If you were alive in February of 1990, you got your picture taken by a  camera 6 billion kilometers away.

Hiding in this pale blue dot is you, and me, and every other human who has ever lived. But you’d never know any of us were here just from looking at that Voyager 1 space probe image.

And at that point, Voyager hadn’t even  made it to interstellar space yet. So you might be thinking: How could aliens beyond our  solar system ever find us? Well it turns out, we have left  a few marks in the universe, both on purpose and by accident, that could announce our presence to anyone out there with the right know-how. [INTRO] As we are recording this video, Voyager 1 is not only the  most distant probe that humans have shot into space, it’s also  still active!

Albeit, barely. And it’s technically been in  interstellar space since 2012, although the special boundary it crossed is still well within our Sun’s  gravitational influence. But it’s not the only one out there.

Voyager 1 is currently one of five human-made artifacts that will eventually sail into  other parts of the galaxy. Its sister probe, Voyager 2,  entered interstellar space in 2018, and two of the Pioneer spacecraft  are headed that way, too. Also the New Horizons mission that flew by Pluto in 2015 is on track to join  them sometime in the 2040s.

New Horizons is carrying a  handful of knick-knacks along with its scientific gear  things like a postage stamp, quarters, and two American flags. But the Voyager and Pioneer probes all contain carefully crafted messages to extraterrestrials. Even though they were mainly designed to explore the outer planets in the 1970s  and ’80s, scientists knew these probes would leave the solar system one day, and added messages just in  case they ever got intercepted.

The Pioneer probes each have a small, metal plaque engraved with two naked humans and a diagram showing the position of our solar system. Meanwhile, the Voyagers each  carry an actual pressed record, as well as instructions on how to play it. An alien that successfully plays  the record will treat themselves to various Earth sounds, music  from cultures around the world, and greetings in different languages, as well as a bunch of images  encoded in analog form.

But even though NASA put all this  effort into crafting these messages, it’s extremely unlikely that  aliens will ever intercept them. At the speeds they’re going, it’ll  take tens of thousands of years for any of these probes to even  approach another solar system. Even then, it would be nearly impossible for extraterrestrials to detect these dark ships the size of a car against the  black backdrop of outer space.

But probes are far from the only things we’re sending out into the universe. Humans have been broadcasting  radio and TV shows since the early 20th century. To beam  those signals to your house, they’re transmitted in the  form of electromagnetic waves.

More specifically, radio waves. And  these days, radio waves are also streaming from our cell phone towers,  transmitting our calls and texts. No matter how or when we created  them, all of our radio signals are now racing into the universe  at the speed of light forever.

Which raises the question: If  there are aliens out there with the technology to detect radio waves  could they eavesdrop on us? In theory, maybe. It’s possible that aliens 75  light-years away could be enjoying early episodes of I Love Lucy.

But  in practice, it’s highly unlikely. Radio signals weaken pretty quickly, just like how a source of visible light gets  dimmer the farther away it is. And there are a bunch of  other, natural radio sources screaming into the universe  that can easily drown ours out.

Plus, when it comes to our radio  and TV programs, the actual content of our broadcasts would probably be  too weak for any aliens to detect, because those signals are super weak. See, when a station sends out a broadcast, it doesn’t send the content of the show directly. It needs to use what’s called a carrier frequency to deliver the show to your TV.

In a way, it’s kind of like the mail truck, and the content of the show  is all the stuff inside. But that carrier frequency is many times stronger than the frequencies that  make up the program itself. So, while aliens probably couldn’t  tune in to our old sitcoms, they might be able to detect the carrier signals that transported those sitcoms.

Assuming there are aliens with  technology similar to ours, one 2025 study suggests that  radio leakage from Earth could be detectable up to 4 light-years away. That’s just shy of the distance  to the next closest star: Proxima Centauri. Which, fun fact, does have at least  one planet in the habitable zone.

So, if there are aliens hanging out over there, and if they are in fact  spying on our solar system, they might recognize some artificial signals coming from Earth with a 4-year delay. And if they’ve been studying us for long enough, they might have noticed a change in our habits, such as people switching from  analog TV to streaming platforms. But they probably couldn’t tune in to the news or binge-watch reality TV…  which is probably for the best.

I'd rather they listen to the music, rather than see what we're getting up to on TV. Also, just trying to picture what music aliens would really like, like my brain says David Bowie, but it's probably Chappell Roan. That said, if any of these hypothetical aliens are listening for radio  signals at just the right time, they could potentially pick up signals that we’ve sent into space on purpose.

For example, in 1999, after  decades of listening for alien signals and getting literal radio silence, a small group of astronomers made the unilateral decision to phone ET’s home themselves. They invented a whole alphabet of symbols to represent numbers, mathematical symbols, chemical elements, and all sorts  of other scientific concepts. Then they crafted an elaborate message to serve as a sort of Rosetta  Stone for understanding Earth.

It included an intro to our math and sciences, a map to our solar system,  and drawings of naked humans. We really love sending aliens drawings of naked humans that’s like our thing. Finally, they translated the  whole message into a binary code that could be represented by  two different radio frequencies, and used a radio transmitter in Crimea to beam it toward five separate star systems.

