In the Search for Extra-Terrestrial Intelligence (SETI) and life beyond our Solar System humanity has always struggled with the challenge of knowing what to look for. If life on Earth is anything to go by, we know that it requires some highly specific conditions in order to emerge, thrive and evolve.
And if the kinds of technologies humanity uses to communicate and explore the Universe are any indication, then some of this activity would be detectable, even from light-years away. Unfortunately, since we have only ourselves and our planet as examples, scientists are forced to make certain theoretical leaps in order to speculate what could be out there.
For instance, given the age of the Universe (13.8 billion years), it seems naive to suspect that some extra-terrestrial intelligences (ETIs) would not have been around a lot longer than us. It would also be foolish to assume that no other civilization would be more technologically advanced than we are.
If that is the case, they would likely be relying on technologies we can only imagine. Lucky for us, imagination can be a powerful tool. And over the years, scientists have come up with some very interesting ideas of what could be possible for humanity someday. And if it is possible for us, why not ETIs too?
For instance, throughout the 20th century, many scientists and science fiction writers conceived of massive structures that could be built to colonize space, encompass an entire planet, and even an entire star system. Known collectively as Megastructures, the possible existence of these has also informed some our of our SETI efforts.
These structures are what we imagine humans might eventually build once we get too big for Earth to hold us. And when it comes to possible extra-terrestrial intelligences (ETIs), its not farfetched to imagine that some of them might have already built structures that were - as Larry Niven put it - "bigger than worlds".
So let's take a look at the possibilities that have been dreamed up over the years. Some of them might already exist out there right now. And who knows? It's possible we've spotted some of them already...
As the name would suggest, the term megastructure is used to describe a possible artificial structure built in space, one that would be observable from other star systems. The first known description was made by British philosopher and science fiction writer, Olaf Stapleton. In his 1937 novel, Star Maker, he described how humanity:
"[B]egan to avail itself of the energies of its stars upon a scale hitherto unimagined. Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy."
The concept became popularized in the 1960s by British-American theoretical physicist and mathematician Freeman Dyson. In his 1960s paper titled "Search for Artificial Stellar Sources of Infrared Radiation", he outlined how an advanced civilization might be able to create a massive spherical structure that encompassed their entire star system.
These types of structures, which are often referred to as "Dyson Spheres" today, would be able to harness a large percentage of a star's energy, thereby meeting the energy requirements of an advanced species once they had grown beyond or exhausted the resources of their home planet.
Since then, multiple variations of the Dyson Sphere and other massive structures have been proposed, ranging from structures in orbit around a planet, to massive space stations capable of supplying their own gravity, to structures capable of drawing energy from an entire galaxy.
Though the particulars may vary, the basic design concept remains the same: Go Big!
An Indication of ETIs:
As noted, humanity's search for ETIs is limited based on what we know works. Since life exists on only one planet that we know of (Earth), we are restricted to looking for "potentially habitable" planets among those that are rocky, have a thick enough atmosphere, are warm enough to support liquid water on their surfaces and experience precipitation (aka. the Water Cycle).
These planets would also need to have the same climate-stabilizing mechanisms that Earth does (like the Carbon Cycle). It's not that we think life is impossible under different circumstances, it's simply that we have no idea how to spot it on worlds that have, say, a methane-nitrogen atmosphere and a methane cycle on their surfaces (like Saturn's moon Titan).
In addition, we are limited in our search for technological activity (aka. "technosignatures") that we know work. In our case, that includes radio transmissions, optical transmissions (lasers), carbon dioxide and methane (pollution), and radioactive isotopes (nuclear testing).
Beyond that, we are forced to speculate based on the kinds of technologies that are at least feasible. And then, based on the kinds of signatures these technologies are likely to produce, scientists search the Universe for them.
When it comes to SETI and speculating about what we might find out there, one name that really stands out is Nikolai Semenovich Kardashev - a Russian astrophysicist and the deputy director of the Astro Space Center, which is overseen by the Russian Academy of Sciences in Moscow.
In addition to his many contributions to the field of Russian SETI research, Kardashev devised the famous classification scheme for ETIs that bears his name. Known as the Kardashev Scale, this scheme classified a civilization's level of development based on the amount of energy they were able to harness and use.
