Potentially-habitable planets discovered by NASA’s TESS spacecraft are becoming targets for SETI, the search for extraterrestrial intelligence, thanks to a new collaboration between TESS scientists and the Breakthrough Listen initiative.
TESS, the Transiting Exoplanet Survey Satellite, is the successor to the fantastically successful Kepler mission, which found a total of 2,717 confirmed exoplanets and a further 3,312 awaiting confirmation, mostly orbiting stars far away.
The sheer distance to most of Kepler’s planets makes studying these planets in more detail – or indeed, looking or listening for signals from any technological societies on them – more difficult.
TESS, however, is surveying the brightest and closest stars in the night sky, which will make follow-up observations of the 10,000 exoplanets that the mission is expected to find much easier.
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Among these 10,000 planets are likely to be 50–100 rocky planets in the habitable zones of their stars. Being in the habitable zone does not necessarily mean that such worlds will be habitable: it is simply defined as the distance from a star that a planet with an Earth-like atmosphere could sustain temperatures suitable for liquid water on its surface.
For example, they may lack an atmosphere, or have the wrong type of atmosphere, or be bombarded with too much radiation. Nevertheless, despite the ambiguities, planets that could potentially be habitable to life as we know it are good starting points for SETI.
Radio and laser beams
Modern SETI began in 1960, when astronomer Frank Drake performed Project Ozma, which was the first search for extraterrestrial radio signals. Radio had been identified as a promising means of interstellar communication in the year before, by astronomers Philip Morrison and Giuseppe Cocconi, who wrote a seminal paper about it that was published in the journal Nature.
Radio has the advantage that it can pass through both interstellar dust and our atmosphere to reach radio telescopes on the ground without being absorbed. It also helped that in 1959–60, radio communication was a mature technology, whereas the laser had only just been invented by Charles Townes.
Today, however, SETI encapsulates a variety of search methods, including radio and optical searches that look for high-powered pulsed lasers that can shine brighter than a star.
These lasers are not science fiction – we have lasers on Earth that can shine this brightly for trillionths of a second, such as the 5.2 petawatt laser at the Shanghai Superintense Ultrafast Laser Facility, which is most powerful laser in the world.
The search for both radio and optical signals has been accelerating since 2016, with the onset of the Breakthrough Listen SETI project, which is being funded to the tune of 15 million dollars per year by philanthropist Yuri Milner’s Breakthrough Foundation.
Looking for leakage
TESS detects exoplanets by watching for their ‘transits’ – that is, the dip in starlight when a planet transits, or moves across, the face of its star as seen from Earth. This depends on a quite specific alignment – we have to be viewing the exoplanetary system edge-on to the plane of an exoplanet’s orbit to be able to see it transit.
Andrew Siemion, who leads the Breakthrough Listen science team at the University of California, Berkeley, says that this alignment is actually a great advantage for SETI.
“We know from our own human activities that exploration of a civilisation’s own planetary system results in excess electromagnetic leakage within that plane,” he says.
Siemion isn’t referring to radio leakage from television signals, for example, as these would be too weak to be detectable. Seth Shostak, who is the Senior Astronomer at the SETI Institute in California, has calculated that if the famous Arecibo radio telescope were placed at alpha Centauri, 4.3 light years away, it would not be able to detect Earth’s television leakage.
Furthermore, he’s shown that an extraterrestrial society would require a radio telescope with an area the size of metropolitan Chicago in order to detect Earth’s television leakage at distances of hundreds of light years (for the record, our leakage has not even travelled that far yet – the broadcast of the 1936 Olympic Games should just be passing the main stars of the constellation Ursa Major, which are 80–85 light years away, by now).
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Instead, Siemion is talking about the high-powered radio signals we transmit to spacecraft exploring the other planets in our Solar System, and the radar transmissions used to survey near-Earth asteroids or map the surfaces of Venus or Mercury.
The logic is that if technological societies exist on planets discovered by TESS, then they too might be exploring their planetary system in the same manner that we do, or otherwise communicating between worlds in their system, and hence we might pick up their radio transmissions and radar beams.
