Astronomers see 2,034 stars in Earth Transit Zone
Scientists said on June 23, 2021, that they’ve identified 2,034 star systems within 326 light-years whose astronomers, if there are any, could find Earth much as we’ve found most known exoplanets. That’s by seeing a transit of our pale blue dot across the face of our sun. At such times, Earth would be backlit by our sun. But Earth’s atmosphere would be visible and would contain clues Earth is inhabited. Co-author of the study Lisa Kaltenegger at Cornell University commented in a statement:
From the exoplanets’ point-of-view, we are the aliens.
Astronomers call this special vantage point in space – the vantage point from which Earth transits can be seen – the Earth Transit Zone. The authors of this study explained:
By watching distant exoplanets transit – or cross – their own sun, Earth’s astronomers can interpret the atmospheres backlit by that sun. If exoplanets hold intelligent life, they can observe Earth backlit by the sun and see our atmosphere’s chemical signatures of life.
What’s Earth Transit Zone?
Astronomers on Earth have various ways of searching for exoplanets, or planets orbiting other stars. Watching exoplanets “transit” or cross the faces of their stars – and recording the tiny dip in light that results – has been the most fruitful of the planet-hunting methods so far. Most of the 4,000+ known exoplanets were discovered via the transit method.
You might see that there’s a special region of the sky from which an extraterrestrial observers would be able to see Earth transits. It’s approximately in the plane of Earth’s orbit around the sun. This is the Earth Transit Zone. The new study not only showed there are 2,034 system within 326 light-years in the Earth Transit Zone. It also showed that 1,715 of those star systems have been in the right position in space to have spotted Earth transiting our sun since the time early human civilization dawned on Earth, about 5,000 years ago.
And an additional 319 stars will enter the Earth Transit Zone in the next 5,000 years.
Moving in space and time
Other studies have considered the Earth Transit Zone. But this is the first study that considers the Earth Transit Zone not only in space, but also in time. It considers changing viewpoints, as the stars and our sun orbit the center of the Milky Way galaxy. As Kaltenegger explained:
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… 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.
Kaltenegger and co-author Jackie Faherty of the American Museum of Natural History were able to perform this study because of a unique data set. It’s the data provided by the European Space Agency’s Gaia space observatory.
Launched in 2013, Gaia is designed for astrometry. It measures star positions, distances and motions of Milky Way stars over and over again. So it provides a glimpse of billions of stars in our Milky Way galaxy, not just where they are today, but how they’re moving over time. Gaia’s ultimate goal is to create a 3-D map of our Milky Way. In the meantime, Gaia data have provided the fuel for many interesting studies. Faherty said:
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.
Earth Transit Zone facts and figures
So, according to Kaltenegger and Faherty’s study, there are 2,034 stars within 326 light-years in the Earth Transit Zone. Within this body of stars, scientists currently know seven with exoplanets. For example, a being on the known exoplanet Ross 128 b – about 11 light-years from Earth – could have witnessed Earth transit the sun in the past. Future alien astronomers on the known exoplanet Teegarden’s Star b – about 12 light-year away – will be in a position to watch Earth transit the sun 29 years from now.
The study found that – of the 2,034 star-systems passing through the Earth Transit Zone over the 10,000-year period examined (5,000 in the past, 5,000 in the future), 117 objects lie within about 100 light-years of our sun.
And it found that 75 of these relatively nearby objects have been in the Earth Transit Zone since commercial radio stations on Earth began broadcasting into space about a century ago.
Using transits to search for life signs
When a planet crosses in front of its star as seen from Earth, we on Earth see that planet as backlit by the star’s light. If the planet has an atmosphere, this is a golden opportunity to analyze the atmosphere for life signs. Certain chemical signatures – called biosignatures – seen in a planet’s atmosphere signal that life exists there.
So, for example, Earth’s atmosphere must be laden with biosignatures.
The James Webb Space telescope, to be launched later this year, will look for biosignatures in the atmospheres of transiting exoplanets. And, in this way (among others), the Webb will search for signs of extraterrestrial life in our Milky Way galaxy.
Have the aliens found Earth?
Let’s go back to known exoplanets in the Earth Transit Zone. Could their astronomers find Earth? Yes. Sometimes.
The known exoplanet Ross 128b orbits a red dwarf star about 11 light-years away in the direction of the constellation Virgo the Maiden. Ross 128 is the second-closest star system to us with an Earth-size exoplanet. Ross 128 b is about 1.8 times the size of Earth. Any alien astronomers on Ross 128 b could have seen Earth transit the sun for a span of 2,158 years, starting about 3,057 years ago. But current occupants of Ross 128 b can no longer see Earth transits: They moved out of the right position to see us about 900 years ago.
Another known exoplanet system, Trappist-1, lies 45 light-years away. Trappist-1 hosts seven transiting Earth-size planets, and four of those lie in the habitable zone of their star. Any being around Trappist-1 currently searching for transiting planets won’t have seen us yet. But that will change in 1,642 years, when their motion takes them into the Earth Transit Zone. At that point they’ll have a view of Earth transiting the sun for 2,371 years before positions shift enough again to move us out of their sight. Kaltenegger said:
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. If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.
The Breakthrough Starshot initiative is a plan to send a nano-sized spacecraft to the closest exoplanet around Proxima Centauri, 4.2 light-years away. Perhaps exoplanets that can detect Earth have similar ambitions. Faherty said:
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.
Bottom line: Scientists have calculated that 2,034 stars within 326 light-years have an opportunity to detect Earth using the transit method.