, the latter scheduled for an October launch.

Juno’s 2022 flyby thus gave us a helpful visual update, one that is complemented by an informative snapshot taken by the spacecraft’s Stellar Reference Unit (SRU) star camera. While we have five high resolution images to work with, the Stellar Reference Unit’s black-and-white image has produced the most detail. The image is intriguing because of its method, for bear in mind that the SRU is designed to track stars for navigation purposes. That makes it a dim light instrument, one that must be handled carefully to avoid washing out the image. The Juno team used it on Europa’s nightside, where the ambient light was sunlight reflected off Jupiter itself and the Sun was safely hidden.

Image: From Juno’s SRU, this image shows the location of a double ridge running east-west (blue box) with possible plume stains and the chaos feature the team calls ‘the Platypus” (orange box). These features hint at current surface activity and the presence of subsurface liquid water on the icy Jovian moon. Credit: NASA/JPL-Caltech/SwRI.

What emerges is a jumble of chaotic terrain cut by ridges and laden with a reddish-brown material familiar from Galileo imagery of the moon. These dark stains have been hypothesized to be the deposits of cryovolcanic plumes. Amidst this terrain, a new feature emerges that interrupts different forms of terrain. The Juno team has christened it the Platypus. Here the ridge topography breaks down as it encounters what is clearly younger material laden with ice blocks, a disrupted area that is some 37 kilometers by 67 kilometers in size. A double ridge line north of the Platypus is also apparent, the complex terrain suggesting the kind of surface change that researchers believe may allow ocean water to come close to the surface in isolated pockets.

The mention of plumes is intriguing because of the possibility of one day collecting samples from a spacecraft during a flyby, although no plumes are evident in the Juno imagery. Both the Platypus and the double ridges suggest recent activity. On the possibility of plumes, the SRU paper notes:

Diffuse discontinuous low-albedo deposits flank double ridges ∼50 km north of the “Platypus” chaos margin, extending radially outward from the lineaments. The morphology of these deposits is similar to features observed elsewhere on Europa that have been associated with hypothesized plume activity, the discontinuous low-albedo spots flanking Rhadamanthys Linea being a prominent example (Quick & Hedman, 2020). Quick and Hedman (2020) surmise that 1–10 m thick deposits can be emplaced by sufficiently compact plumes and detected by high-resolution visible wavelength cameras. The radii of the deposits observed by the SRU are ∼2–5 km, which Quick & Hedman’s models associate with <10 km high plumes.

We can also compare the Juno imagery with that of Galileo, as the JunoCam paper does:

The number of documented craters larger than 1 km on Europa has gone from 41 to 40 craters. Careful comparisons of the JunoCam images with overlapping images from Galileo show no surface changes due to plume deposits or ongoing geologic activity over time intervals of 23–26 yr, though admittedly the images are not well matched in resolution, viewing geometry, and wavelength. No active eruptions were detected. Finally, from the Europa data set taken on 2022 February 24, we can say that the north polar cap of Europa at this image scale looks similar to lower latitudes.

It’s worth adding here that a recent search using the Atacama Large Millimeter/submillimeter Array (ALMA) collected data over four days to examine the moon’s entire surface, coming up with no evidence of plume activity. We’re clearly not dealing with a geyser phenomenon anywhere as active as what we find at Enceladus, and thus far evidence from the Hubble instrument has been the most compelling, but even the data from its 2013 observations remain at the edge of detection. Clearly the search for active plumes will continue given their exciting implications.

Meanwhile, evidence for surface activity of other kinds on Europa continues to emerge, presenting new targets for Europa Clipper as well as Juice. Juice (Jupiter Icy Moons Explorer) launched on April 14, 2023 and will arrive in July of 2031, while Europa Clipper is scheduled to reach the giant planet in April of 2030. The new imagery suggests that Europa’s outer ice shell moves freely over the ocean (“true polar wander”), capturing steep depressions up to 50 kilometers wide near the equator. These ovoid features are similar to those found in other parts of Europa. Candy Hansen, who leads JunoCam planning at the Planetary Science Institute in Tucson, AZ, notes their relevance:

“True polar wander occurs if Europa’s icy shell is decoupled from its rocky interior, resulting in high stress levels on the shell, which lead to predictable fracture patterns. This is the first time that these fracture patterns have been mapped in the southern hemisphere, suggesting that true polar wander’s effect on Europa’s surface geology is more extensive than previously identified.”

