Each summer for the past 25 years, tens of thousands of people have flocked to Barcelona, Spain, to witness Sónar, a three-day festival dedicated to electronic music, art, and design. Something of a cross between a TED talk, Burning Man, and Coachella, Sónar has evolved from a small experiment into an event that the New York Times described as a “European institution” in 2017. It’s also the closest thing we have to an extraterrestrial envoy.

To celebrate Sónar’s 25th anniversary in 2018, the festival partnered with the Catalonia Institute for Space Studies and the nonprofit METI International to send a series of interstellar messages to Luyten’s star, a red dwarf about 12 light-years from Earth. Although red dwarfs are the most common stellar objects in our galaxy, Luyten’s star is remarkable for hosting GJ237b, the closest potentially habitable planet outside of our own solar system. No one knows for sure whether GJ237b hosts life, intelligent or otherwise, but if ET does call the planet home, Sónar wants to rock its socks off.

Over the course of several nights in late 2017 and early 2018, a radar system in Tromsø, Norway, blasted a custom message from Sónar toward GJ237b. Like any good correspondence, the message began with a greeting: In this case, the first 33 prime numbers repeated on two alternating radio frequencies functioned as a stand-in for “hello.” This was followed by a brief tutorial that the message designers hoped would teach ET to extract the music written by Sónar-affiliated musicians and embedded in the message.

Each song in the Sónar messages is only a few seconds long and might only be called music in the loosest sense of the word. One track was created by feeding an algorithm music and letting it remix the notes as it saw fit, which resulted in something that sounds like a horror movie sound effect. Another uses the atomic numbers of a handful of oxygen, silicon, and other elements as the frequencies for pure tones. These arrangements don’t make for easy listening, but that’s not the point. Instead, the artists use music as a way of conveying information, whether it’s about our aesthetic sensibilities, our technology, or our physiology—all topics that would presumably be of interest to an extraterrestrial recipient.

In many respects, the Sónar messages are on well-trodden ground. The first human-made object to make it to interstellar space, the Voyager 1 spacecraft, carries a gold-plated phonographic record that includes Mexican folk music, early rock and roll, a Peruvian wedding song, and more. In 2001, a message sent from the Evpatoria radar in Ukraine included theremin renditions of Beethoven, Vivaldi, and Gershwin; a few years later, NASA blasted a Beatles song at a star 400 light-years away.

But the Sónar messages are unique insofar as they are the only interstellar transmissions to use songs designed by musicians specifically for communicating with ET. That the messages include a substantial information content places them firmly in the tradition of messaging extraterrestrial intelligences, or METI, a term coined by the Russian radio astronomer Alexander Zaitsev to differentiate the practice from other modes of interstellar communication. The Search for Extraterrestrial Intelligence, or SETI, is focused on listening for ET signals rather than sending them, and “active SETI” is about creating beacons that lack information but signal to alien intelligences that we exist.

Historically, interstellar communication has tended toward formalism, or systems in which elements are manipulated according to stringent rules. (So a game like chess is highly formalized, whereas natural language is less so.) For example, the lingua cosmica developed in 1960, the first artificial language for interstellar communication, is based on a mixture of logic, mathematics, and natural language syntax. (The Cosmic Call transmissions in 1999 and 2003 used a custom symbolic language based on the lingua cosmica.) More recently, the Dutch computer scientist Alexander Ollongren proposed a second generation of the lingua cosmica that was derived from lambda calculus, a highly formalized logical system.

These systems lend themselves to straightforward analysis—the idea is that aliens could tease out the rules of the system without understanding what the symbols themselves mean. Music is sometimes perceived as the opposite, ineffable, something that is not so much understood as felt. But as any musician will tell you, there is also deep logic inherent to music: There are equal distances between notes in a scale, notes can be combined in certain ways called harmonics, rhythm can be expressed in numerical ratios called time signatures, and so on. Music is a hybrid of logic and emotion, the yin and yang of the human experience.

