Don’t be too alarmed, but you can see Earth from more than 1,700 stars up to 300 light years away. You know, if alien astronomers exist.
In a new study published Wednesday in Nature, two astronomers at Cornell University and the American Museum of Natural History took a look at stars within 100 parsecs (about 326 light-years) of the Sun to see which ones might be able to see the Earth through the transit method. And the results could eventually help us find life in space.
The transit method is what Kepler and the Transiting Exoplanet Survey Satellite use to find planets that pass in front of their stars from our point of view, causing tiny — but detectable — dips in light.
Because of the vast number of planets found through transits, they inferred that the transit method is also the most likely way for astronomers orbiting distant stars to spot our little rocky blue world. And that could have implications for how astronomers on Earth search for life on distant worlds.
What’s New — The team, consisting of Cornell’s Lisa Kaltenneger and AMNH’s Jackie Faherty, looked for the “Earth Transit Zone” — the area of space where observers around distant stars might have a chance to discover our planet with the transit method.
The key takeaways of the study:
- Planets orbit their stars in a plane, called the ecliptic
- Astronomers can only see transits if they happen to be lined up with that solar system’s ecliptic.
- Today, 1,402 stars are in just the right position to watch our planet pass across the Sun
- But in the last 5,000 years, at least 1,715 stars within 300 light years of Earth could have observed our transit at some point
- An additional 319 stars will be able to see us in the next 5,000 years
Everything in the universe is moving relative to everything else, so stars change position over time. As a result, the night sky looked different thousands of years ago than it does now.
And just as our view of the stars has changed, so has their (potential) view of us. Some of the stars in the Earth Transit Zone now weren’t there a few centuries ago, and some stars that aren’t lined up right to watch our tiny transit now had a great view during the heyday of the Roman Empire.
Kaltenneger and Faherty worked that out by modeling the relative movements of 2,034 stars within 300 light years of Earth. What’s especially impressive: seven of these stars that could have peeked in on us have known planets in their habitable zone.
They discovered, for example, that Ross-128 — a red dwarf star about 11 light years away with a tidally-locked exoplanet just at the inner edge of its habitable zone — could have observed Earth’s transits from 900 to 3,000 years ago, or from roughly between the time of ancient Egypt and the Middle Ages.
"Any civilization with our level of technology could have seen us already on Ross 128b, but lost that vantage point about 900 years ago,” Kaltenneger tells Inverse. “Would anyone have concluded that there was intelligent life on Earth 900 years ago?"
One important system that could soon see us is TRAPPIST-1, another red dwarf about 39 light years away with 4 rocky exoplanets in its habitable zone. It will have the chance in another 1,642 years, if there’s anyone there to point a telescope.
But there’s more: of the stars that can see us transit, 75 stars could have received our radio signals already.
Humans started unwittingly sending radio waves out into the galaxy about 100 years ago, so the first technological signals from our planet will have reached star systems within 100 light years of us by now.
In 29 years, Teegarden’s Star, a star about 12 light years away with two Earth-mass planets in its habitable zone, will finally be in position to observe Earth passing across the Sun after receiving our leaked radio waves for more than a century.
It wouldn’t take a lot of work to figure out that those signals were coming from the small rocky world transiting its star every 365 days. One clue would be that the radio waves decrease in strength every time Earth passes around the other side of the Sun.
Here’s The Background — Of the 4,422 planets outside the Solar System we know of, around 75 percent have been discovered through transit. This is because telescopes like TESS can stare at many stars at once.
Right now, we can get a few things out of a transit. For instance, we can figure out roughly the radius of the transiting planet. Follow-up methods can help us nail down a mass, which tells us if an Earth-sized planet also has a similar mass to our home planet.
But telescopes like Hubble can also watch how a star’s spectra— the wavelengths of light absorbed and emitted by particular chemicals — changes when a planet with an atmosphere transits. That offers some clues about which worlds might be habitable.
This goes for alien astronomers too — they could find us through transits, then perform follow-ups to determine our mass, distance from the Sun, and, if they have a big enough telescope, find out that our atmosphere is rich in oxygen and water vapor.
“The combination of oxygen and methane on Earth is the golden fingerprint for life in Earth’s atmosphere, and has been detectable for about 2 billion years on our planet,” Kaltenneger says. “If there are curious alien observers out there, they probably would draw those conclusions as well and could have found out there is life on Earth for a long time.”
In other words, if you were an astronomer on Wolf 359 (of Star Trek: The Next Generation infamy) looking for alien life, Earth would seem like a pretty good candidate. And if you’re an astronomer here on Earth, looking for alien life, stars that have been in a position to notice us are a good place to look for messages headed our way.
Why It Matters — If astronomers at projects like Breakthrough Listen or the SETI Institute’s COSMIC (which uses the same Very Large Array made famous in the movie Contact) want to scan the skies for alien radio signals, it makes sense to look – or listen – in a direction we’ve got some reason to expect a message from.
That means looking in the Earth Transit Zone, and especially at the 75 stars which could have seen us transit and received our radio signals.
"The concept of the Earth Transit Zone doesn't guarantee that we'll 'meet' an alien intelligence, but it's a good place to search," University of California, Berkeley astronomer Steve Croft tells Inverse.
Looking at stars that were in the Earth Transit Zone in the past helps expand the list of potential targets.
Louisiana State University astronomer Tabetha Boyajian tells Inverse that the recent study also helps SETI researchers make a connection between our history here on Earth and when exoplanets could have picked up signatures of life and technology coming from our planet.
What’s Next — TESS has recently entered its extended mission phase, and Kaltenneger and her colleagues plan to use the telescope to observe more stars in the Earth Transit Zone. The Breakthrough Listen project recently used the Green Bank Telescope to search 20 stars in the Earth Transit Zone for radio signals, with no luck, but the search continues.
"If a star closer than 50 light years to Earth detected some of our earliest radio transmissions, and responded right away with their own radio signal, we might be just about to receive that reply," Croft tells Inverse.
Abstract — In the search for life in the cosmos, transiting exoplanets are currently our best targets. With thousands already detected, our search is entering a new era of discovery with upcoming large telescopes that will look for signs of ‘life’ in the atmospheres of transiting worlds. Previous work has explored the zone from which Earth would be visible while transiting the Sun. However, these studies considered only the current position of stars, and did not include their changing vantage point over time. Here we report that 1,715 stars within 100 parsecs from the Sun are in the right position to have spotted life on a transiting Earth since early human civilization (about 5,000 years ago), with an additional 319 stars entering this special vantage point in the next 5,000 years. Among these stars are seven known exoplanet hosts, including Ross-128, which saw Earth transit the Sun in the past, and Teegarden’s Star and Trappist-1, which will start to see it in 29 and 1,642 years, respectively. We found that human-made radio waves have already swept over 75 of the closest stars on our list.