The landscape of ice blocks and troughs near Europa’s equator broken by depressions tells a tale that must be interpreted in terms of light and shadow. The feature called Gwern, for example, an apparent impact crater found in Galileo imagery, turns out under different lighting to be nothing more than an oval shadow caused by the intersection of prominent ridges. Cross-cut ridges and the dark stains that may mark the residue from ancient (or recent) plumes offer a compelling landscape. New features like the Platypus will get a particularly hard look from our incoming spacecraft.

Image: Jupiter’s moon Europa was captured by the JunoCam instrument aboard NASA’s Juno spacecraft during the mission’s close flyby on Sept. 29, 2022. The images show the fractures, ridges, and bands that crisscross the moon’s surface. Credit: Image data: NASA/JPL-Caltech/SwRI/MSSS. Image processing: Björn Jónsson (CC BY 3.0).

The SRU paper is Becker et al., “A Complex Region of Europa’s Surface With Hints of Recent Activity Revealed by Juno’s Stellar Reference Unit,” JGR Planets 22 December 2023 (full text). The paper on the JunoCam imagery is Hansen, “Juno’s JunoCam Images of Europa,” Planetary Science Journal Vol. 5, No. 3 (21 March 2024), 76. Full text. The paper on the ALMA observations is Cordiner et al., “ALMA Spectroscopy of Europa: A Search for Active Plumes,” submitted to IAU Symposium 383 conference proceedings (preprint).

New Horizons: Mapping at System’s Edge

Dust between the stars usually factors into our discussions on Centauri Dreams when we’re considering its effect on fast-moving spacecraft. Although it only accounts for 1 percent of the mass in the interstellar medium (the other 99 percent being gas) its particles and ices have to be accounted for when moving at a substantial fraction of the speed of light. As you would expect, regions of star formation are particularly heavy in dust, but we also have to account for its presence if we’re modeling deceleration into a planetary system, where the dust levels will far exceed the levels found along a star probe’s journey.

Clearly, dust distribution is something we need to learn more about when we’re going out from as well as into a planetary system, an effort that extends all the way back to Pioneers 10 and 11, which included instruments to measure interplanetary dust. Voyager 1 and 2 carry dust detecting instruments, and so did Galileo and Cassini, the latter with its Cosmic Dust Analyzer (CDA).

And I’m reminded by a recent news release from the New Horizons team that the Student Dust Counter (SDC) aboard New Horizons is making a significant contribution as it moves ever deeper into the Kuiper Belt. You’ll recall that the SDC played the major role in identifying what may be an extended Kuiper Belt in findings published in January (citation below). Alex Doner (University of Colorado Boulder) is lead author of the paper on that work. He serves as SDC lead:

“New Horizons is making the first direct measurements of interplanetary dust far beyond Neptune and Pluto, so every observation could lead to a discovery. The idea that we might have detected an extended Kuiper Belt — with a whole new population of objects colliding and producing more dust – offers another clue in solving the mysteries of the solar system’s most distant regions.”

Image: Artist’s impression of a collision between two objects in the distant Kuiper Belt. Such collisions are a major source of dust in the belt, along with particles kicked up from Kuiper Belt objects being peppered by microscopic dust impactors from outside of the solar system. Credit: Dan Durda, FIAAA.

We have to account for the variable distribution and composition of dust not only in terms of spacecraft design but also for fine-tuning our astronomical observations. Scattering and absorbing starlight, dust produces what astronomers refer to as extinction, dimming and reddening the light in significant ways. It’s a part of the cosmic optical background, which on the largest scale includes light from extragalactic sources as well as our own Milky Way. This background can tell us about galactic evolution and even dark matter if we know how to adjust for its effects.

Joel Parker (SwRI) is a New Horizons project scientist who notes that even as the craft continues to make observations within the Kuiper Belt (and the search for potential flyby targets continues), its instruments are also returning data with implications for astrophysics at large:

“New Horizons is uniquely positioned to make astrophysical observations that are difficult or impossible to make here on Earth or even from orbit. Many things can obscure observations, but one of the biggest problems is the dust in the inner solar system. It may not be obvious when you look up into a clear night sky, but there is a lot of dust in the inner part of the solar system. There is also a great deal of obscuration at certain ultraviolet wavelengths at closer distances due to the hydrogen that pervades our planetary system, but which is much reduced out in the Kuiper Belt and the outer heliosphere.”

