New study examines how extraterrestrial civilizations could become ‘stellivores’

One of the most challenging aspects of astrobiology and the Search for Extraterrestrial Intelligence (SETI) is anticipating what life and extraterrestrial civilizations will look like. Invariably, we have only one example of a planet that supports life (Earth) and one example of a technologically advanced civilization (humanity) upon which to base our theories.

As for more advanced civilizations, which statistically seems more likely, scientists are limited to projections of our own development. However, these same projections offer constraints on what SETI researchers should search for and provide hints about our future development.

In a series of papers led by the Blue Marble Space Institute of Science (BMSIS), a team of researchers examines what Earth’s level of technological development (aka “technosphere”) will look like in the future. In the most recent installment, available on the arXiv preprint server, they offer a reinterpretation of the Kardashev Scale, which suggests that civilizations expand to harness greater levels of energy (planet, host star, and galaxy).

Instead, they suggest that the Kardashev Scale establishes upper limits on the amount of stellar energy a civilization can harness (a “luminosity limit”) and that civilizations might circumvent this by harnessing stellar mass directly.

As with the previous study in this series, the research was led by Jacob Haqq-Misra, the Senior Research Investigator at the Blue Marble Space Institute of Science. He was joined by George Profitiliotis, an Affiliate Research Scientist at the BMSIS and a Research Member of the Working Group on SETI and Law at the International Institute of Space Law (IISL), and Clement Vidalb, a researcher with the Center Leo Apostel (CLEO) at the Free University of Brussels. The paper “Projections of Earth’s Technosphere: Luminosity and Mass as Limits to Growth” is being reviewed for publication in Acta Astronautica.

The Kardashev Scale, named after Soviet-Russian astrophysicist and radio astronomer Nikolai Kardashev (1932–2019), was first proposed in his seminal paper, “Transmission of Information by Extraterrestrial Civilizations,” released in 1964. In it, Kardashev suggested what types of radio frequencies (and at what energies) scientists should search for to discern possible transmissions of an extraterrestrial civilization (ETC). In keeping with the idea that there may be civilizations billions of years older than humanity, he reasoned that these civilizations could harness levels of energy beyond human capabilities.

To characterize the level of an ETC’s development, Kardashev proposed a three-level scale based on the amount of energy they could harness. This included:

• Type I—Planetary Civilizations: ETCs that have developed the means to harness and store all of their home planet’s energy, an estimated 4×10¹⁹ erg/sec.
• Type II—Stellar Civilizations: ETCs that have evolved to the point where they can harvest all the energy emitted by their star—4×10³³ erg/sec.
• Type III—Galactic Civilizations: ETCs able to harness the energy of an entire galaxy—4×10⁴⁴ erg/sec.

However, this scale reflected the assumption that civilizations and their energy needs will grow exponentially. This is in keeping with observations of humanity’s own “technosphere,” which refers to the human-made infrastructure, machinery, communications, and other indications of technological activity (aka “technosignatures”). Basically, it reflects our limited perspective when it comes to the kinds of behaviors advanced ETCs would exhibit. As Haqq-Misra told Universe Today via email:

“Earth is our only known example of a planet with technology, so the search for extraterrestrial civilizations must begin by thinking about how to search for analogs to Earth’s technosignatures today and possible technosignatures that could arise in Earth’s future. We should also try to stretch our minds to consider other, non-terrestrial, and more exotic possibilities, but even such imaginative possibilities will always either begin with (or contrast with) what we know is possible based on existing or known physics on Earth.”

Traditional applications of the Kardashev Scale predict that growth will be exponential and have even considered how this could give rise to a civilization capable of utilizing the energy output of all stars in the universe—a Type IV Cosmic Civilization. This application has motivated many searches for civilizations that have reached these scales of vast energy utilization, as indicated by megastructures (e.g., Dyson Spheres, Clarke Bands, etc.) and other advanced technospheres. For their study, Haqq-Misra and his colleagues took a different approach:

“Our study re-examines these assumptions by noting that civilizations can follow different trajectories for their expansion in space and their energy consumption. This involves tradeoffs between ‘exploration’ and ‘exploitation,’ and there are many possibilities for how a civilization might develop along these two dimensions.

“Some civilizations may prioritize exploration in physical distance without ever needing to expand their energy consumption to Kardashev Type I or Type II scales. Other civilizations may focus on exploitation and increase their energy use more locally. Some civilizations may attempt to find an optimal balance between exploration and exploitation.

“We also point out that the Kardashev scale is better considered as a theoretical limit to a civilization that utilizes stellar energy (luminosity). Rather than describing a trajectory that advanced civilizations will follow, the Kardashev scale is the uppermost limit for a civilization’s energy use, as it relates to expansion in physical distance, but a limit that may never actually be achieved due to thermodynamic efficiency limits.

“In other words, the Kardashev scale describes an upper-limit to the tradeoffs between exploration and exploitation, and a civilization that is dependent on stellar luminosity for its energy needs will always fall below the energetic and spatial limits described by the Kardashev scale.”

The scenario Haqq-Misra and his colleagues proposed presents some new and interesting possibilities for advanced civilizations. For example, suppose humanity ever reaches the limit of how much energy it can harness from our sun. In that case, it may not choose to explore and settle other star systems (with the intent of harnessing the energy of more planets and more stars). Instead, they may turn to harvesting stellar mass itself.

“Civilizations like this that consume stars, which we call ‘stellivores,’ would be able to expand in energy use beyond the luminosity limits of the Kardashev scale,” said Haqq-Misra. “We are not at this level as a civilization on Earth yet, but we can at least think about the possibility that harvesting mass and converting it into energy (as Einstein’s famous equation describes) provides a way for a civilization to reach energy use scales beyond those envisioned by the Kardashev scale.”

Like all projections on humanity’s future development, this study also has implications for future SETI surveys. This is in keeping with the assumption that ETCs in our galaxy would be older and more advanced than humanity at this point. It’s also consistent with the principle that “if we can conceive of it, someone else has probably done it already.” As Haqq-Misra explained, future SETI surveys should examine “accreting binaries,” closely orbiting binary stars with mass flowing from one star to another.

Maqq-Misra and his colleagues recommend that scientists observe accreting binaries to search for abnormal behavior, which could indicate technological activity:

“If some civilizations actually do evolve into stellivores, then some of these may look like such accreting binary star systems. We cannot claim that all, or even most, accreting binaries are actually technological civilizations, but we also cannot rule out the possibility that some of them could in fact be technological. It is worth keeping our minds open and actually searching for such evidence of advanced and exotic civilizations rather than ruling them out before we look.”