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.

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