In this respect, music is an ideal medium for interstellar communication, but it must be tailored for transmission across billions of miles of empty space. When I hear music on Earth my ear is registering the compression of the surrounding air, but there’s no air in space so ET can’t hear a musical message directly. The music must first be encoded into the radio wave in either an analog or digital format. (Both have been used to send music across interstellar space.) Music’s inherent formalism suggests that an ET that lacks the ability to hear could gainfully analyze various elements of music—its rhythm, pitch, and so on—by studying the way these elements are encoded in radio waves.

If the goal of METI is to convey information about Earth, neglecting to include music would be a major oversight.

Douglas Vakoch, the founder of the METI Institute and the director of the Sónar messaging effort, the composer Andrew Kaiser, and Ollongren have all proposed unique ways for encoding musical concepts in interstellar messages. For example, Vakoch has suggested a method to use icons to teach musical concepts to aliens. (Unlike symbols, which bear no resembles to the thing they represent, icons directly resemble the thing they represent.) So to teach the concept of rhythm, an interstellar message could be pulsed rhythmically. And what might an ET learn from its analysis of the formal elements of music encoded in an interstellar message? According to Vakoch, musical messages can teach ET quite a bit about human physiology. For example, the number of notes in a scale can be used to establish how sensitive we are to differences between notes.

Beyond the practicalities of using music as a basis for an interstellar message, it’s also worth considering its role in the human experience. Aspects of music are found in nearly every culture on Earth. Unlike language, anyone—at least, any human who can hear and/or perceive rhythm—can “understand” music, even if those who cannot play an instrument or interpret the notes upon a staff. If the goal of METI is to convey information about Earth and the people who inhabit it, neglecting to include music would be a major oversight.

The ubiquity of music on Earth is a good thing, but when it comes to interstellar messaging it poses a problem: How do we select which songs to send to ET? Historically, the musical contents of interstellar messages have been extremely biased toward Western classical music, which hardly captures the diversity of musical styles found on our planet. This bias arises from the lack of diversity in the small committees of individuals responsible for selecting the music for interstellar transmission. (Jon Lomberg, who helped design the Voyager golden records, attempted to create a more diverse message for the New Horizons mission, which will be the next to enter interstellar space. But it was not included on the spacecraft.)

But any selection process that only considers already existing music is bound to suffer from cultural biases. It is simply impossible to create a corpus of music that represents every cultural group on Earth or every genre of music. This suggests that intentionally designing music for interstellar transmission is the most promising path forward insofar as it would effectively be creating an entirely new genre of music. Not only would it avoid selection bias, but it opens the possibility of creating music that carries a maximum amount of information about the species that created it. This is a radical departure from music’s typical function of connecting humans with one another; the new extraterrestrial music would be composed to connect two entirely different intelligent species across vast expanses of time and space.

The musical elements of the 2018 Sónar messages were a first tentative step in the direction of extraterrestrial music, but they won’t be the last. Earlier this year, the SETI Institute announced its Earthling project, which is crowdsourcing original music from all around the globe. These samples will form the basis of a seven-part music composition called “Earthling” that will be performed at the Allen Telescope Array in Northern California, the only telescope in the U.S. dedicated to the search for extraterrestrial intelligence. The Earthling project aims to create a collective composition that represents humanity writ large and will hopefully avoid the bias that has plagued past musical interstellar messages. Although there is no plan to broadcast the composition into the cosmos, the project is can teach us a great deal about how to compose music with an eye toward interstellar communication.

When Carl Sagan set about designing the Voyager Golden Record, he understood humanity’s first musical interstellar message was unlikely to ever be intercepted by an extraterrestrial intelligence. Nevertheless, he recognized that “launching this bottle into the cosmic ocean says something very hopeful about life on this planet.” The same holds true for all future musical interstellar messages, even if our terrestrial melodies never grace an extraterrestrial ear.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.