A team of astronomers from Cornell University and the American Museum of Natural History has identified 1,715 stars within 100 parsecs (326 light-years) from the Sun that are in the right position to have spotted life on a transiting Earth since early human civilization (about 5,000 years ago), and additional 319 stars that will enter this special vantage point in the next 5,000 years; among these stars are seven known exoplanet-hosting stars, 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; human-made radio waves have already swept over 75 of the closest stars on the team’s catalog.
This artist’s impression shows the view from the surface of one of the exoplanets in the TRAPPIST-1 planetary system. Image credit: ESO / M. Kornmesser / Spaceengine.org.
“From the exoplanets’ point-of-view, we are the aliens,” said Profeesor Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University.
“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.”
Professor Kaltenegger and her colleague, Dr. Jackie Faherty from the American Museum of Natural History, used positions and motions of stars from ESA’s Gaia eDR3 catalog to determine which stars enter and exit the Earth Transit Zone — and for how long.
Of the 2,034 star-systems passing through the Earth Transit Zone over the 10,000-year period examined, 117 objects lie within about 100 light-years of the Sun and 75 of these objects have been in the Earth Transit Zone since commercial radio stations on Earth began broadcasting into space about a century ago.
“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,” Dr. Faherty said.
The team’s catalog includes seven known exoplanet-hosting stars. Each one of these systems has had or will have an opportunity to detect Earth, just as terrestrial astronomers have found thousands of worlds orbiting other stars through the transit technique.
One of these systems is Ross 128, a red dwarf host star located 11 light-years away in the constellation of Virgo. It is the second-closest system with an Earth-size exoplanet.
Any inhabitants of this world, Ross 128b, could have seen Earth transit our own Sun for 2,158 years, starting about 3,057 years ago; they lost their vantage point about 900 years ago.
The TRAPPIST-1 system, at 45 light-years from Earth, hosts seven transiting Earth-size planets — four of them in the temperate, habitable zone of that star.
While astronomers have discovered the exoplanets around TRAPPIST-1, they won’t be able to spot us until their motion takes them into the Earth Transit Zone in 1,642 years.
Potential TRAPPIST-1 astronomers will remain in the cosmic Earth transit stadium seats for 2,371 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,” Professor Kaltenegger said.
“If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
The team’s paper was published in the June 24, 2021 issue of the journal Nature.
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L. Kaltenegger & J.K. Faherty. 2021. Past, present and future stars that can see Earth as a transiting exoplanet. Nature 594, 505-507; doi: 10.1038/s41586-021-03596-y
