On blind dates, we search for others that resemble us, at least at some level. This is true in our personal life but even more so on the galactic dating scene, where we have been seeking a companion civilization for a while without success. While developing our own radio and laser communication over the past seven decades, the Search for Extraterrestrial Intelligence (SETI) focused on radio or laser signals from outer space—two kinds of electromagnetic “messenger” that astronomers use to study the cosmos.
Over the same period, we have been also launching probes, like Voyager 1 and 2, Pioneer 10 and 11 and the New Horizons spacecraft, towards interstellar space. These could eventually reach alien civilizations, passively announcing our existence. But in 1960, at the dawn of the space age, Ronald Bracewell noted in a Nature paper that a physical space probe could also search for technological civilizations across interstellar distances. SETI should therefore explore this technique as well—a timely notion in the era of multimessenger astronomy, ushered most recently by the detection of gravitational waves.
This sort of exploration obviously could work both ways. Thanks to data collected by the Kepler space telescope, we now know that about half of all sunlike stars host a rocky Earth-size planet in their habitable zone. Within this zone, the planet’s surface temperature can support liquid water and the chemistry of life. The famous Drake equation quantifies (with large uncertainties) the likelihood of receiving a radio signal from another civilization in our Milky Way galaxy. But it does not apply to physical probes that might arrive at our doorstep. The distinction resembles the difference between a cell phone conversation at the speed of light and the exchange of letters through surface mail.
It also suggests an addendum to the Drake equation: the number of probes in a volume of interstellar space can be expressed as the number of stars times the average number of probes produced per star, N. The nearest star system, Alpha Centauri, contains a close pair of sunlike stars (A and B) bound to a more distant dwarf star (C). This triple star system is about four light years away—but the nearest probe could be much closer—at a distance that is smaller by a factor of (3N)1/3. In fact, this probe would be within the Earth-sun separation if civilizations produce on average a quadrillion (N~1015) probes per star over their lifetime.
If each probe weighs a gram, similar to what has been proposed by the Breakthrough Starshot initiative, then the total mass of a quadrillion probes would be comparable to the weight of a kilometer-size asteroid—completely negligible in the planetary mass budget. Such a meteor strikes the Earth every half a million years, and its size is smaller by a factor of several tens than the Chicxulub K/Pg impactor that killed the dinosaurs about 66 million years ago. Clearly, the actual number of interstellar probes would depend on the abundance and lifetime of technological civilizations per star, as well as the weight of each probe and the sophistication of its production technology.
My forthcoming book, titled Extraterrestrial, tells the story of the discovery of `Oumuamua, meaning “scout” in the Hawaiian language, by the Pan-STARRS facility in Hawaii on October 2017. As the first interstellar object detected near Earth from outside the solar system, it looked weird, unlike any comet or asteroid seen before within the solar system. The book details the unusual properties of `Oumuamua: it had a flattened shape with extreme proportions—never seen before among comets or asteroids, as well as an unusual initial velocity and a shiny appearance. It also lacked a cometary tail, but nevertheless exhibited a push away from the sun in excess of the solar gravitational force.
As a regular comet, `Oumuamua would have had to lose about a tenth of its mass in order to experience the excess push by the rocket effect. Instead, `Oumuamua showed no carbon-based molecules along its trail, nor jitter or change in its spin period—as expected from cometary jets. The excess force could be explained if `Oumuamua was pushed by the pressure of sunlight; that is, if it is an artificially-made lightsail—a thin relic of the promising technology for space exploration that was proposed as early as 1924 by Friedrich Zander and is currently being developed by our civilization. This possibility would imply that `Oumuamua could be a message in a bottle.
In September 2020, another unusual “asteroid” was discovered by-Pan STARRS, showing an excess push by sunlight without a cometary tail. This object, labeled by the astronomical name 2020 SO, was not unbound like `Oumuamua but instead on an Earthlike orbit around the sun. After integrating its orbit back in time, it was found that 2020 SO is a stray rocket booster, left over from a crash of the Surveyor 2 lunar lander on the surface of the Moon in 1966.
Nevertheless, its discovery lends credibility to the notion that thin artificial objects with a large surface-to-mass ratio can be distinguished from natural objects based on their excess push away from the sun without a cometary tail. There is no way that `Oumuamua could have originated from our planet based on its high local speed, its large size and the inclination of its trajectory. Another way to put it is that `Oumuamua spent a fraction of a year within the orbit of the Earth around the sun, and we know of no human-made object that was propelled to its trajectory over the year preceding its discovery.
When taking a vacation near a beach, I enjoy studying natural seashells, but on rare occasions I encounter an artificially made plastic bottle. Similarly, astronomers regularly spot naturally made rocks when monitoring comets or asteroids from the solar system, but perhaps `Oumuamua represents our first encounter with a plastic bottle, manufactured by an advanced technological civilization. Lightsails can be designed to weigh a gram per tens of meters squared of surface area, comparable to the area of `Oumuamua.
Interstellar probes could also maneuver to preferred trajectories that are not drawn from a random distribution. In particular, it is beneficial to bring them to rest relative to the star they intend to probe. In that case, the gravitational attraction by the star will pull them straight towards it. The focusing of their trajectories will enhance their density in the vicinity of the star, allowing more of them to travel through the habitable zone and spy for any technological signatures there. In the outer envelope of the solar system, such slow-moving probes would be hidden among the numerous icy rocks of the Oort cloud, that are loosely bound to the sun halfway to Alpha Centauri.
If the senders of the probes prefer to remain anonymous, they might choose to deposit them in the galactic parking lot, the so-called local standard of rest, which averages over the random motions of all stars in the vicinity of the sun. In this neutral frame of reference, it is not possible to identify where they came from. Surprisingly, `Oumuamua started in that frame before entering the solar system.
The data we gathered on `Oumuamua are incomplete. To learn more, we must continue to monitor the sky for similar objects. The realization that we are not alone will have dramatic implications for our goals on Earth and our aspirations for space. When reading the news every morning, I cannot help but wonder whether we are “the sharpest cookies in the jar.” Are there extraterrestrials smarter than us in the Milky Way? The only way to find out is by surveying the sky for the multitude of messengers that they might be using.