“By observing other systems that are edge-on relative to Earth, we can dramatically increase our probability of detecting leakage from other civilisations,” says Siemion.
There are caveats to this. The radar beams that we use to seek out asteroids tend to be one-off events, rather than a repeated, long-term pattern of signals in a specific direction. Meanwhile, transmissions to a spacecraft exploring another planet will be sent in different directions relative to the background stars as the planet orbits its star.
This would make confirming a discovery problematic since we may only receive one radio burst in our direction, and one of the golden rules of SETI is that a signal must be seen to repeat for it to be taken seriously, otherwise we end up with unsolved mysteries, such as 1977’s Wow! signal.
Furthermore, radar signals don’t contain any intrinsic data, so they wouldn’t be carrying a message. However, should an anomalous burst of powerful radio waves be detected from a TESS planet, it would certainly prompt us to look more closely and see if we can detect any other, perhaps fainter, signals.
Listening for radio signals is a very typical SETI activity, but in 2015 there was a discovery that has helped to usher in a new era searching for technosignatures, which is a phrase that refers to extraterrestrial technology in general.
Citizen scientists scrutinising data from the Kepler mission on the Planet Hunters website found a star with a bizarre sequence of transits. There was no pattern to them, and the transits varied in size – at one point a quarter of the star’s light was being blocked!
Named Boyajian’s Star, after astronomer Tabetha Boyajian of Louisiana State University, we now know that the transits are being caused by giant clumps of dust, but for a time it was thought that the transits could be being caused by an alien megastructure, such as an incomplete Dyson sphere.
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Technosignatures had been considered by SETI scientists before. The legendary physicist Freeman Dyson developed his Dyson sphere concept, which envisioned a swarm of solar energy collectors surrounding a star in 1960.
Later, in 2005, French astronomer Luc Arnold proposed how a technological extraterrestrial society may build giant geometric structures in orbit – pyramids, cubes, rings, dodecahedrons – that would produce a distinctive dip in a star’s light when they transit that we’d recognise, as a way of communicating their existence.
However, even though Boyajian’s Star proved not to be a technosignature, the attention it received from both the scientific community and the media has helped to raise interest in the concept of looking for technosignatures of this kind.
One way to do so is to use machine learning to study transit data. ‘Clustering algorithms’ can be designed to clump objects with similar properties together – these would be your typical transiting planets, or perhaps stars with a plague of starspots, or eclipsing binary stars, anything that could cause a star’s light to dip. Objects that do not fit into these groups would be seen as outliers, quite possibly signifying something new.
“We’re most interested in algorithms that look for generic anomalies – that is, any photometric behaviour that isn’t expected,” says Siemion. Sometimes an outlier might be found to be the result of a new or unexpected natural phenomenon, like Boyajian’s Star was.
Other times, the anomaly might be the result of a quirk of the observing instrument itself, which is why it’s important to be able to collaborate directly with TESS scientists, says Siemion, because they know their instrument better than anyone and will be able to identify such instrument artefacts.
Of course, it is also hoped that one or two outliers might prove to be bonafide extraterrestrial technology. While the odds are low that we will find anything, we won’t know until we’ve looked, and a by-product of the search might be discovering new natural phenomena, as in the case of Boyajian’s Star.
Work on the partnership has already begun, with Breakthrough Listen’s coterie of observatories following up on discoveries made by TESS.
Among the thirteen observatories involved in the search are radio telescopes such as the Allen Telescope Array and the Green Bank Telescope in the United States, the Lovell Telescope at Jodrell Bank in the UK, and the Five Hundred Metre Aperture Telescope (FAST) in China, which is the largest single-dish radio telescope in the world.
Then there’s the search for optical laser signals, involving the VERITAS telescope array in Arizona, in Hawaii and the Automated Planet Finder at Lick Observatory in California.
Thanks to Breakthrough Listen, as well as the increasing number of exoplanet discoveries, SETI research is really beginning to take off. “Interest in the field of SETI is definitely growing, both in the US and internationally,” says Siemion.
Although the odds of a successful detection are still stacked against us, given the sheer number of stars to search in the Galaxy, slowly but surely ET is running out of places to hide.