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When I was a kid, and survived on a diet of 100% trashy sci-fi, I would sometimes go outside at night, look up at the stars, and wonder how many worlds were out there. More to the point, who was out there looking back at me. I'll admit the thought made me uneasy, like I was vulnerable, exposed, examined by calculating alien brains contemplating Earth as an easy target. For what, exactly, didn't matter; the feeling was still discomfiting.
My, how things have changed. We now know of over 4,000 exoplanets, alien worlds orbiting alien stars, and statistically speaking it's fair to say that there are as many planets out there as stars in the sky. And while I no longer worry about aliens coming to steal our water and maybe eat us, the question of whether they're out there and know about us is an interesting one.
How would they know? Well, we know about most of these exoplanets via the transit method: If we see the planet's orbit edge-on, then once per orbit it passes between us and the star, dimming it a bit. It's a relatively easy way to find lots of planets, using technology that should be readily available to aliens.
Thinking of it that way, are there stars out there in the sky that see Earth as a transiting exoplanet?
Yes! The Earth orbits the Sun in a flat ellipse. From our viewpoint that means the Sun moves around the sky once per year on a line, a literal reflection of the Earth's orbit on the sky, which we call the ecliptic. Any stars in the sky close to the ecliptic would see the Earth transit the Sun once a year (well, once an Earth year) and could easily detect us*.
New research shows there are lots of them. And they make interesting targets for people involved with the Search for Extraterrestrial Intelligence (SETI). After all, if we're looking for planets that are like Earth, and can support life — and maybe, hopefully, possibly, finding that life, even intelligent life — then there may come a time when we want to talk to them. Aliens living on planets close to our ecliptic might already know we're here. That makes the first phone call a lot less awkward.
So how many stars are there like that?
Attempts have been made to make a list, but it's not all that easy. First you have to work out the geometry; how far from the ecliptic can you be and still see the Earth transit? The trig isn't too bad, and the answer is about a quarter of a degree, making a strip on the sky centered on the ecliptic half a degree wide (for comparison, and if you look at the geometry you'll realize not uncoincidentally that's about the same width as the Sun in the sky).
However, that includes transits where the Earth barely nicks the Sun, called a grazing transit. Those are hard to detect. Let's restrict the search to stars that would see us as passing no more than half the Sun's radius above and below the Sun's equator. That reduces the width of the strip to about 0.13° above and below the ecliptic.
Plus, we might want to restrict the stars by distance, as well. Too far away and the Sun appears too dim to easily spot a transit (yes, this depends on alien tech, but we have to start somewhere); plus our own techniques are not terribly reliable past a distance of about 1,000 parsecs (3,260 light years).
Previous catalogs have found some dozens of stars within the critical zone around the ecliptic and within that distance. But they used catalogs that are incomplete (they can't see really faint stars).
The new research aimed to fix that. A team of astronomers turned to TESS, the Transiting Exoplanet Survey Satellite, which is scanning the skies looking for other worlds. It has an input catalog of 1.7 billion stars to examine! Of those, the new work focused on those in the right part of the sky, and used data from Gaia to restrict the distance. They also ignored stars that were red giants and such, assuming any planets they once had are now vaporized.
When they were done, they had a list of 1,004 normal stars that could see Earth transit, and 508 from which the transit would be near the Sun's equator. That's a lot!
They could break them down by type, too. 77% were red dwarfs (the most common kind of star in the galaxy), 12% were slightly cooler than the Sun, 6% were stars like the Sun, and the remainder were hotter, more massive stars.
The closest is Ross 64, a red dwarf just under 28 light years away. We know red dwarfs are good at making Earth-sized planets, so that's a pretty interesting target all by itself.
Of all the stars found, just two are known to have exoplanets. K2-155 is 270 light years away, and has three known transiting planets, and K2-240 at 230 light years distant, which has two known transiting planets. None of these planets is Earth-sized, sadly.
But, we do know statistically speaking that roughly 10% of stars in the sky at least will have Earth-sized planets in their host stars' habitable zones (where temperatures are supportive for water-based life), so there may be 100 or so stars in the wider sample that could see us transit the Sun.
Mind you, this list changes over time. Stars move as they orbit the Milky Way, for one thing. Teegarden's star, a nearby red dwarf with planets, currently can't see the Earth transit, but in 2044 it'll move into a position where it can, and will for 450 years! Plus, the Earth's orbital plane moves slowly (about 0.5° every 3,800 years, meaning the entire zone of the sky we're talking about here changes over that time). So for most of these stars, the time window to spot Earth is limited. Civilizations don't last forever, so that's something to consider.
This is kind of a funny bit of research. The purely scientific value of it is limited, but the idea of helping astronomers look for intelligent life is an interesting one. The odds are low, of course, but the search for other intelligent creatures out there doesn't take a massive effort, and narrowing down where to look is a worthy goal.
So who knows? Perhaps when I was a kid I had the right idea. We don't know how many intellects vast and cool, perhaps unsympathetic, regard this Earth with envious eyes, and are slowly and surely drawing their plans against us. Or maybe they aren't planning a war of the worlds, but instead, like us, seek out new life and new civilizations in the hopes of making contact.
But what we do know is how many there could be. And that's a nice start.
*There are lots of ways, actually, to look for other planets. Our tech is pretty good, but it wouldn't take major advances to make it way better. We can assume aliens might be more advanced technologically than us, but for now this is a good place to start the logical process of how they might search for us, since it's the easiest way.