A pair of researchers from the Harvard–Smithsonian Center for Astrophysics and Princeton University have developed a new means by which, in the future and with the requisite telescopic power, it may be possible to detect artificial lights from cities on other planets inhabited by extraterrestrial intelligence. In the meantime, say Professors Abraham Loeb and Edwin Turner, the technique can be put to the test by searching for artificially illuminated objects in our own Kuiper Belt.

The idea of looking for an artificial body in the Kuiper Belt isn’t quite as outlandish as it sounds. In 1950 the nuclear physicist Enrico Fermi, at the Los Alamos National Laboratory, famously posed his extraterrestrial puzzler that subsequently has become know as the Fermi Paradox: if they exist, where are they? Fermi’s rationale was that it would potentially take millions of years to colonise the Galaxy, but that is no time at all compared to the age of the Galaxy. Therefore if extraterrestrial intelligence exists, it should have had plenty of time to have reached Earth long ago. Given that they are not here, that might lead one to think they don’t exist. However, one possible solution (of many) to Fermi’s Paradox is that they have been here, but parked a probe hidden somewhere in our Solar System to keep an eye on us, and the Kuiper Belt has been cited as one possible location.

The Kuiper Belt is a disc of icy bodies beyond the orbit of Neptune, extending between 30–50 astronomical units from the Sun (an astronomical unit, AU, is 149.6 million kilometres, which is the average distance between Earth and the Sun). Among its denizens are long period comets and frozen dwarf planets such as Eris and Pluto. Loeb and Turner have calculated that the lights of a city such as Tokyo would appear at a magnitude of +23.7 at a distance of 30AU. This is very faint, but the faintest objects ever seen – distant galaxies in the Hubble Ultra Deep Field – were detected at magnitude +31.3 by the Hubble Space Telescope. This is the equivalent of Tokyo being placed at thousands of astronomical units from us, far beyond the Kuiper Belt. Therefore, propose Loeb and Turner, a survey of the Kuiper Belt is definitely within our capabilities.

“Observing a city in the Kuiper Belt, 30AU away, can be done with a modest size telescope and observing time, since this city will be almost a thousand times brighter [than the faintest object seen by Hubble],” Loeb tells Astronomy Now.

Even with Hubble, taking a picture would likely be out of the question, for a typical Kuiper Belt object at that distance would appear as little more than a point source. Instead, Loeb and Turner describe in a paper submitted to the journal Astrobiology that we should instead examine the spectra of Kuiper Belt objects. Artificial light on Earth, be it incandescent sodium lamps or light emitting diodes [LEDs] have different spectral properties to natural starlight (or indeed, the light of natural fires or volcanoes). While the intensity of light from sunlight-illuminated objects drops off inversely with distance to the fourth power, artificial objects would drop off with the inverse square law.

“Since the orbits of Kuiper Belt objects are known to exquisite precision, we can forecast how their distance will change with time and then measure how their corresponding flux changes with time,” says Loeb. “Measuring this light curve is straight-forward, and will allow us to separate those objects that are artificially illuminated.”

Working off the back of existing or future observatories, such as the 8.4-metre Large Synoptic Survey Telescope that it is hoped will be operational by the end of the decade, a search of the Kuiper Belt would not involve any extra financial investment beyond manpower and computer processing time. The chances of finding something in the Kuiper Belt is pretty thin, but if we don’t look we will never know. Moreover, a Kuiper Belt survey would provide a testbed for detecting the lights of alien cities in perhaps a more probable location: exoplanets.

Alien homeworlds
In order to detect lights on exoplanets, we’ll need to build the next generation of giant telescopes, such as the 39.3-metre European Extremely Large Telescope set to be constructed at the European Southern Observatory in Chile and the Thirty Metre Telescope to be built in Hawaii, or perhaps the 6.4-metre James Webb Space Telescope in orbit. These telescopes will watch for transiting planets and then isolate the light of the planet by subtracting the light of the star by itself (when the planet is hidden in eclipse behind it) from the light of the star and planet combined. This technique has already been used by astronomers to detect the signatures of the likes of hydrogen, oxygen, water vapour and carbon dioxide in exoplanet atmospheres.

