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Stars and solar physics

Stars and solar physics

Are dusty quasars masquerading as Dyson sphere candidates?

30 May 2024
Dyson swarm superstructure
Mistaken identity Have astronomers found evidence of Dyson swarms, or are their findings due to distant, dusty quasars? (Courtesy: CC BY 2.0/Kevin Gill)

Seven candidate Dyson spheres found from their excess infrared radiation could be a case of mistaken identity, with evidence for dusty background galaxies spotted close to three of them.

The seven candidates were discovered by Project Hephaistos, which is coordinated by astronomers at Uppsala University in Sweden and Penn State University in the US.

A Dyson sphere is a hypothetical construct: a swarm of energy collectors capturing all of a star’s radiant energy to provide huge amounts of power for its builders. As these energy collectors – basically huge arrays of solar panels – absorb sunlight, they must emit waste heat as infrared radiation to avoid overheating. While a complete Dyson swarm would hide a star from view, this waste heat would still be detectable.

The caveat is that to build a complete Dyson swarm, a lot of raw material is required. In his 1960 paper describing the concept, Freeman Dyson calculated that dismantling a gas giant planet like Jupiter should do the trick.

Given that this is easier said than done, Project Hephaistos has been looking for incomplete Dyson swarms “that do not block all starlight, but a fraction of it,” says Matías Suazo of Uppsala University, who is leading the project.

Suazo’s team searched five million objects in archival data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Two Micron All-Sky Survey (2MASS), and cross-checked them against photometry and distance data accrued by the European Space Agency’s Gaia mission. This resulted in seven candidates exhibiting a suspicious infrared excess, as reported in Monthly Notices of the Royal Astronomical Society.

All the candidates are M-dwarfs, which are the smallest, coolest and most common type of star in the Universe. The closest is located 466 light years away.

Debris discs and Hot DOGs

Yet questions have now arisen over the real nature of these candidates.

“They could be an astrophysical phenomenon such as extreme debris discs, or something more exotic,” says Ann Marie Cody, an astronomer at the SETI Institute in California who is not involved in Project Hephaistos, but has conducted her own Dyson swarm search. “However, the data published thus far cannot discriminate between these scenarios.”

Debris discs are the dusty remnants of planet formation, but while many M-dwarfs have been found to have planets, only a handful have been found with sizeable debris discs, leading Suazo to be sceptical of the debris disc explanation.

“Observationally, M-dwarf debris discs are really uncommon,” Suazo tells Physics World. “There are different theories about why, including observational biases, formation mechanisms and so on. Those few cases that have been confirmed are in the submillimetre/radio regime, which means they are way cooler than the temperature range of our models.”

Tongtian Ren and Michael Garrett of the Jodrell Bank Centre for Astrophysics at the University of Manchester, and Andrew Siemion of Breakthrough Listen and the University of Oxford, have proposed that the candidates have another explanation: background contamination from distant, dusty quasars.

They found strong radio sources very close in the sky to three of the candidates. Each radio source is attributed to an active supermassive black hole at the centre of a very distant, dusty quasar known as a “Hot DOG”, or hot dust-obscured galaxy. Because they are dusty, they radiate infrared and are large enough in the sky to extend behind the Dyson swarm candidates.

Although no coincident radio sources were found near the four remaining candidates, the density of Hot DOGs in the sky leads Ren, Garrett and Siemion to conclude that they most probably are also contamination from Hot DOGs, but ones that are radio quiet.

But Garrett isn’t completely ruling out Dyson swarms.

“We still think the sources are worth pursuing with new observations across the electromagnetic spectrum to see which interpretation is correct,” he tells Physics World.

JWST to the rescue?

More bad news for the candidates comes from Cody’s Dyson swarm search, which used NASA’s Transiting Exoplanet Survey Satellite (TESS) to look for anomalous transits that could potentially signpost large, artificial structures.

“Our optical photometry pipeline did not classify the Dyson sphere candidates as having any anomalous variability,” says Cody. “In fact, I personally examined the available TESS data for each of the seven objects in this paper, and none of them appear to be significantly variable.”

To settle the matter, Suazo, Garrett and Cody all agree that spectroscopic observations are now vital. If dust is present, either in a disc or a background galaxy, it would produce specific absorption lines. Alternatively, spectroscopy could measure the energy distribution of a candidate star’s photosphere (its visible surface) so that a best-fit model can be applied to determine whether a candidate really is consistent with a Dyson swarm.

“James Webb Space Telescope data would be ideal, since it could quickly rule out or confirm the debris disc or the galaxy contamination explanations,” says Suazo.

If the Hot DOGs explanation turns out to be correct, it leaves the hunt for Dyson swarms in a difficult place. An infrared excess is a Dyson swarm’s calling card, but if contamination from background galaxies is the probable answer each time, how can one distinguish artificial megastructures from natural phenomena?

“It’s a good question,” says Garrett. “Hot DOGs would be detected in deep near-infrared observations. There might also be some subtle aspects of the data that would be a tell-tale sign of contamination – we will start looking at that now.”

Having already probed 60 million stars, Cody’s search continues and her team is still vetting about a thousand events that are probably eclipsing binaries, but you never know.

For Cody, a multifaceted approach is essential in hunting for Dyson swarms. “With photometric data alone, it can be challenging to distinguish between rare astrophysical phenomena and Dyson swarms,” she says. “However, I believe that optical and infrared spectroscopic data may assist with the task.”

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