For centuries, human beings have speculated about the existence of planetary systems (much like our own) orbiting other stars. However, it has only been in the past few decades that scientists have been able to detect and study these distant worlds. To date, astronomers have used various methods to confirm the existence of 4,422 extrasolar planets in 3,280 star systems, with an additional 7,445 candidates awaiting confirmation.
Naturally, this raises some questions. If there is intelligent life out there that has similar capabilities to our own – and the same burning sense of curiosity – could it be watching us too? Equally important is the question of how many of be able to detect us. According to new research conducted by a team from Cornell and the American Museum of Natural History, there are 2,034 star systems within 326 light-years of Earth that would be watching us right now!
This research was conducted by Lisa Kaltenegger, a professor of astronomy and director of Cornell’s Carl Sagan Institute, and astrophysicist Jackie Faherty, a senior scientist at the American Museum of Natural History. A paper that describes their findings, titled “Past, present and future stars that can see Earth as a transiting exoplanet,” was recently published in the journal Nature.
To date, the vast majority of extrasolar planets have been detected and confirmed through indirect means. Of these, the majority were found using Transit Photometry (aka. the Transit Method), where astronomers monitor stars for periodic dips in brightness that are possible indications of a planet is passing in front of the star (aka. transiting) relative to the observer.
In addition to being a highly effective means of detection, this method also provides relatively accurate constraints on an exoplanet’s size and orbital period. At times, astronomers are even able to obtain spectra from light that has passed through the exoplanet’s atmosphere during a transit, allowing them to determine its chemical composition. One minor drawback of this method is that exoplanets must orbit their parent stars edge-on from our point of view.
The same holds true for any extraterrestrial observers that may be out there. To planets orbiting other stars, Earth will only be detectable making transits if it is edge-on relative to them. This is what is known as the Earth Transit Zone (ETZ), a special region of the sky from which an extraterrestrial observer would be able to detect Earth as it passes in front of the Sun (makes a transit). As Kaltenegger explained in a Cornell Chronicle news release:
“From the exoplanets’ point-of-view, we are the aliens. We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light. And because stars move in our dynamic cosmos, this vantage point is gained and lost.”
This artist’s impression shows the planet orbiting the Sun-like star HD 85512 in the southern constellation of Vela (The Sail). Credit: NASA
For the sake of their study, Kaltenegger and Faherty relied on survey data obtained by the European Space Agency’s Gaia Observatory. Specifically, they consulted data on the positions and proper motions of nearby stars that were part of the early third data release (eDR3 catalog). This data, said Faherty, is a game-changer when it comes to how other star systems would view our own:
“Gaia has provided us with a precise map of the Milky Way galaxy, allowing us to look backward and forward in time, and to see where stars had been located and where they are going. Our solar neighborhood is a dynamic place where stars enter and exit that perfect vantage point to see Earth transit the Sun at a rapid pace.”
Using this data, Kaltenegger and Faherty created a catalog of stars that have (or are yet to) pass through the ETZ during a 10,000 year period. The beginning of this period was selected to coincide with the birth of human civilization (ca. 5,000 years ago) and extend another 5,000 years into the future. Ultimately, they found that in the past 5,000 years, 1,715 star systems passed through the ETZ while another 319 will do so in the next 5,000 years.
They further found that 117 of these stars are within about 100 light-years of the Sun while 75 have been in the ETZ during the last century, coinciding with the invention of radio communications. For scientists engaged in the Search for Extraterrestrial Intelligence (SETI), radio waves are considered a viable indication of technological activity (aka. technosignature). For planets orbiting those 75 stars, our own radio waves would have been detectable.
The Green Bank Telescope monitoring the galaxy for Fast Radio Bursts (FRBs). Credit: UC Berkeley
Among the 2,034 star systems included in the catalog, Kaltenegger and Faherty also determined that seven are known to host exoplanets. These include Ross 128 b, an exoplanet comparable in size to Earth (1.8 Earth radii) that orbits a red dwarf star located about 11 light-years from Earth in the Virgo constellation. For a civilization living on this exoplanet, Kaltenneger and Faherty found that they would be able to see Earth transits from 3,057 to 900 years ago (a period of 2,158 years).
Then there’s the seven rocky exoplanets that orbit TRAPPIST-1, a red dwarf star 45 light years from Earth. Four of these planets orbit within the star’s habitable zone, meaning that they could be potentially habitable. While scientists detected these planets between , it appears as though they won’t be able to detect Earth for another 1,642 years. However, their window of opportunity for spotting us will last for 2,371 years.
Trappist-1 system, at 45 light-years from Earth, hosts seven transiting Earth-size planets – four of them in the temperate, habitable zone of that star. While we have discovered the exoplanets around Trappist-1, they won’t be able to spot us until their motion takes them into the Earth Transit Zone in 1,642 years. Potential Trappist-1 system observers will remain in the cosmic Earth transit stadium seats for 2,371 years.
“Our analysis shows that even the closest stars generally spend more than 1,000 years at a vantage point where they can see Earth transit,” said Kaltenegger. “If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
Each of these worlds has had (or will have) the opportunity to detect Earth as it passes in front of our Sun relative to them. If these potential observers were able to observe Earth being backlit by the Sun, they would also have been able to obtain spectra from our atmosphere. From this, they could discern the presence of chemical elements we associate with life (aka. biosignatures), like oxygen gas, carbon dioxide, and water vapor.
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
Moreover, if they observed our planet in the past century, they might have also noticed the signs of industrial activity (chemical pollutants) and nuclear testing (radioactive isotopes), both of which are clear technosignatures. Later this year, the James Webb Space Telescope (JWST) will launch to space and dedicate its advanced infrared instruments towards the characterization of exoplanets and the search for potential signs of life.
It will be followed in 2024 by the Nancy Grace Roman Space Telescope, named after the “mother of Hubble.” This next-generation telescope will have optics comparable to that of the venerable Hubble, but will also have 100 times the field of view. Working in tandem with the JWST, Roman is expoected to aid in the discovery and characterization of tens of thousands of exoplanets.
In the coming decades, Breakthrough Starshot will attempt to send a nano-sized spacecraft to Proxima Centauri to study Proxima b, the closest exoplanet beyond the Solar System. Using a lightsail and directed energy propulsion system, this mission aims to make the journey to Proxima b in just 20 years and fully characterize the exoplanet upon arrival. Alas, says Faherty, someone from these stars may have similar plans for our planet, or even been here already and seen all there was to see:
“One might imagine that worlds beyond Earth that have already detected us, are making the same plans for our planet and solar system. This catalog is an intriguing thought-experiment for which one of our neighbors might be able to find us.”
Further Reading: Cornell Chronicle, Nature