The basics of this scheme were detailed in Kadashev's 1964 paper, "Transmission of information by extraterrestrial civilizations", where he stated that civilizations could be classified based on three types.
Type I Civilizations: Also known as "planetary civilizations", intelligent species in this category are those that can harness and store all of the energy of their home planet. According to Kardashev, this would amount to the consumption of 4 x 1019 erg/sec which would likely be in the forms of fusion power, antimatter, and renewable energy on a global scale.
Type II Civilizations: Also called as a "stellar civilization", intelligent species in this category would have evolved to the point where they could harvest all the energy emitted by their star - which Kardashev speculated would likely involve a structure like a Dyson Sphere. In this case, this would work out to a consumption of 4 x 10³³ erg/sec.
Type III Civilizations: Also known as a "galactic civilization", an intelligent species belonging to this category would be able to harness the energy of an entire galaxy, which would work out to energy consumption on the order of 4 x 1044 erg/sec.
Using this scheme, it stands to reason that species that have developed the means to harness energy on a stellar, interstellar, or galactic scale would be capable of producing artificial structures exponentially larger than anything a Type I civilization could produce.
Over time, the Kardashev Scale has undergone some expansion as scientists and theorists have proposed other categories and methods of classification. For starters, some have suggested that there be a Type 0 that would apply to all civilizations that had not yet achieved mastery of their planet and its resources.
According to Carl Sagan's 1973 book, The Cosmic Connection: An Extraterrestrial Perspective, humanity fit into this category, having not yet achieved a Type I level of development:
"A Type I civilization is able to muster for communications purposes the equivalent of the entire present power output of the planet Earth – which is now used for heating, electricity, transportation, and so on; a large variety of purposes other than communication with extraterrestrial civilizations. By this definition the Earth is not yet a Type I civilization... A combined energy/information characterization of our present global terrestrial society is Type 0.7"
Similary, there have been those who have suggested the inclusion of Type IV and Type V rankings, which would apply to civilizations that have control over their entire Universe or multiple Universes. Since the power output of the visible Universe is incalculable, there is no way to estimate how much energy civilizations in these categories would consume.
There have even been suggestions that different metrics be used to measure the level of a civilization's development. For instance, Sagan suggested in The Cosmic Connection that the amount of information available to a civilization would be a better means of gauging how advanced they had become.
Famed aerospace engineer and author Robert Zubrin also suggested that a more wholistic metric that extends beyond energy use be used - something along the lines of planetary, stellar, or galactic "mastery" instead of usage.
British cosmologist John D. Barrow has even suggested inverting the scale by classifying species based on their mastery of ever-smaller scales (i.e. microtechnology, nanotechnology, picotechnology, and femtotechnology).
Types of Megastructures:
While countless types of megastructures have been theorized over the years, some are more widely-known than others. For the most part, these structures would only be feasible for Type II civilizations; moreover, they are the means through which that reached this level of development. Here are some of the most popular concepts that have been proposed so far.
Alderson Disk (Discworld):
An Alderson Disk is essentially a massive disc-shaped that would surround a central star and maximize living space within the star's habitable zone. The idea takes its name from Dan Alderson, a scientist with the NASA Jet Propulsion Laboratory who wrote the software used to navigate the Voyager 1 and 2 probes.
The disc itself would be several thousand kilometers thick and have a radius of several astronomical units (AU) (roughly the distance between the Sun and Mars/Jupiter). The star which would reside in a hole in the center, which would means that all points on the disc would experience perpetual twilight - unless the star were made to bob up and down.
The mass of the disc would provide its own gravity, allowing for habitation on both sides, while the atmosphere would be contained by placing a thousand-km high wall at the inner edge. Assuming the presence of sufficient technology, the entire disk could be habitable. But even if life was restricted to the star's habitable zone, it would still be the equivalent to tens of millions of Earths.
The mechanical stresses on the disk mean that no known material would be sufficiently strong. Therefore, the construction of such a disk would require that various supermaterials be invented and mass-produced beforehand. Additionally, the construction of such a disk would require more matter than exists around any known stars, which means its entire system of planets and several others would have be disassembled for building materials.
