A Simple Map
40 years ago, NASA launched the Voyager spacecraft and a plan was devised in the event that intelligent life wanted to find their origin point. That plan involved the creation of a map that would lead the finders of the Voyager probes back to Earth. Now, it couldn't be any old map that used directions like North, South, East, West, or vague locations like "the third planet from the Sun."
Instead, astrophysicist Frank Drake decided to create a map that used pulsars — massive neutron stars that can live for millions of years. They often look like they're flickering, but are actually spinning constantly, and slow down with age, and by timing those flickers, you can figure out their spin rate. As explained by Nadia Drake at National Geographic, an intelligent being who found the Voyager and the accompanying map could measure the current spin rate of a pulsar, and compare it to the spin rate noted on the map, informing them of how long the probe had been traveling.
Frank Drake and fellow astrophysicist Carl Sagan decided on this in 1971, six years before either of the Voyagers were launched. 14 pulsars were used for the original map, which contains lines connecting each pulsar to the Sun as the central point. The pulsars' individual spin rates are written on the lines in binary code, with the entire map inscribed on the Voyager Golden Record.
"There was a magic about pulsars … no other things in the sky had such labels on them," explained Drake. "Each one had its own distinct pulsing frequency, so it could be identified by anybody, including other creatures after a long period of time and far, far away."
More Than One Way to Find a Planet
The Pulsar map isn't the only way we've provided extraterrestrial life with a way to track us down. It's widely known that we've sent radio messages and signals to space, including the Aceribo Message which was initially sent in 1974. Even unintentional signals have been sent from various radio and TV broadcasts over the years.
Presently, organizations like Messaging Extraterrestrial Intelligence (METI) are putting more funding into sending additional messages to the stars, while the Breakthrough Message initiative is encouraging a new round of debates about what should be said if/when we find alien life (or it finds us). These efforts are going so far as to hold a competition in which people come up with the "best" digital messages, though there are no plans to send them just yet.
Some are against the idea of continually letting the Universe know we're here, and how to find us. With regards to the Voyager probe, though, it's unlikely the map will ever reach anyone that can read it.
"The thing is going something like 10 kilometers (6 miles) per second, at which speed it takes—for the typical separation of stars—about half a million years to go from one star to another," said Drake. "And of course, it’s not aimed at any star, it’s just going where it’s going."