More than 1,700 stars could have seen Earth in the past 5,000 years.
BY LISA KALTENEGGER
Do you ever feel like someone is watching you? They could be. And I’m not talking about the odd neighbors at the end of your street.
This summer, at the Carl Sagan Institute at Cornell University and the American Museum of Natural History in NYC, my colleague Jacky Faherty and I identified 1,715 stars in our solar neighborhood that could have seen Earth in the past 5,000 years.1 In the mesmerizing gravitational dance of the stars, those stars found themselves at just the right place to spot Earth. That’s because our pale blue dot blocks out part of the sun’s light from their view. This is how we find most exoplanets circling other stars. We spot the temporary dimming of their star’s light.
The perfect cosmic front seat to Earth with its curious beings, is quite rare. But with about the same technology as we have, any nominal, curious aliens on planets circling one of the 1,715 stars could have spotted us. Would they have identified us as intelligent life?
All of us observe the dynamics of the cosmos every night. Stars rise and set—including our sun—because Earth rotates among the rich stellar tapestry. Our night sky changes throughout the year because Earth moves in orbit around the sun. We only see stars at night when the sun doesn’t outshine them. While circling the sun, we glimpse the brightest stars in the anti-sun direction only. Thus, we see different stars in different seasons.
The discussion rolls around to: “Does life on Earth qualify as advanced?”
If we could watch for thousands of years, we could watch the dynamic dance of the cosmos unfold in our night sky. But alternatively, we can use the newest data from the European Space Agency’s GAIA mission and computers to fast-forward the time before our eyes, with decades unfolding in mere minutes. While we can only see the light of the stars, we already know that more than 4,500 of these stars are not alone. They host extrasolar planets. Several thousand additional signals indicate even more new worlds on our cosmic horizon.
Astronomers found most of these exoplanets in the last two decades because of a temporary dimming of their stars when a planet, by chance, crossed our line of sight on its journey around its star. The planet temporarily blocks out part of the hot star—and its light—from our view. Telescopes on the ground and from space, including NASA’s Kepler and TESS (Transiting Exoplanet Survey Satellite) mission, found thousands of exoplanets by spotting this dimming, which repeats like clockwork.
The time between dimming tells us how long the planet needs to circle its star. That allows us to figure out how far away an exoplanet wanders from its hot central star. Most known exoplanets are scorching hot gas balls. We can tell when planets orbit closer to a central star than others because they need less time to circle it—we also find those faster than the cooler ones farther away. But about three dozen of these exoplanets are already cool enough. They orbit at the right distance from their stars, where it is not too hot and not too cold. Surface temperatures could allow rivers and oceans to glisten on the surfaces of these planets in this so-called Habitable Zone.
This vantage point—to see a planet block part of the hot stellar surface from view—is special. The alignment of us and the planet must be just right. Thus, these thousands of known exoplanets are only the tip of the figurative exoplanet iceberg. The ones we can most easily spot hint at the majority waiting to be discovered.
But what if we change that vantage point? If anyone out there were looking, which stars are just in the right place to spot us?
Our powers of observation have been boosted by the European Space Agency’s Gaia mission. Launched in 2013, the Gaia spacecraft is mapping the motion stars around the center of our galaxy, the Milky Way. The agency aims to survey 1 percent of the galaxy’s 100 billion stars. It has generated the best catalog of stars in our neighborhood within 326 light-years from the sun. Less than 1 percent of the 331,312 catalogued objects —stars, brown dwarfs, and stellar corpses—are at the right place to see Earth as a transiting exoplanet. This special vantage point is held by only those objects in a position close to the plane of Earth’s orbit. Roughly 1,400 stars are at the right place right now to see Earth as a transiting exoplanet…