The last decade has seen the discovery of thousands of exoplanets (planets orbiting stars other than the Sun), a handful of which have the potential to be inhabited. In the journal Nature, astronomers have flipped the focus, identifying the exoplanets in our galactic neighborhood whose theoretical inhabitants might be able to see the Earth passing in front of the Sun and recognize its suitability for life.
Cornell Professor Lisa Kaltenegger is trying to answer the question: What are the chances that aliens at similar technological levels know we're here? Unless they are close enough to detect our radio signals, the answer is probably low, but there are better prospects they'd at least know the Earth was suited to life.
"From the exoplanets' point-of-view, we are the aliens," Kaltenegger said in a statement. "We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun's light,"
Although several methods have been used to discover exoplanets, by far the most successful has been to look for the dips in brightness when a planet transits across its star's face. There are limitations to the transit method, however, principally that we can only see planets whose orbital plane is suitably aligned with our location.
Kaltenegger and Dr Jackie Faherty of the American Museum of Natural History looked first for all the stars within 100 parsecs (326 light-years) positioned so the Earth would transit from their perspective. This number comes to 1,402, but Kaltenegger noted that “Stars move in our dynamic cosmos, this vantage point is gained and lost.” The pair used Gaia data on stellar movements to add all those that have been in a position to see Earth transits in the last 5,000 years (or will be in the next 5,000), bringing the total to 2,034.
Most of these stars are M-type red dwarfs, whose prospects for hosting life are greatly debated. Another 14 are A or B type-stars with lifespans too short to host complex life. However, that still leaves 194 G-type stars (the Sun's category), and another 189 only a little warmer or cooler.
So far we know that seven of these stars have rocky planets orbiting them, but the wealth of data being collected by the TESS satellite is increasing that fast.
The closest known planet fitting Kaltenegger and Faherty's criteria is Ross 128 b, which is just 11 light-years away. Ross 128 b doesn't transit from Earth – we found it using other methods – and we currently don't transit from its vantage point either. However, for more than 3,000 years, ending 900 years ago, the Earth did transit from Ross 128 b's perspective. Any alien astronomers living there might have noticed the dips in brightness from the Sun and identified the presence of a world a little smaller than their own with a temperature suited to liquid water. With equipment not much more advanced than ours, they might also have detected signs of life in Earth's atmosphere.
The two planets around Teegarden's star (12.5 light-years) cannot currently see Earth transiting, but they'll be able to do so in just 29 years' time, although any aliens there might take a little while to notice. The famous Trappist-1 seven-planet system (45 light-years) will be positioned to see Earth transiting in 1,642 years.
Knowing we find only a fraction of exoplanets, Kaltenegger and Faherty extrapolate to estimate 508 Earth-sized planets within their stars' habitable zone that could witness Earth's transiting. Of these, 29 are within the 100 light-year radius sphere that radio signals from Earth have reached since we started broadcasting them.