Image: New Horizons mission scientists and external colleagues are taking advantage of the New Horizons spacecraft’s position in the distant Kuiper Belt to make unique astrophysical and heliospheric observations. Alice, the ultraviolet spectrograph on the spacecraft, performed 75 great circle scans of the sky in September 2023, for a total of 150 hours of observations. These data focus on the light from hydrogen atoms in the ultraviolet Lyman-alpha wavelength across the sky as seen from New Horizons’ vantage point in the distant solar system. This map shows the data from the scans overlaid on a smoothed model of the expected Lyman-alpha background. (Credit: NASA/Johns Hopkins APL/SwRI).

We’ve recently talked about hydrogen-alpha transitions, which are a factor in astronomical observations (we saw this in the Project Hephaistos Dyson sphere papers). The Lyman-alpha transitions referred to above produce different emissions as electrons change energy levels within the atom, and are primarily useful for studies of the interstellar and intergalactic medium. So New Horizons is on point for helping us clarify how local dust levels may affect our observations of these distant sources.

Parker puts it this way:

“It’s like driving through a thick fog, and when you go over a hill, it’s clear. Suddenly, you can see things that were obscured. When you’re trying to look for a very faint light far outside our solar system or beyond our galaxy, that obscuration creates a challenge. If we measure how the ‘fog’ changes as we move farther out, we can make better models for our observations from Earth. With more accurate models, we can more easily subtract the effects of light and dust contamination.”

New Horizons records the cosmic ultraviolet background and maps hydrogen distribution as it moves through the outer regions of the heliosphere and eventually through the heliopause and into the local interstellar medium. This is going to be useful in telling astronomers something about the evolution of galaxies by yielding data on star formation rates as we learn how to remove the contaminating signature of interplanetary dust from our observations.

It’s fascinating to see how a single spacecraft can, as have the Voyagers, function as a kind of Swiss army knife with tools useful well beyond a single target. Successors to New Horizons will one day extend these observations as we learn more about dust distribution all the way out to the Oort Cloud.

The paper on a possible extended Kuiper Belt is Doner et al., “New Horizons Venetia Burney Student Dust Counter Observes Higher than Expected Fluxes Approaching 60 au,” The Astrophysical Journal Letters Vol. 961, No. 2 (24 January 2024), L38 (abstract).

Seven Dyson Sphere Candidates

I’m enjoying the conversation about Project Hephaistos engendered by the article on Dyson spheres. In particular, Al Jackson and Alex Tolley have been kicking around the notion of Dyson sphere alternatives, ways of preserving a civilization that are, in Alex’s words, less ‘grabby’ and more accepting of their resource limitations. Or as Al puts it:

One would think that a civilization that can build a ‘Dyson Swarm’ for energy and natural resources would have a very advanced technology. Why then does that civilization not deploy an instrumentality more sly? Solving its energy needs in very subtle ways…

As pointed out in the article, a number of Dyson sphere searches have been mounted, but we are only now coming around to serious candidates, and at that only seven out of a vast search field. Two of these are shown in the figure below. We’re a long way from knowing what these infrared signatures actually represent, but let’s dig into the Project Hephaistos work from its latest paper in 2024 and also ponder what astronomers can do as they try to learn more.

Image: This is Figure 7 from the paper. Caption: SEDs [spectral energy distributions] of two Dyson spheres candidates and their photometric images. The SED panels include the model and data, with the dashed blue lines indicating the model without considering the emission in the infrared from the Dyson sphere and the solid black line indicating the model that includes the infrared flux from the Dyson sphere. Photometric images encompass one arcmin. All images are centered in the position of the candidates, according to Gaia DR3. All sources are clear mid-infrared emitters with no clear contaminators or signatures that indicate an obvious mid-infrared origin. The red circle marks the location of the star according to Gaia DR3. Credit: Suazo et al.

We need to consider just how much we can deduce from photometry. Measuring light from astronomical sources across different wavelengths is what photometry is about, allowing us to derive values of distance, temperature and composition. We’re also measuring the object’s luminosity, and this gets complicated in Dyson sphere terms. Just how does the photometry of a particular star change when a Dyson sphere either partially or completely encloses it? We saw previously that the latest paper from this ongoing search for evidence of astroengineering has developed its own models for this.

