I often think of Dyson structures around stars as surprisingly benign places, probably motivated by encountering Larry Niven’s wonderful Ringworld when it was first published by Ballantine in 1970. I was reading it in an old house in Iowa on a windy night and thought to start with a chapter or two, but found myself so enthralled that it wasn’t until several hours later that I re-surfaced, wishing I didn’t have so much to do the next day that I had to put the book aside and sleep.

I hope I’m not stretching the definition of a Dyson construct too far when I assign the name to Niven’s ring. It is, after all, a structure built by technological means that runs completely around its star at an orbit allowing a temperate climate for all concerned, a vast extension of real estate in addition to whatever other purposes its creators may have intended. That a technological artifact around a star should be benign is a function of its temperature, which makes things possible for biological beings.

But a Dyson sphere conceived solely as a collection device to maximize the civilization’s intake of stellar energy would not be built to biological constraints. For one thing, as retired UCLA astrophysicist Ben Zuckerman points out in a new paper, it would probably be as close to its star as possible to minimize its size. That makes for interesting temperatures, probably in the range of 300 K at the low end and reaching perhaps 1000 K, which sets up emissions up to 10 µm in wavelength, and as Zuckerman points out, this would be in addition to any emission from the star itself.

Zuckerman refers to such structures as DSRs, standing for Dyson spheres/rings, so I guess working Niven in here fits as well. Notice that when talking about these things we also make an assumption that seems reasonable. If civilizations are abundant in the galaxy, they are likely long-lived, and that means that stellar evolution has to be something they cope with as yellow dwarf stars, for example, turn into red giants and, ultimately, white dwarfs. The DSR concept can accommodate the latter and offers a civilization the chance to continue to use the energy of its shrunken star. Whether this would be the route such a civilization chooses is another matter entirely.

Image: Artist depiction of a Dyson swarm. Credit: Kevin McGill/Wikimedia Commons.

One useful fact about white dwarfs is that they are small, around the size of the Earth, and thus give us plenty of transit depth should some kind of artificial construct pass between the star and our telescopes. Excess infrared emission might also be a way to find such an object, although here we have to be concerned about dust particles and other potential sources of the infrared excess. Zuckerman’s new paper analyzes the observational limits we can currently derive based on these two methods.

The paper, published in Monthly Notices of the Royal Astronomical Society, uses data from Kepler, Spitzer and WISE (Wide-field Infrared Survey Explorer) as a first cut into the question, revising Zuckerman’s own 1985 work on the number of technological civilizations that could have emerged around main sequence stars that evolved to white dwarfs within the age constraints of the Milky Way. Various papers exist on excess of infrared emissions from white dwarfs; we learn that Spitzer surveyed at least 100 white dwarfs with masses in the main sequence range of 0.95 to 1.25 solar masses. These correspond to spectral types G7 and F6, and none of them turned up evidence for excess infrared emission.

As to WISE, the author finds the instrument sensitive enough to yield significant limits for the existence of DSRs around main sequence stars, but concludes that for the much fainter white dwarfs, the excess infrared is “plagued by confusion…with other sources of IR emission.” He looks toward future studies of the WISE database to untangle some of the ambiguities, while going on to delve into transit possibilities for large objects orbiting white dwarfs. Kepler’s K2 extension mission, he finds, would have been able to detect a large structure (1000 km or more) if transiting, but found none.

It’s worth pointing out that no studies of TESS data on white dwarfs are yet available, but one ongoing project has already observed about 5000, with another 5000 planned for the near future. As with K2, a deep transit would be required to detect a Dyson object, again on the order of 1000 kilometers. If any such objects are detected, we may be able to distinguish natural from artificial objects by their transit shape. Luc Arnold has done interesting work on this; see SETI: The Artificial Transit Scenario for more.