When transiting, the planet’s night side is pointed towards us. Ninety degrees farther around its orbit, we’ll see the planet in its ‘quarter’ phase, with half the visible world in daylight and half in the darkness of night. Consequently, as the planet travels around its star exhibiting different phases, the levels of daylight and artificial light from the night-side would be seen to wax and wane. However, to detect the flux in light from alien cities on the night-side of a planet would require the extraterrestrials to run up an immense electricity bill.

“What matters is how much power is associated with the artificial light on the night-side relative to the day-side, and detecting the artificial illumination on planets around other stars requires that the night side will be of comparable luminosity to the day side,” says Loeb. “If the illuminated area is the same in both cases, the luminosity per unit area should be comparable. If the illuminated area is small [for instance, a concentrated region such as a conurbation] its brightness should be higher than the dayside.” In other words, we would have to look for a world with a gargantuan planet-wide city, rather than the pockets of urbanisation on Earth. Such mega-cities may not be feasible, according to Edwin Turner.

“Note they cannot illuminate the night side of their planet to levels comparable to the day-side without roughly doubling the rate at which it is heated by the incoming starlight alone,” he says. “Their global warming issue would dwarf any we might currently have.”

This leads to some interesting consequences. Recently there has been a schism amongst the SETI community regarding whether we should transmit our own messages into space, an activity known as Active SETI. Proponents of Active SETI have countered that because advanced extraterrestrial civilisations would have the technology to see our city lights, they would know we are here and consequently there is little point in continuing to hide our existence. However, according to Loeb and Turner, Earth’s city lights would be far too feeble to see, at least using their method. “The light produced by our civilisation on the night-side of Earth is only a millionth of the solar power impinging on the day-side,” says Loeb. “It will be extra difficult for extra-solar civilisations to see us with similar telescopes.”

The Sun as a lens
Within the next few decades, proposed missions such as NASA’s Terrestrial Planet Finder and ESA’s Darwin mission should be able to image nearby terrestrial planets directly, but there is another route that a more technologically advanced civilisation may take to detect our city lights.

As any cosmologist will tell you, a large mass can bend space-time to such an extent that any light travelling through that patch of space-time becomes magnified, creating a natural telescope. It is by this method that cosmologists probe some of the deepest recesses of the Universe, by studying faraway galaxies that have been ‘gravitationally lensed’ by foreground galaxies or clusters of galaxies. In the same way but on a much smaller scale, our Sun becomes a gravitational lens at a distance of 550AU.

Reaching such a distance from the Sun is currently beyond our capabilities – Voyager 1 is only about 40AU from the Sun after travelling for 35 years. However, a number of scientists, including Dr Claudio Maccone of the International Academy of Astronautics, are exploring the potential for such a mission, nicknamed FOCAL, that could perhaps launch later this century. Using the Sun as a gravitational lens, FOCAL would be able to directly image Earth-like planets, cities and all. “If they [extraterrestrials] are using their own star as a gravitational lens, which is something we might do in the next 100 years, then they can pick up the lights of London,” says the SETI Institute’s Dr Seth Shostak.

The technological and engineering obstacles will be formidable, not just in getting out to 550AU but also in building multiple telescopes to stare at the sky in all directions. “Otherwise a big problem with a solar gravitational lens telescope is that is is very hard and slow to point it at different targets,” says Turner. “To do so, you have to move around by hundreds of astronomical units or more. “That said, if we ever do implement this inventive technique, then it could be used to search for artificial illumination on exoplanets among many other things.”

A gravitational lens telescope is for the future, whereas Loeb and Turner’s proposal is for the here and now and the realities of the economic climate in which we live. “The effort required to realise our proposal is orders of magnitude less expensive than putting a telescope at 550AU,” says Loeb. “The trick, especially in an economic recession, is to find ways to obtain exciting results with limited resources. The science that we propose to do within the Solar System does not require investments of new funds. We should simply do it without a prejudice.”

Equally as great as the detection of a radio signal from ET, finding artificial lights on another planet would be conclusive proof that we are not alone. As Loeb says, “The discovery of an alien city will change our perception of reality.”