A view of Earth and Sun from thousands of miles above our planet. Stars that enter and exit a position where they can see Earth as a transiting planet around our Sun are brightened. (OpenSpace/American Museum of Natural History)

Exoplanet scientists and enthusiasts spend a lot of time trying to find, measure and understand distant planets that can — under specific conditions — be detected as passing in front of their host star.  A majority of the 4000-plus exoplanets discovered so far were indirectly detected this way, by measuring the diminishing of stellar light as the exoplanet passes between the star and us.

In a conceptual turnaround, two researchers have now asked the question of how common it might be for beings on distant exoplanets to be able to similarly detect and measure Earth as it transits in front of our sun.

Astronomers call this special vantage point in space – the point from which Earth transits can be seen – the Earth transit zone.  Because the cosmos is dynamic and ever-changing, they looked for not only stars that are in that zone now, but have also passed through over the past 5,000 years and will in the next 5,000 years.

“From the exoplanets’ point-of-view, we are the aliens,” said 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.”

Transit data are rich with information. By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet. The orbital period of the planet can be determined by measuring the elapsed time between transits. Once the orbital period is known, Kepler’s Third Law of Planetary Motion can be applied to determine the average distance of the planet from its stars. (NASA/Ames)

How many stars (and their orbiting planets) have this proper vantage point, have had in the past and will in the future?

In Kaltenegger’s paper, published in Nature with Jackie Faherty of the astrophysics department of the American Museum of Natural History, the numbers reported are quite low.

They found that since the earliest human civilization about 5,000 years ago, only 1,715 stars among the 300,000-plus that shine within 300 light years of our sun are in the right geometric alignment for an observation of Earth passing in front of our sun.  An additional 319 stars will enter this special vantage point in the next 5,000 years.

They arrived at these numbers using the geometry of the ecliptic — the pancake plane on which all our solar system’s planets orbit — and data from the European Space Agency’s Gaia space telescope on where the stars are and will be and have been located. (Gaia is an ambitious mission to chart a three-dimensional map of our galaxy.)

“To see Earth block light from the sun – from your point of view – you have to be located in the ecliptic plane. That is the plane when you take Earth’s orbit around the sun and lie a plane through it and extend it outwards. You can be a little bit above or below that plane and still see Earth darken the sun, but not by much.”

“The Earth transit zone around the ecliptic is very small, so it is not surprising that there are so few stars in it,” Kaltenegger said.

The ecliptic plane is the horizontal band on which most of our solar system planets orbit. Pluto has an orbit off the ecliptic, but of course Pluto is no longer considered a planet for other reasons.

Of those handful of stars, only seven  are currently known to have orbiting planets where intelligent life potentially exists (although some are giant gas planets where such life clearly does not exist.) The number of exoplanets orbiting Earth transit zone stars might be significantly higher but the necessary planet hunting has not be done on many of those stars.

Some percentage of these worlds has had, or will have, an opportunity to detect Earth if technologically intelligent life is present, just as Earth’s scientists have found thousands of worlds orbiting other stars through the transit technique.

Some examples:  The Ross 128 system, with a red dwarf host star, is about 11 light-years away and is the second-closest system with an Earth-size exoplanet (about 1.8 times the size of our planet). Any inhabitants of this exoworld 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 we know the exoplanets are there,  any potential intelligent beings won’t be able to spot us until their motion takes them into the Earth Transit Zone in 1,642 years. These possible (if unlikely) Trappist-1 system observers 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,” 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.”

By watching distant exoplanets transit – or cross – their own sun, Earth’s astronomers can interpret the atmospheres backlit by that sun and can potentially detect chemical signatures of life. If exoplanets hold intelligent life, they can in theory observe Earth backlit by our sun and see our atmosphere’s chemical biosignatures.

Lisa Kaltenegger is director of the Carl Sagan Institute at Cornell University, where she is an associate professor.  Born and trained in Austria, Kaltenegger is known for her studies of the atmospheres of extrasolar planets, especially Earth-like ones, and she is a pioneer in the study of the Earth as an astronomical object evolving in time. (Cornell University)

I asked Kaltenegger, via email, why she wanted to research this question of which stars might line up to allow for an extraterrestrial view of Earth crossing our sun.

She said that since the 1960s, SETI observers were already discussing the enhanced probability of an extraterrestrial civilization (if any exist) sending out a detectable message if they know a planet is there. And currently the easiest  way for us – and anyone with our level of technology – to find planets is searching for transits.

“So I started to wonder what stars could see us if we change the vantage point. I wrote a paper (which ran in the Monthly Notices of the Royal Astronomical Society: Letters) at the end of last year on what stars could see Earth transit right now.”

“But while writing, I started to wonder how that vantage point changes with time. The cosmos is dynamic, so the vantage point is not forever, it is gained and lost. And I was wondering how long that front row seat to find Earth through the dip in brightness of the star lasts. No one knew. And I was wondering if there were any known exoplanets in the habitable zone that could see us transit as well.”

“And then the incredible star catalog that the European Space Agency’s Gaia mission got released, which contains position and the motion of the stars. So, you can spin their movement into the future and trace it back into the past.”

The data she and her colleagues are collecting is of interest to SETI researchers now and also has been shared with operators of space telescopes such as the Transiting Exoplanet Survey Satellite.   That space telescope is an all-sky survey mission that will discover thousands of exoplanets around nearby bright stars, and especially in the crowded ecliptic region.

Kaltenegger, who is a co-investigator on the TESS science team, said that the telescope will indeed be used to search for exoplanets orbiting some of the relatively nearby stars recently identified as being in the Earth transit zone.

The American Museum of Natural History,  where Nature co-author and astrophysicist Faherty is a senior scientist and senior education manage, produced the animation above.  It helps further explain the transit technique for finding exoplanets and visualizes what has to happen to allow a potential alien being to view Earth transiting the sun.

“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,” Faherty said in a release.

“One might imagine that worlds beyond Earth that have already detected us, are making the same plans for our planet and solar system.” The paper and the accompanying catalogue of stars in the Earth transit zone, she said,  constitute an “intriguing thought-experiment for on how one of our neighbors might be able to find us.”