Earlier Kepler data are likewise consulted. From the paper:

From Equation 4 we see that about a billion F6 through G7 stars that were on the main sequence are now white dwarfs. Studies of Kepler and other databases by Zink & Hansen (2019) and by Bryson et al. (2021) suggest that about 30% of G-type stars are orbited by a potentially habitable planet, or about 300 million such planets that orbit the white dwarfs of interest here. If as many as one in 30 of these planets spawns life that eventually evolves to a state where it constructs a DSR with luminosity at least 0.1% that of its host white dwarf, then in a sample of 100 white dwarfs we might have expected to see a DSR. Thus, fewer than 3% of the habitable planets that orbit sun-like stars host life that evolves to technology, survives to the white dwarf stage of stellar evolution, and builds a DSR with fractional IR luminosity of at least 0.1%.

Science fiction writers will want to go through Zuckerman’s section on the motivations of civilizations to build a Dyson sphere or ring, which travels deep into speculative territory about cultures that may or may not exist. It’s an imaginative foray, though, discussing the cooling of the white dwarf over time, the need of a civilization to migrate to space-based colonies and the kind of structures they would likely build there.

There are novels in the making here, but our science fiction writer should also be asking why a culture of this sophistication – able to put massive objects built from entire belts of asteroids and other debris into coherent structures for energy and living space purposes – would not simply migrate to another star. The author only says that if the only reason to travel between the stars is to avoid the inevitable stellar evolution of the home star, then no civilization would undertake such journeys, preferring to control that evolution through technology in the home system. This gives me pause, and Centauri Dreams readers may wish to supply their own reasons for interstellar travel that go beyond escaping stellar evolution.

Also speculative and sprouting fictional possibilities is the notion that main sequence stars may not be good places to look for a DSR. Thus Zuckerman:

…main sequence stars suffer at least two disadvantages as target stars when compared to white dwarfs; one disadvantage would be less motivation to build a substantial DSR because one’s home planet remains a good abode for life. In our own solar system, if a sunshield is constructed and employed at the inner Earth-Sun Lagrange point – to counter the increasing solar luminosity – then Earth could remain quite habitable for a few Gyr more into the future. Perhaps a more important consideration would be the greater luminosity, say about a factor of 1000, of the Sun compared to a typical white dwarf.

Moreover, detecting a DSR around a main sequence star becomes more problematic. In the passage below, the term τ stands for the luminosity of a DSR measured as a fraction of the luminosity of the central star:

For a DSR with the same τ and temperature around the Sun as one around a white dwarf, a DSR at the former would have to have 1000 times the area of one at the latter. While there is sufficient material in the asteroid belt to build such an extensive DSR, would the motivation to do so exist? For transits of main sequence stars by structures with temperatures in the range 300 to 1000 K, the orbital period would be much longer than around white dwarfs, thus relatively few transits per year. For a given structure, the probability of proper alignment of orbital plane and line of sight to Earth would be small and its required cross section would be larger than that of Ceres.

So we are left with that 3 percent figure, which sets an upper limit based on our current data for the fraction of potentially habitable planets that orbit stars like the Sun, produce living organisms that produce technology, and then construct a DSR as their system undergoes stellar evolution. No more than 3 percent of such planets do so.

There is a place for ‘drilling down’ strategies like this, for they take into account the limitations of our data by way of helping us see what is not there. We do the same in exoplanet research when we start with a star, say Proxima Centauri, and progressively whittle away at the data to demonstrate that no gas giant in a tight orbit can be there, then no Neptune-class world within certain orbital constraints, and finally we do find something that is there, that most interesting place we now call Proxima b.

As far as white dwarfs and Dyson spheres or rings go, new instrumentation will help us improve the limits discussed in this paper. Zuckerman points out that there are 5000 white dwarfs within 200 parsecs of Earth brighter than magnitude 17. A space telescope like WISE with the diameter of Spitzer could improve the limits on DSR frequency derived here, its data vetted by upcoming 30-m class ground-based telescopes (Zuckerman notes that JWST is not suited, for various reasons, for DSR hunting). The European Space Agency’s PLATO spacecraft should be several times more sensitive than TESS at detecting white dwarf transits, taking us well below the 1000 km limit.

The paper is Zuckerman, “Infrared and Optical Detectability of Dyson Spheres at White Dwarf Stars,” Monthly Notices of the Royal Astronomical Society stac1113 (28 April 2022). Abstract / Preprint.

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