The first was about 69 light years  away, in the constellation Cygnus. Which means it won’t arrive until 2068. And if any Cygnans decide to send a message back, we won’t get that response until 2137.

As this message started its interstellar journey, the scientific community argued about whether sending any message to aliens was a good idea. After all, we don’t have a great track record when it comes to civilizations  meeting each other on Earth. But good or idea or not, it was definitely not the last time we tried to send a  message to extraterrestrials.

There have been more than a  dozen messages beamed into space since then, and not all of them are  exactly serious works of science. In 2008, NASA transmitted the  Beatles song “Across the Universe” toward Polaris, the current  North Star, and Doritos paid a European research organization to beam an advertisement toward the Big Dipper. I mean no one can resist Doritos...

Just like the message to  Cygnus, all of these messages are still in-transit, so no  one’s called us back yet. But while we’ve only been  sending radio signals into space since the 20th century,  there's another signal or set of signals that human civilizations have been producing for much longer. Ever since the Industrial Revolution kicked off, human technology has altered our atmosphere in some very non-subtle ways.

They’re called technosignatures, and I’m not just talking about all the CO₂ we’ve pumped into our atmosphere. To aliens, that alone might  not be especially interesting, because you don’t need life to get CO₂. I mean, Venus’s atmosphere is almost all CO₂, with no help from living things.

We THINK. But other compounds we pump into the atmosphere aren’t so common in nature,  like the chlorofluorocarbons used in refrigerants and cleaning supplies. These are combinations of  carbon, chlorine, and fluorine that basically never form naturally and could be a pretty dead giveaway to  any aliens spying on Earth.

Assuming, that is, they’re  advanced enough to analyze the spectrum of light passing  through Earth’s atmosphere. An aligned correctly to see that. A spectrum is what you get  when you pass light through something like a prism that splits it  into all of its constituent colors.

Not just the stuff in a visible rainbow, either. The invisible wavelengths, too. The molecules in Earth’s  atmosphere absorb some of those wavelengths, creating little  dark lines in the spectrum.

And that pattern of dark lines reveals which elements and compounds are doing the absorbing. Spectra are how we figure out what  planets’ atmospheres are made of, and a species with the right  technology could do the same to us. There are tens of billions  of planets in our galaxy that could potentially study Earth this way.

But if they’re more than a  few hundred light years away, they won’t see much. Even assuming their technology  is better than ours, the most obvious technosignatures have only been in our atmosphere since the Industrial Revolution. That light hasn’t gotten very  far yet, in galactic terms.

But if you want to be obnoxiously optimistic, some research has measured  an uptick in atmospheric carbon dioxide and methane  as early as 6,000 years ago. Scientists aren’t sure if these higher levels can be tied to human activity, but  this was a point in history where agriculture was taking  hold around the world. Forests were being converted  to farmland on a large scale, which absorbs less carbon.

Meanwhile, livestock may have been farting and burping out that extra methane. But even if humans really are to blame for those ancient measurements, they’re also probably too subtle for anyone 6,000 light years away to detect. Still, it’s fun to think about just  how far away aliens could detect life on Earth.

Especially when you  realize it doesn’t take an advanced species to alter the makeup of the atmosphere. Much simpler life forms have been doing it for far longer than humans could ever hope to. Ever since plants colonized the  land half a billion years ago, they changed the nature of the light that our planet reflects back into space.

Photosynthesizing plants  absorb a lot of visible light and they strongly reflect infrared light. That light has now traveled well beyond our galaxy and crossed through hundreds  of thousands of other galaxies. So if aliens were out there with powerful enough telescopes to study Earth’s spectrum, they’d see a steep curve right  around the point where the light goes from visible to infrared.  Astronomers call it a red edge.

Of course, if those aliens don’t have  plants of their own on their planet, or if those plants don’t do  photosynthesis, maybe they won’t have any idea what they’re looking at. But they might at least clock  it as something unusual. And that’s not the only sign of  these plants in our atmosphere.

Here on Earth, photosynthesizing  plants also pump out huge amounts of oxygen. Some of that oxygen also goes through reactions in the  atmosphere to create ozone. But if it weren’t for plants, as  well as other photosynthesizing organisms like algae, the  oxygen in our atmosphere would disappear pretty quickly because of  how it reacts with other molecules.

The fact that it stays at high  levels, alongside other molecules it would normally react with,  is a good sign that something alive is constantly topping  off Earth’s oxygen supply. These bio- and technosignatures  would be detectable from far across the galaxy to a  species with the right tools. But if aliens ever did get suspicious  enough to take a closer look, and send a spacecraft to the  edge of our solar system, they might see more than that pale  blue dot that Voyager captured.

If it were outfitted with both  a telescope and a spectroscope, a tool for splitting light into  spectra, they might be able to see something even more interesting  than all the molecules we and our technology have  pumped into the atmosphere: If they looked at our planet’s night side, they might also pick up  some light from our cities. A lot of that light comes  from sodium street lamps, so it would have a telltale  signature in the spectroscope. And any aliens that are at least advanced as we are would be able to tell it’s artificial.

Earth would still be a glowing  smudge from that distance. But they’d know we’re down here ready to say, “We're not really ready for first contact. Can you maybe do next Thursday?” [OUTRO]