Suppose you were an alien astronomer living on a planet as advanced as ours, just beginning to discover exoplanets. We've been able to detect exoplanets orbiting stars since the early 1990s using a process known as photometry, which is the precise measurement of the flux (brightness) of light coming from a star over a period of time.
If we're fortunate enough to see a planet transit in front of a star, the starlight will dim infinitesimally — but measurably. The time between dimmings gives us a reasonable estimate of how long it takes the planet to orbit its sun.
So far, using various Earth and space-based telescopes, we've been able to detect about 500 planets (out of the more than 6,000 confirmed exoplanets) in the habitable zone.
Where would an alien astronomer have to be relative to Earth to catch our planet transiting our sun, and thus be able to detect it? It doesn't happen often. Astronomers Lisa Kaltengger and Jacqueline Faherty identified 1,715 stars in our solar neighborhood that could have seen Earth in the past 5,000 years.
Our knowledge of exoplanets is in its infancy. The Kepler telescope, which scanned approximately 1% of the night sky, identified around 4,500 stars with planets using photometry. Thousands more potential candidates have yet to be studied.
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.
That special vantage point doesn't last forever, given the motion of stars over a period of centuries. Eventually, those stars that could see us transiting our sun will lose sight of us, and others will take their place.
The 1,715 stars that astronomers believe could see Earth transit the Sun have been in position to observe its dimming for only the last 5,000 years. Even if alien astronomers could detect that Earth orbits in the Sun’s habitable zone, they would have no idea that anyone is waiting to hear from them.
Of those 1,715 stars that could have discovered Earth, there are just three planets orbiting all those stars that lie in the habitable zone.
The three systems that host planets in the Habitable Zone in the Earth transit zone are close enough to detect radio waves from Earth. Because radio waves travel at light speed, they have only washed over 75 of the stars on our list so far. These stars are within 100 light-years from Earth—because light had 100 years to travel since Earth first started to leak radio signals.
Ross 128b, an exoplanet a mere 11 light-years away from us, could have seen Earth block the sun’s light about 3,000 years ago. But it lost this bull’s-eye view about 900 years ago. Another exoplanet, Teegarden’s Star b, which is a bit heavier than Earth, and circles a red sun, is about 12.5 light-years away, and will start to see Earth transit in 29 years. And the fascinating Trappist-1 system, with seven Earth-size planets at 40 light-years distance, will be able to see Earth as a transiting planet but only in about 1,600 years.
Needless to say, even if alien scientists were able to detect Earth transiting the Sun, the chances of them deducing that intelligent life existed on that planet are remote.
The more we discover about the universe, the more we understand why, while we may be part of a large cosmic family of intelligent beings, finding them, much less communicating with them, is extraordinarily difficult.