The model draws on earlier work from some of the co-authors of the paper we’re studying now. It relies on two approaches to the effect of a Dyson sphere on a star’s photometry. First, we need to model the obscuration of the star by the sphere itself. Beyond this, it’s essential to account for the re-emission of absorbed radiation at much longer wavelengths, as the megastructure – if we can call it that – gives off heat.

“[W]e model the stellar component as an obscured version of its original spectrum and the DS component as a blackbody whose brightness depends on the amount of radiation it collects,” write the authors of the 2022 paper I discussed in the last post. The modeling process is worth a post of its own, but instead I’ll send those interested to an even earlier work, a key 2014 paper from Jason Wright and colleagues, “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result.” The citation is at the end of the text.

The recently released 2024 paper from Hephaistos examined later data from Gaia (Data Release 3) while also incorporating the 2MASS and WISE photometry of some 5 million sources to create a list of stars that could potentially host a Dyson sphere. In the new paper, the authors home in on partial Dyson spheres, which will partially obscure the star’s light and would show varying effects depending on the level of completion. The waste heat generated in the mid-infrared would depend upon the degree to which the structure (or more likely, ‘swarm’) was completed as well as its effective temperature.

So we have a primary Dyson sphere signature in the form of excess heat, thermal emission that shows up at mid-infrared wavelengths, and that means we’re in an area of research that also involves other sources of such radiation. The dust in a circumstellar disk is one, heated by the light of the star and re-emitted at longer wavelengths. As we saw yesterday, all kinds of contamination are thus possible, but the data pipeline used by Project Hephaistos aims at screening out the great bulk of these.

Seven candidates for Dyson spheres survive the filter. All seven appear to be actual infrared sources that are free of contamination from dust or other sources. The researchers subjected the data to over 6 million models that took in 391 Dyson sphere effective temperatures. They modeled Dyson spheres in temperature ranges from 100 to 700 K, with covering factors (i.e., the extent of completion of the sphere) from 0.1 to 0.9. Among many factors considered here, they’re also wary of Hα (hydrogen alpha) emissions, which could flag the early stage of star growth and might be implicated in observations of infrared radiation.

Image: IC 2118, a giant cloud of gas and dust also known as the Witch Head Nebula. H-alpha emissions, which are observed over most of the Orion constellation, are shown in red. This H-alpha image was taken by the MDW Survey, a high-resolution astronomical survey of the entire night sky not affiliated with Project Hephaistos. I’m showing it to illustrate how pervasive and misleading Hα can be in a Dyson sphere search. Credit: Columbia University.

I want to be precise about what the authors are saying in this paper: “…we identified seven sources displaying mid-infrared flux excess of uncertain origin.” They are not, contra some sensational reports, saying they found Dyson spheres. These are candidates. But let’s dig in a bit, because the case is intriguing. From the paper:

Various processes involving circumstellar material surrounding a star, such as binary interactions, pre-main sequence stars, and warm debris disks, can contribute to the observed mid-infrared excess (e.g. Cotten & Song 2016). Kennedy & Wyatt (2013) estimates the occurrence rate of warm, bright dust. The occurrence rate is 1 over 100 for very young sources, whereas it becomes 1 over 10,000 for old systems (> 1 Gyr). However, the results of our variability check suggest that our sources are not young stars.

Are the candidate objects surrounded by warm debris disks? What’s interesting here is that all seven of these are M-class stars, and as the authors note, M-dwarf debris disks are quite rare, with only a few confirmed. Why this should be so is the object of continuing study, but both the temperature and luminosity of the candidate objects differs from typical debris disks. The questions deepen and multiply:

Extreme Debris Disks (EDD) (Balog et al. 2009), are examples of mid-infrared sources with high fractional luminosities (f > 0.01) that have higher temperatures compared to that of standard debris disks (Moór et al. 2021). Nevertheless, these sources have never been observed in connection with M dwarfs. Are our candidates’ strange young stars whose flux does not vary with time? Are these stars M-dwarf debris disks with an extreme fractional luminosity? Or something completely different?

The authors probe the possibilities. They consider chance alignments with distant infrared sources, and offsets in the astrometry when incorporating the WISE data. There is plenty to investigate here, and the paper suggests optical spectroscopy as a way of refuting false debris disks around M-dwarfs, which could help sort between the seven objects here identified. Stellar rotation, age and magnetic activity may also be factors that will need to be probed. But when all is said and done, we wind up with this:

…analyzing the spectral region around Hα can help us ultimately discard or verify the presence of young disks by analyzing the potential Hα emission. Spectroscopy in the MIR [mid-infrared] region would be very valuable when determining whether the emission corresponds to a single blackbody, as we assumed in our models. Additionally, spectroscopy can help us determine the real spectral type of our candidates and ultimately reject the presence of confounders.