The classic megastructure that was the first to be popularized. This theoretical structure is named from Freeman Dyson, and represents what he theorized a sufficiently advanced civilization might someday build in order to accommodate their energy needs and need for more habitable space.
The advantages of such a structure are that it could be built within a star's habitable zone. In the Sun's case, this corresponds to about 1 AU (or somewhere between Venus and Mars). This way, all sections of the sphere would be habitable, providing the equivalent of billions of Earths.
Another advantage is the fact that every section of the sphere would be pointed towards the Sun, resulting in perpetual daylight and the ability to meet all energy needs with solar arrays. Assuming the sphere itself was thick enough, it could also provide its own gravity.
Otherwise, artificial gravity could be created through centripetal force, caused by the sphere's rotation around the star. However, the latter scenario would mean that the strongest gravitational force would be experienced around the equatorial band, with little gravity around the poles.
While a Dyson Sphere is a classic example of a Type II civilization, it has been suggested that the concept is scalable and could be built by Type III civilizations (or higher). An example of this is outlined in a 2011 study by Inoue and Yookoo, who speculate that a civilization could be capable of building a Dyson Sphere around the supermassive black hole at the center of their galaxy.
Many variations of a Dyson Sphere have been theorized over the years, leading to the more broad term "Dyson Structure". These include the Dyson Swarm, which consists of a large number of independent constructs ranging from satellites to habitats that orbit in a dense formation around the star.
There's also the Dyson Ring, which is similar in concept but represents a scaled-down version of the Sphere (see Ringworld, below). Then there is the Dyson Bubble, which is similar to a swarm and ring in that it consists of many independent structures that co-orbit around the star.
A slight variation on the Dyson Sphere, this concept envisions megastructures arranged in concentric layers around a star (like a matrioshka doll). This "brain" would essentially be a massive super-computer, where each layer uses the heat generated by the previous layer for computational purposes.
While the innermost layer would draw energy directly from the star, every subsequent layer would draw waste heat from the adjoining one. The concept was originally proposed by Robert Bradbury as an alternative to a "Jupiter Brain" - a similar idea but on a smaller scale.
This concept envisions how a species may eventually need to rely on computational resources that are so massive, they have become stellar in scale. Alternately, the species may have chosen to shed their physical bodies and live indefinitely as part of a virtual existence powered by the Brain.
Stellar Engine (Shkadov Thruster):
Also similar to a Dyson Sphere is the concept of the Shkadov Thruster, a megastructure designed to focus the energy of a star in a single direction (thus generating thrust). The idea was first proposed by Fritz Zwicky, a Swiss astronomer, who indicated during a lecture at Oxford University in May 1948 where he addressed the possibility of:
“…accelerating…[the Sun] to higher speeds, for instance 1000 km/s directed toward Alpha Centauri A in whose neighborhood our descendants then might arrive a thousand years hence. [This one-way trip] could be realized through the action of nuclear fusion jets, using the matter constituting the Sun and the planets as nuclear propellants.”
However, it was Russian aeronautical engineer Dr. Leonid Shkadov who provided a detailed description and calculations with his 1987 study, "Possibility of controlling solar system motion in the Galaxy". His proposal for the stellar engine that would bear his name consisted of a giant, curved reflective surface placed close enough to the star to be attracted by its gravitational force.
As sunlight struck the reflective surface, it would create a repulsive force, pushing the megastructure away. The star's gravitational pull would cause it to be pulled along for the ride and the entire system would slowly begin to move. Over time, the stellar engine would accumulate tremendous speeds and be able to leave its part of the galaxy. As Shkadov explained:
"It is shown, that if a screen reflecting solar rays is positioned stationarily at some distance from the sun, the central symmetry of solar radiation in the sun-screen system will be violated and a force disturbing the sun motion will arise... It is shown that during one orbital period of the Sun a radial deflection of the sun from its reference orbit by the value of some 10-12 parsec is possible. Lateral deviation of the sun by 4.4 parsec from its orbital plane is also possible, when the screen axis is normal to the orbital plane and has a constant orientation."
Within this concept, it is conceivable that a system of planets would still orbit the star. If placed well-outside a distance of 1 AU from the star, an Earth-like planet would still be able to orbit without complications. Habitats could also be established around the rim of this massive structure, allowing for billions of inhabitants to travel through space.
