An intelligent species of extraterrestrials might have telescopes far more powerful than ours.

Scientists have been trying to discover planets around other stars for generations. They finally succeeded in the 1990s, and more than 4,000 have been catalogued since then.

But could aliens have found our planet? Is Earth cataloged by even a single population of extraterrestrials? If so, what do they really know about terra firma?

You may consider this an idle question, of no greater importance than asking if gerbils enjoy oboe concertos. But the answer is of real consequence for those who scan the skies for signals from intelligent aliens. After all, if extraterrestrials are unaware of our world and its properties, what would spur them to transmit signals in our direction?

Additionally, if you’re among the many folks who are convinced that aliens are sailing through the troposphere, it might help your self-esteem to know that extraterrestrials could have learned enough about us to pay a visit.

It seems a safe bet that if advanced aliens do exist in our galaxy, they would at least know our planet is here. If human astronomers can find thousands of worlds in two dozen years, how many exoplanets—planets around other stars — will the denizens of other solar systems find in, say, a millennium of slogging away?

The real challenge for these exo-catalogers isn’t finding the planets, but discovering details beyond the gross characteristics — mass, size and approximate temperature. To learn more, the aliens will need really big telescopes.

Is that unreasonable? Hardly. Sure, some aliens would have instruments comparable to ours, but forget those guys. We’re talking about sophisticated Klingons (or whatever their species is). So let’s begin by considering what they could see with scopes whose optical systems are approximately as big as the largest we are building ourselves: tens of meters in size.

That class of instrument would allow them to observe many exoplanets as dots of light — maybe just a single pixel. No detail, in other words. But they could direct that light through a prism, analyze its spectral fingerprint and detail the planet’s atmosphere.

Recently, researchers used spectroscopy to find water vapor in the atmosphere of planet K2-18b, which is a daunting 110 light-years from Earth. That was hard for us but would be a piece of Klingon cake for some otherworldly astronomers. They could easily find the oxygen in Earth’s air, a tipoff to the fact that this is a living planet. The oxygen betrays photosynthesis.

Another thing that enterprising extraterrestrials might do is simply monitor the total amount of light coming from Earth — even seen in a single pixel. They’d note that it changes in regular ways over the course of days, and that alone would tell them that Earth is a rotating world. Deeper analysis of this changing brightness might clue them in to the presence of oceans and continents.

But let’s not hold back. If the aliens’ telescopes are truly top-of-the-line, their observations could be considerably more newsworthy. A telescope with a mirror that’s kilometers in size would allow the postulated gray guys to make actual images of Earth. These wouldn’t be high-res pix, but would be enough to map out the general size and shape of the continents.

Researchers using data from NASA’s Deep Space Climate Observatory recently simulated what one of these primo instruments might see of our planet after observing it for two years to average out the cloud cover. (The clouds come and go, but the geography is fixed.) The resulting image is hardly Google Earth quality, but the aliens would still have a better world map than the Greeks did.

None of this research is too much to ask of an alien society — even one that’s no more than a few centuries beyond our own. If we can imagine it, some of them have probably done it. Of course, the most interesting thing these hypothesized neighbors might find is not the outlines of the Americas or even the oxygen in our atmosphere. They might find us.

They wouldn’t see us, but if they’re within 70 light-years or so they could pick up our radar or television — signals we began launching into space during World War II. The extraterrestrial eavesdroppers would need big antennas to do this, but not impossibly big ones.

Roughly 15,000 star systems lie within 70 light-years. No one knows if that’s a large enough sample to contain some technologically nimble aliens, but it’s hardly inconceivable that somebody has not only found Earth but learned that we inhabit it.

One supposes they would find this of some interest.

This article was originally published for NBC News MACH at https://www.nbcnews.com/mach/science/we-keep-looking-space-aliens-are-they-looking-us-ncna1054271