So the hunt for Dyson spheres proceeds. Various pieces need to fall into place to make the case still more compelling, and we should remember that “The MIR data quality for these objects is typically quite low, and additional data is required to determine their nature.” This layman’s guess – and I am not qualified to do anything more than guess – is that rather than Dyson spheres we are glimpsing interesting astrophysics regarding M-dwarfs that this investigation will advance. In any case, do keep in mind that among some five million sources, only seven show compatibility with the Dyson sphere model.

If Dyson spheres are out there, they’re vanishingly rare. But finding just one would change everything.

The paper on Dyson sphere modeling is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result,” The Astrophysical Journal Vol. 792, Issue 1 (September, 2014), id 27 (abstract). The 2022 paper from Project Hephaistos is Suazo et al., “Project Hephaistos – I. Upper limits on partial Dyson spheres in the Milky Way,” Vol. 512, Issue 2 (May 2022), 2988-3000 (abstract / preprint). The 2024 paper is Suazo et al., “Project Hephaistos – II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE,” MNRAS (6 May 2024), stae1186 (abstract / preprint).

Project Hephaistos and the Hunt for Astroengineering

For a project looking for the signature of an advanced extraterrestrial civilization, the name Hephaistos is an unusually apt choice. And indeed the leaders of Project Hephaistos, based at Uppsala University in Sweden, are quick to point out that the Greek god (known as Vulcan in Roman times) was a sort of preternatural blacksmith, thrown off Mt. Olympus for variously recounted transgressions and lame from the fall, a weapons maker and craftsman known for his artifice. Consider him the gods’ technologist.

Who better to choose for a project that pushes SETI not just throughout the Milky Way but to myriads of galaxies beyond? Going deep and far is a sensible move considering that we have absolutely no information about how common life is beyond our own Earth, if it exists at all. If the number of extraterrestrial civilizations in any given galaxy is scant, then a survey looking for evidence of Hephaistos-style engineering writ large will comb through existing observational data from our own galaxy but also consider what lies beyond. Which is why Project Hephaistos’s first paper (2015) searched for what the authors called ‘Dysonian astroengineering’ in over 1000 spiral galaxies.

More recent papers have stayed within the Milky Way to incorporate data from Gaia, the 2 Micron All Sky Survey (2MASS) and the accumulated offerings of the Wide-field Infrared Survey Explorer, which now operates as NEOWISE, analyzing the observational signatures of Dyson spheres in the process of construction and calling out upper limits on such spheres-in-the-making in the Milky Way. Such objects could present anomalously low optical brightness levels yet high mid-infrared flux. This is the basic method for searching for Dyson spheres, identifying the signature of waste heat while screening out young stellar objects and other factors that can mimic such parameters.

This article is occasioned by the release of a new paper, one that homes in on Dyson sphere candidates now identified. And it prompts reflection on the nature of the enterprise. Key to the concept is the idea that any flourishing (and highly advanced) extraterrestrial civilization will need to find sources of energy to meet its growing needs. An obvious source is a star, which can be harvested by a sphere of power-harvesting satellites. The notion, which Dyson presented in a paper in Science in 1960, explains how a search could be conducted in its title: “Search for Artificial Stellar Sources of Infrared Radiation.” In other words, comb the skies for infrared anomalies.

I strongly favor this ‘Dysonian’ approach to SETI, which makes no assumptions at all about any decision to communicate. As we have no possible idea of the values that would drive an alien culture to attempt to talk to us – or for that matter to any other civilizations – why not add to the search space the things that we can detect in other ways. However it is constructed, a Dyson sphere should produce waste heat as it obscures the light from the central star. Infrared searches could detect a star that is strangely dim but radiant at infrared wavelengths, and we might also find changes in brightness as such a ‘megastructure’ evolves that vary on relatively short timeframes.