In this way, the population would be able to use their star as a means of transporting planets, moving throughout the galaxy and colonizing other planets. Scientists have speculated that this is already the case when it comes to hypervelocity stars that have been kicked out of our galaxy due to interaction with our supermassive black hole (Sagittarius A*).
Yet another megastructure inspired by Dyson's proposal is the Ringworld or Niven Ring, a concept which is named for its inventor (science-fiction author Larry Niven) and the 1970 novel that popularized it (Ringworld).
As the name would suggest, this megastructure consists of an artificial ring that orbits a star with a radius roughly equal to Earth's orbit (1 AU). The ring spins to create artificial gravity while inner walls to prevent atmosphere from escaping. As Niven described it in the foreword to the novel:
“I myself have dreamed up a structure intermediate between Dyson spheres and planets. Build a ring 93 million miles in radius - one Earth orbit - around the sun. If we have the mass of Jupiter to work with, and if we make it a thousand miles wide, we get a thickness of about a thousand feet for the base."
As with other megastructures, the advantage of such a structure is that it would multiply the amount of livable space within a star's habitable zone. While the concept has been criticized for being unstable and improbable from an engineering standpoint, it remains one of the most popular examples of a megastructure to date.
Other Possible Megastructures:
Beyond these classic examples, there are many other types of megastructures that have been theorized and proposed over the years. In most cases, these are smaller concepts that would be possible for Type I civilizations, but could still be detectable by civilizations living in star systems located light-years away.
Banks Orbital/Bishop Ring (Halo):
Similar to a Niven Ring, a Banks Orbital and a Bishop Ring are smaller versions of a ring-shaped space habitat that rotates to provide artificial gravity, as well as a day-night cycle. The former concept takes its name from science fiction writer Ian M. Banks, who wrote about such structures in his Culture series.
Based on the descriptions from his novels, a Banks Orbital would measure approximately 10 million km (62 million mi) in circumference and have widths varying between 1000 and 6000 km (620 and 3730 mi), giving them a surface area of between 20 and 120 times that of the Earth.
The Bishop Ring takes its name from Forest Bishop of the Institute of Atomic-Scale Engineering, who detailed his proposal in a 1997 study titled "Open Air Space Habitats". The original proposal called for a structure measuring 1000 km (620 mi) in radius and 500 km (310 mi) in width.
These megastructures can be built in orbit of a planet, or within a system's Lagrange Points, and daylight can be provided by angling the ring towards the system's star or by positioning an angled mirror (or artificial Sun) in the ring's center.
Bernal Sphere (O'Neill Cylinder/Topopolis):
The Bernal Sphere was proposed by John Desmond Bernal in his 1929 study titled "The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul". In Section II: The World, he spoke of the challenges of conquering space and what it would take to create a lasting human presence there.
To this, Bernal suggested the creation of a hollow spherical space habitat measuring 1.6 km (1 mile) in diameter, filled with air, and able to accommodate a population of 20,000 to 30,000 people:
"Imagine a spherical shell ten miles or so in diameter, made of the lightest materials and mostly hollow; for this purpose the new molecular materials would be admirably suited. Owing to the absence of gravitation its construction would not be an engineering feat of any magnitude. The source of the material out of which this would be made would only be in small part drawn from the earth; for the great bulk of the structure would be made out of the substance of one or more smaller asteroids, rings of Saturn or other planetary detritus."
This concept is said to have been one of the inspirations for American physicist Gerard K. O’Neill and his students to come up with the idea of the O'Neill Cylinder. The concept was described in his 1976 book, The High Frontier: Human Colonies in Space, where O'Neill described how humanity could expand throughout the Solar System by building "islands in space":
"Although the total volume of the asteroids is far smaller than Earth's, it is a volume much more accessible than the depths of our planet. On Earth only a thin skin of material is available to us without deep mining under high pressures and intense heat... [W]e would have to disfigure the entire Earth to obtain only a hundredth of the material contained in three now-useless, lifeless asteroids; and there are thousands of those minor planets."
