For centuries, people have speculated about the existence of planetary systems (similar to our own) orbiting other stars. However, it has only been possible to discover and study these distant worlds in the last few decades. 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.
This, of course, raises some questions. If there is intelligent life that has capabilities similar to ours – and the same burning curiosity – could it be watching us too? Equally important is the question of how many of us can recognize us. According to new research done 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 carried out by Lisa Kaltenegger, 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 describing their findings, entitled “Past, Present and Future Stars Who Can See Earth as a Transiting Exoplanet,” was recently published in the journal Nature.
To date, the vast majority of extrasolar planets have been discovered and confirmed indirectly. Of these, most were found using transit photometry (also known as the transit method), in which astronomers monitor stars for periodic decreases in brightness, which are possible signs that a planet is passing in front of the star relative to the observer (also known as transit ). .
This method is not only a highly effective means of detection, but also provides relatively precise constraints on the size and orbital period of an exoplanet. Sometimes astronomers are even able to obtain spectra of light that has passed through the exoplanet’s atmosphere during a transit and thus determine its chemical composition. A small disadvantage of this method is that, from our point of view, exoplanets have to orbit their parent stars from the edge.
The same goes for any alien observers who might be out there. For planets orbiting other stars, the earth is only recognizable when it is making transits, when it is relative to them. This is the so-called Earth Transit Zone (ETZ), a special region of the sky from which an extraterrestrial observer could see the earth when it passes in front of the sun (makes a transit). As Kaltenegger stated in a Cornell Chronicle press release:
“From the perspective of the exoplanets, we are the extraterrestrials. We wanted to know which stars are in the right angle to see Earth as it blocks the sun’s light. And because stars move in our dynamic cosmos, this vantage point is won and lost. “
This artist’s impression shows the planet orbiting the sun-like star HD 85512 in the southern constellation Vela (The Sail). Photo credit: NASA
For their study, Kaltenegger and Faherty relied on survey data from the Gaia Observatory of the European Space Agency ESA. In particular, they consulted data on the positions and self-motions of nearby stars that were part of the early third data release (eDR3 catalog). This data, according to Faherty, is groundbreaking when it comes to how other star systems would see our own:
“Gaia provided us with an accurate map of the Milky Way that enables us to look back and forth in time and see where stars were and where they are moving. Our solar neighborhood is a dynamic place where stars enter and leave this perfect vantage point to watch the earth pass the sun at a rapid pace. “
With this data, Kaltenegger and Faherty compiled a catalog of stars that have passed (or will pass through) the ETZ during a period of 10,000 years. The beginning of this period was chosen to coincide with the birth of human civilization (about 5,000 years ago) and to extend another 5,000 years into the future. Ultimately, they found that 1,715 star systems have passed ETZ in the past 5,000 years, while another 319 will do so in the next 5,000 years.
They also found that 117 of these stars were within about 100 light years of the Sun, while 75 were in the ETZ in 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 useful indicator of technological activity (also known as technosignature). For planets orbiting these 75 stars, our own radio waves would have been detectable.
The Green Bank Telescope monitors the galaxy for Fast Radio Bursts (FRBs). Photo credit: UC Berkeley
Of the 2,034 star systems in the catalog, Kaltenegger and Faherty also found that seven are home to exoplanets. These include Ross 128b, an exoplanet comparable in size to Earth (1.8 Earth radii) orbiting a red dwarf star located about 11 light years from Earth in the constellation Virgo. For a civilization that lives on this exoplanet, Kaltenneger and Faherty found that they could see Earth transits 3,057 to 900 years ago (a period of 2,158 years).
Then there are the seven rocky exoplanets orbiting TRAPPIST-1, a red dwarf star 45 light years from Earth. Four of these planets orbit within the star’s habitable zone, which means they could potentially be habitable. As scientists discovered these planets in between, it seems like they won’t be able to discover Earth for another 1,642 years. However, your window of time to discover us will be 2,371 years.
The Trappist-1 system, 45 light years from Earth, is home to seven Earth-sized transit planets – four of them in the temperate, habitable zone of this star. Although we have discovered the exoplanets around Trappist-1, they will not be able to discover us until their movement brings them into the Earth transit zone in 1,642 years. Potential observers of the Trappist-1 system will remain in the seats of the cosmic earth transit stadium 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 the passage of the earth,” said Kaltenegger. “Assuming the opposite is the case, this provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
Each of these worlds had (or will have) the ability to see the earth as it passes in front of our sun relative to them. Had these potential observers been able to observe how the earth is backlit by the sun, they could also have obtained spectra from our atmosphere. From this, they were able to identify the presence of chemical elements that we associate with life (also known as biosignatures), such as 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. Photo credit: ESO / M. Grain knife
Additionally, if they had been observing our planet over the past century, they might also have 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 into space and dedicate its advanced infrared instruments to characterizing exoplanets and looking for potential signs of life.
The Nancy Grace Roman Space Telescope, named after the “mother of Hubble”, will follow in 2024. This next-generation telescope will have optics comparable to that of the venerable Hubble, but will also have a 100x field of view. Working with the JWST, Roman is expected to assist in the discovery and characterization of tens of thousands of exoplanets.
In the coming decades, Breakthrough Starshot will attempt to send a nano spacecraft to Proxima Centauri to study Proxima b, the closest exoplanet beyond the solar system. Using a light sail and a directional energy propulsion system, this mission aims to make the journey to Proxima b in just 20 years and fully characterize the exoplanet upon arrival. Unfortunately, says Faherty, someone from these stars could have similar plans for our planet, or even have been here and seen all there was to see:
“One could imagine that worlds beyond Earth that have already discovered us are making the same plans for our planet and our solar system. This catalog is an exciting thought experiment that one of our neighbors might find us for. ”
Further reading: Cornell Chronicle, Nature
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