Astronomers Define The Star Systems That Might Be Observing Earth From Space
Are alien civilizations in distant star systems capable of detecting our existence on Earth? This is a question that may open up new avenues for searching for signs of extraterrestrial intelligence, but it is not one that is easily answered.
Nonetheless, an astronomical team has identified 2,034 star systems within 100 parsecs (326 light-years) of Earth that would provide an optimal vantage point for detecting Earth life as our home planet orbits the Sun.
“From the exoplanets’ point of view, we are the aliens,” astronomer Lisa Kaltenegger of Cornell University’s Carl Sagan Institute explained.
Using data from the Gaia space observatory – a project that is currently mapping the Milky Way in three dimensions with unprecedented precision – Kaltenegger and her colleagues sought to determine whether any alien civilizations out there could find humanity using the tools we use to discover exoplanets.
We have a number of these, but the most fruitful is what we refer to as the transit method. When an exoplanet orbits a star perfectly, it will pass between us and its host star, a phenomenon known as a transit. As a result of the exoplanet’s transit, the starlight dims and brightens fractionally.
The depth of the light curve provides an estimate of the exoplanet’s size, which can be used to rule out exoplanets that are unlikely to support life as we know it, such as gas giants like Jupiter.
Additionally, if the exoplanet has an atmosphere, scientists can stack transits to enhance the spectrum of light from the host star that passes through it. The way certain wavelengths are amplified or absorbed by the atmosphere can provide information about its composition, including the presence of gases indicative of life.
If those exoplanets, like Earth, developed technology that polluted their atmospheres, we might be able to detect it as well (although we have not yet).
Kaltenegger and her team used the Gaia data to look for star systems that may have been capable of doing the same thing to us across a 10,000-year period, from 5,000 years in the past to 5,000 years in the future. The Earth Transit Zone is the name given to this vantage point.
“We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light,” she explained. “And because stars move in our dynamic cosmos, this vantage point is gained and lost.”
According to their examination of Gaia data, the researchers determined that over the previous 5,000 years, there have been 1,715 star systems in the Earth Transit Zone that may have detected biosignatures when human technological civilizations arose. Within the next 5,000 years, an extra 319 star systems will enter the Earth Transit Zone.
“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,” said astrophysicist Jackie Faherty of the American Museum of Natural History.
Seven of the systems that have or will pass through the Earth Transit Zone are known to house exoplanets, some of which may be habitable. Ross-128, TRAPPIST-1, and Teegarden’s star are among them.
Ross-128 spent 2,158 years in the Earth Transit Zone in the past. TRAPPIST-1 will enter it in approximately 1,642 years and remain for a further 2,371 years. Teegarden’s star, which is scheduled to enter the zone in 29 years, will have a far narrower window of only 410 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,” Kaltenegger stated. “If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
The scientists also looked at whether stars would be capable of detecting technosignatures – the technogenic radiation released by Earth – as part of their investigation. We began emitting radio waves into space only approximately 100 years ago, which implies that those signals are detectable within a 100 light-year radius of us.
Within that radius and throughout a 100-year period, the Earth Transit Zone has had 75 systems.
Our quest for extraterrestrial civilizations have so far yielded no results. The team’s research indicates that our detection methods have considerable limitations, not the least of which are the ever-changing configurations of the stars in our neighborhood. However, with enough time and chance, locating our cosmic neighbors may be conceivable.