Measuring 8 km (5 mi) in diameter and 32 km (10 mi) long, an O'Neill cylinder would consist of two counter-rotating cylinders. These would provide artificial gravity while at the same time canceling out any gyroscopic effects and keep the habitat aimed at the Sun.
The advantages of these types of habitats, according to O'Neill, is that they would relieve population pressures here on Earth. The cylinders could also be positioned through the Solar System at the L3, L4, and L5 Lagrange Points, creating habitats for millions of people without having to colonize other planets.
“A nonindustrial Earth with a population of perhaps one billion people could be far more beautiful than it is now," he wrote. "Tourism from space could be a major industry, and would serve as a strong incentive to enlarge existing parks, create new ones, and restore historical sights."
A Shellworld is similar to a Dyson Sphere except that it encompasses an entire planet instead of a star system. The concept was proposed by Kevin Roy - an engineer with the US Department of Energy - in his 2009 study, "Shell Worlds - An Approach To Terraforming Moons, Small Planets and Plutoids".
The concept was proposed as a means of making terraforming more effective. This was to be accomplished by building a large “shell” around an uninhabitable world to ensure that atmospheric gases that were introduced would not be lost to space.
This would allow for long-term changes to take root, which would include the introduction of microorganisms, plants and other complex life forms that would ensure that habitable conditions would be stabilized and reinforced.
This process, which is an extension of paraterraforming, would enable planets that do not have the proper initial conditions (i.e. being warm enough, wet enough, in possession of an atmosphere and a magnetic field, etc.) to be terraformed.
Here is a concept that has been immensely popularized in science fiction, and is even the subject of detailed research. First proposed in 1959 by Russian scientist Yuri N. Artsutanov, the concept is based on the idea of geostationary satellite and counterweight in orbit being connected to the Earth by a massive tensile structure (aka. "the beanstalk").
Between Earth and the geostationary satellite, rocket-powered robot cars could ferry payloads, materials, and people up and down the beanstalk. From there, they could be shipped off-world to the Moon, to Mars, or any number of locations throughout the Solar System.
In addition to being a massive feat of engineering, the concept would significantly reduce the cost of launching crew and payloads to space. For example, a 2017 report from NASA's Ames Research Center indicated that it cost roughly $400 million to launch 16,000 kg worth of payload to Low Earth Orbit (LEO) - which works out to about $25,000 per kg ($11,365 per lbs).
Stanford Torus (Von Braun Wheel):
Like many of its counterparts, this concept calls for a massive space habitat that rotates in order to provide artificial gravity. The concept was originally proposed by Russian rocket scientist Konstantin Tsiolkovsky, who wrote about using rotation to create artificial gravity in space in 1903.
This was flowed by Slovenian rocket engineer Herman Potočnik with his 1928 book, The Problem of Space Travel: The Rocket Motor. In this comprehensive study, he described a rotating "Habitat Wheel" that would be placed in geostationary orbit (GSO) around Earth.
During the 1950s, German-American rocket scientist Wernher von Braun proposed a rotating torus-shaped station (known as a Von Braun Wheel), which was featured in a series of articles in the national magazine Collier’s titled, “Man Will Conquer Space Soon!”
The idea became more widely-known after being proposed as a part of the 1975 NASA Summer Study, a collaborative effort between NASA’s Ames Research Center and Stanford University. The resulting design would thereafter be known as a "Stanford Torus".
Searching for Megastructures:
All of these concepts are exciting on their own, but even more exciting is the prospect of finding them or something similar within our galaxy - or possibly other galaxies. But how would we go about searching for them? What "technosignatures" would indicate the presence of a megastructure and hence, a highly-advanced civilization?
As Professor Abraham Loeb - the Frank D. Baird Jr. Chair of Physics and the Chair of the Astronomy Department at Harvard University - told Interesting Engineering via email:
"It is likely that advanced civilizations will modify their natural environment, by building megastructures, producing artificial lights, polluting atmospheres and redistributing heat. This has two benefits. It provides us with many possible flags that would signal their existence so that we will know that we are not alone. And second, by studying the burnt-up surfaces of planets with dead civilizations, we will learn how to get our act together and avoid a similar fate."
