To date, researchers have identified over 5,000 exoplanets in the Milky Way, a small fraction of the billions hypothesized to exist in our galaxy. Among them, researchers catalog so-called “Earth-like planets,” or planets with a similar size, structure, and distance from the Sun as Earth. While of great interest to modern astronomers, no conclusive evidence of past or present extraterrestrial life on these planets has been found.
But what if one of these planets was inhabited by beings like us, also searching space for signs of life? If an extraterrestrial space agency with similar technical capabilities as NASA pointed their telescopes toward Earth, would they detect our existence? To answer this question, we can adopt the perspective of the closest known exoplanet and “super Earth” Proxima Centauri b.
NASA Visualization Technology Applications and Development (VTAD)
Before we can look through the lens of this analog space agency, or NASA b, we might first need to consider how long humans have been around. Because our perception of objects in space depends on the amount of time it takes light from that object to reach us, all local observers looking towards Earth will view Earth at some point in its past. Significant human impacts on the planet have only occurred within the last 100 years, so far away observers would be unlikely to detect signals of human presence. But at only four light years away, Proxima Centuari b is close enough that, if observing today, NASA b would perceive Earth in 2020.
To detect Earth itself, NASA b could use a few different techniques. One popular method relies on a concept called transiting. Similar to an eclipse, when Earth passes in front of the Sun from an appropriate vantage point in space, the amount of light (or “light curve“) detected from the Sun will dim for the length of that transit, or time it takes to pass in front of the Sun. From observing this transit, NASA b could apply their laws of planetary motion to determine the size of Earth, the length of our year, and the distance between Earth and the Sun. If they had similar criteria for a “habitable zone” as we do, they could then identify Earth as a candidate planet for life development.
After identifying Earth’s planetary characteristics, detailed observations of the Sun’s spectra during Earth’s transit could provide even more clues towards its inhabitants. As the Earth transits, light from the Sun interacts with the gasses in its atmosphere, creating a unique dispersion pattern. Using this method, NASA b could identify key component gasses in Earth’s atmosphere, namely nitrogen, oxygen, and even water vapor. The presence of these gasses could serve as another clue for the existence of life.
Due to their close proximity to Earth, they might also be able to identify so-called “technosignatures”, or remnants of industrial activity located in the atmosphere. Nitrogen dioxide, an abundant byproduct of fossil fuel burning, is a likely candidate, along with pollutant chemicals and infrared emissions. After recording the abundance of these emissions, NASA b could run models to determine that the amount present in our atmosphere is far greater than could be created by natural processes. This would almost certainly be conclusive evidence for our existence.
While these would certainly serve as important discoveries, this might be the extent of their observations. Without significantly larger and more advanced telescopes, NASA b would not be able to resolve any specific features like spacecraft or city lights on Earth. (Consider that if the Earth was the size of a bowling ball, its largest satellite, the ISS, would be imperceptible to the human eye.) So NASA b would not be able to spy on individual Earthlings quite yet.
But they might be able to hear us.
Radio signals, the preferred method for deep-space communication, have been emitted from Earth since the early 1900s. Not only do deep space missions like Voyager-1 require complicated systems of data relaying that send unmistakably artificial broadcasts into space, there have been a handful of concerted, yet controversial efforts to broadcast radio transmissions from Earth toward nearby celestial objects–although Proxima Centauri b has not been among the list of recipients. Still, it is not implausible that NASA b could have picked up on stray radio transmissions over the years. They might even be crafting a response.
While this discussion may seem like a fun thought experiment, several researchers have taken this question seriously. Scientists have already found 1,715 stars in the right position to have spotted a transiting Earth since the beginning of human civilization. Meanwhile, other researchers conclude about 46 nearby stars, with roughly 29 potentially habitable planets, have the correct vantage point to both view Earth’s transit and detect our radio transmissions. And we have no way of knowing if any alien civilization is far ahead of us in observational capabilities.
As scientists and engineers continue to search for evidence of communication from extraterrestrials and develop more advanced telescopes, we on Earth continue to broadcast our location in the universe a little further every day. So the next time you look out into space and wonder, “is there life out there?”, consider that there might be someone looking back.
About the Author
Sahana is an undergraduate at the University of Georgia studying Astrophysics with a Sustainability certificate. When she's not pretending to do her homework, she can be found playing ultimate frisbee or adventuring around Athens. Connect with her at sahana@uga.edu.