Funding plays into our science in inescapable ways, so the fact that Dysonian SETI can be conducted using existing data is welcome. It’s also helpful that in-depth studies of particular Dyson sphere candidates may prove useful for nailing down astrophysical properties that interest the entire community, especially since there is the possibility of ‘feedback’ mechanisms on the star from any surrounding sphere of technology. We go looking for extraterrestrial megastructures but even if we don’t find them, we produce good science on unusual stellar properties and refine our observational technique. Not a bad way forward even as the traditional SETI effort in radio and optics continues.

The number of searches for individual Dyson spheres is surprisingly large, and to my knowledge extends back at least as far as 1985, when Russian radio astronomer Vyacheslav Ivanovich Slysh searched using data from the Infrared Astronomical Satellite (IRAS) mission, as did (at a later date) M. Y. Timofeev, collaborating with Nikolai Kardashev. Richard Carrigan, a scientist emeritus at the Fermi National Accel­era­tor Laboratory, looked for Dyson signatures out to 300 parsecs.

But we can go earlier still. Carl Sagan was pondering “The Infrared Detectability of Dyson Civilizations” (a paper in The Astrophysical Journal) back in the 1960s. In more recent times, the Glimpsing Heat from Alien Technologies effort at Pennsylvania State University (G-HAT) has been particularly prominent. What becomes staggering is the realization that the target list has grown so vast as our technologies have improved. Note this, from a Project Hephaistos paper in 2022 (citation below):

Most search efforts have aimed for individual complete Dyson spheres, employing far-infrared photometry (e.g., Slysh 1985; Jugaku & Nishimura 1991; Timofeev et al. 2000; Carrigan 2009) from the Infrared Astronomical Satellite (IRAS: Neugebauer et al. 1984), while a few considered partial Dyson spheres (e.g., Jugaku & Nishimura 2004). IRAS scanned the sky in the far infrared, providing data of ≈ 2.5 × 105 point sources. However, nowadays, we rely on photometric surveys covering optical, near-infrared, and mid-infrared wavelengths that reach object counts of up to ∼109 targets and allow for larger search programs.

The Project Hepaistos work in the 2022 paper homed in on producing upper limits for partial Dyson spheres in the Milky Way by searching Gaia DR2 data and WISE results that showed infrared excess, looking at more than 108 stars. We still have no Dyson sphere confirmations, but the new Hephaistos paper adds 2MASS data and moves to Gaia data release 3, which aids in the rejection of false positives. Gaia also adds to the mix its unique capabilities at parallax, which the authors describe thus:

…Gaia also provides parallax-based distances, which allow the spectral energy distributions of the targets to be converted to an absolute luminosity scale. The parallax data also make it possible to reject other pointlike sources of strong mid-infrared radiation such as quasars, but do not rule out stars with a quasar in the background.

Notable in the new 2024 paper is its description of the data pipeline focusing on separating Dyson sphere candidates from natural sources including circumstellar dust. The authors make the case that it is all but impossible to prove the existence of a Dyson sphere based solely on photometric data, so what is essentially happening is a search for sources showing excess infrared that are consistent with the Dyson sphere hypothesis. The data pipeline runs from data collection through a grid search methodology, image classification for filtering out young stars obscured by dust or associated with dusty nebulae, inspection of the signal to noise ratio, further analysis of the infrared excess and visual inspection from all the sources to reject possible contamination.

This gets tricky indeed. Have a look at some of the ‘confounders,’ as the authors call them. The figure shows three categories of confounders: blends, irregular structures and nebular features. In blends, the target is contaminated by external sources within the WISE coverage. The nebular category is a hazy and disordered false positive without a discernible source of infrared at the target’s location. Irregulars are sources without indication of nebulosity whose exact nature cannot be determined. All of these sources would be considered unreliable at the conclusion of the pipeline:

Image: This is Figure 5 from the paper. Caption: Examples of typical confounders in our search. The top row features a source from the blends category, the middle row a source embedded in a nebular region, and the bottom row a case from the irregular category. On these scales, the irregular and nebular cases cannot be distinguished, but the nebular nature can be established by inspecting the images at larger scales. Credit: Suazo et al.

In the next post, I want to take a look at the results, which involve seven interesting candidates, all of them around a type of star I wouldn’t normally think of in Dyson sphere terms. The papers are Suazo et al., “Project Hephaistos – I. Upper limits on partial Dyson spheres in the Milky Way,” Vol. 512, Issue 2 (May 2022), 2988-3000 (abstract / preprint) and Suazo et al., “Project Hephaistos – II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE,” MNRAS (6 May 2024), stae1186 (abstract / preprint).

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

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