Sounds totally worth it, doesn't it? But of course, the search for ETIs and technosignatures presents some very big challenges. In a recent essay published in Scientific American, Prof. Loeb also addressed what is perhaps the greatest of them all.
"If other civilizations do exist," he wrote, "one key in becoming aware of them is whether we are intelligent enough to adequately interpret their signals or to identify a piece of their technology if it should appear in our solar system."
Considering we have no frame of reference when to comes to ETIs or the technology they might use, it is entirely possible the signs are out there and we are just missing them. Luckily, the same sense of wonder and imagination that has allowed us to speculate about megastructures has also offered some suggestions on how to look for them.
Intrinsic to Freeman Dyson's study, where he first proposed the concept of a spherical megastructure, was an idea of how we might go about finding them among the cosmos. According to Dyson, megastructures that harnessed the energy of an entire star would radiate massive amounts of waste heat out into space.
This heat could be detected using the infrared instruments on some of the world's largest telescopes. The same holds true for any orbital structure that harnesses energy from a star. In short, any Type II megastructure could be spotted by examining stars closely for signs of thermal anomalies.
Another method would be to monitor star systems closely for signs of periodic dips in brightness. Ordinarily, this method (Transit Photometry) is used to discern the presence of exoplanets around stars, where dips are caused by exoplanets passing in front of the star relative to the observer.
Using this method, astronomers are able to not only detect exoplanets but also place accurate estimates on their size and orbital period. In much the same way, the transit of a megastructure in front of a star would lead to a significant drop in brightness that would not be easily explained.
In 2015, this possibility was considered when an international team of astronomers noticed that a distant star - KIC 8462852 (now Boyajian's Star or Tabby's Star, after the team's lead researcher) - was dimming periodically. Even more curious was the fact that no natural cause could be discerned.
Multiple follow-up studies were conducted that led to various explanations, ranging from comets and dust rings to a consumed planet or planets with large dust rings. A 2018 study by an international team of 100 astronomers - and led by Assistant Professor Tabetha Boyajian herself - appeared to settle the matter by showing that Tabby's Star was likely being obscured by dust.
However, the mystery of Tabby's star endures, as many more dips in brightness have been recorded which may or not confound this explanation. In the meantime, other stars have been undergoing similar dimming patterns.
These include EPIC 204278916, a young star that showed periodic dips in its light curve throughout 2016, and VVV-WIT-07, a variable star that experienced several dips and an eclipsing event in 2012. Here too, natural explanations have been made that appear to fit the facts, but there are still those who believe that the "alien megastructure theory" cannot be ruled out.
In the case of Type I megastructures, their smaller size would make them more difficult to detect. Nevertheless, these could be spotted in the same way that astronomers hope to be able to detect constellations of satellites around distant exoplanets (Clarke Belts) in the coming years.
In this respect, next-generation telescopes that will allow for the Direct Imaging of exoplanets could also help scientists detect orbital structures. These include the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT), the Extremely Large Telescope (ELT), all of which are scheduled to finish construction sometime in the next decade.
In the same way, the deployment of the James Webb Space Telescope (JWST), and the Wide-Field Infrared Survey Telescope (WFIRST) in the coming decade will make it easier to detect large-scale infrared signatures light-years away, which could indicate the presence of Type II megastructures.
And thanks to the recently-retired Kepler Space Telescope and current missions like the Transitting Exoplanet Survey Satellite (TESS), scientists are able to gather light curves from thousands of stars at a time, all of which are monitored closely for possible transits.
With all of these instruments at our disposal, an army of astronomers and countless citizen scientists, we will not be alone in the Universe forever. If there are any Type I, Type II, or even more civilizations out there, we will sniff them out... sooner or later!
*A special shout out to Neil Blevin for much of the cool artwork in this article. You can find his work here - ArtOfSoulburn (DeviantArt)
- Wikipedia - Megastructures
- LarryNiven.net - Megastructures
- Larry Niven - Bigger than Worlds
- EarthSky - What is a Dyson Sphere?
- Centauri Dreams - The Star as Starship
- NSS - Stanford Torus Space Experiment
- Listverse - Top 10 Theoretical Megastructures
- Universe Today- What are Alien Megastructures?
- Scientific American - Are We Really the Smartest Kid on the Cosmic Block?