December 13, 2022 2:12 PM EST

The James Webb Space Telescope (JWST) has ticked a lot of boxes in the near year it’s been aloft. Fly safely to its appointed spot in space 1.6 million km (1 million mi.) from Earth? Check. Successfully deploy its mirror, scientific instruments, and tennis court-sized sun shield? Check. Begin returning eye-popping images like none ever seen before? Check.

Now, the Webb has delivered on its biggest promise to date. According to a new, not-yet peer-reviewed paper on the pre-publication website arXiv, and presented on Dec. 12 at the Space Telescope Science Institute in Baltimore, Webb’s mission control, the telescope has spotted and confirmed the four oldest galaxies ever seen—galaxies which date back an average of just 400 million years (some even earlier) after the Big Bang, which occurred 13.8 billion years ago.

“This is the way the galaxies would have appeared 13.4 billion years ago,” says lead author Brant Robertson, professor of astronomy and physics at the University of California, Santa Cruz. “[With Webb] you can rewind the clock and see them as they were back then. That’s what we’re trying to do by taking these observations: we’re looking back in time.”

The new findings show not only that galaxies started forming as early as 325 million years after the Big Bang, but that there are likely ones that are older still—bringing astronomers closer to discovering the actual birth date of the very first galaxies.

Read more: The James Webb Telescope Team: Innovator of the Year 2022

This discovery is the result of work conducted by the JWST Advanced Deep Extragalactic Survey (JADES), an international team of more than 80 astronomers from 10 countries who used 10 days of observation to study a deep field of 100,000 galaxies first imaged by the Hubble Space Telescope in 2010. Apparent size, faintness, and distance of many of the galaxies suggested that they were fantastically old, but Hubble, which sees principally in visible light, didn’t have the instrumentation to image them clearly or determine their age. That’s because visible light from so far away can’t easily penetrate the intervening dust of interstellar space. Infrared radiation, however, cuts right through the dust. This allows Webb— a telescope which detects energy in that frequency—to see as far as 13.6 billion light-years distant.

Using two instruments aboard the Webb—the Near Infrared Camera (NIRCam) and the Near Infrared Spectrograph (NIRSpec)—Robertson and his colleagues focused on four galaxies that appeared especially small, faint, and distant, studying what is known as their red shift. As objects move toward us in space, the wavelength of light they emit is compressed, shifting it to the bluer end of the visible spectrum. As objects move away from us, the wavelength is stretched, shifting it toward the red end. The redder an object appears, the more distant and old it is in our still-expanding universe.

Red shift is a tricky thing to measure, because it has no particular units like inches or nanometers. Instead, it is just a number that indicates how stretched the wavelength of the light is. An object like Jupiter, which is pretty much stationary in the sky relative to Earth, has a red shift of zero. The higher the number, the greater the movement of an object away from Earth. The cosmic background radiation, a burst of universe-wide energy that was released just 380,000 years or so after the Big Bang has a red shift of about 1,100.

“For most galaxies,” says Robertson, “the highest red shifts [or the oldest galaxies] we had spectra for were at six, seven, or eight.”

Against those relatively modest standards, the four galaxies Robertson’s team imaged blew the doors off the old record, weighing in with red shifts of 10.38, 11.58, 12.63, and 13.2. Those numbers put the galaxies on a continuum from about 450 million years after the Big Bang—13.35 billion years ago—to 325 million years after the Big Band, or 13.475 billion years ago.

“These are well beyond what we could have imagined finding before [Webb],” said Robertson in a statement accompanying the release of the paper.

Read more: Scientists Learn More About the History of Stars in Latest Webb Telescope Images

The galaxies are impressive by dint their age, but not by dint of their size and mass. “The Milky Way is a few tens of billions of times the mass of the sun in stars,” says Robertson. “These galaxies are 100 million to a billion times the mass of the sun in stars.”

The explanation for that is a straightforward one. Astronomers don’t yet know exactly when the first dust and gas began to accrete into galactic clouds, and stars then accreted within them, but the newly measured quartet emerged very early in that process. “The universe just wasn’t building big galaxies at that point,” Robertson says. “There was not a lot of time [for larger galaxies to form].”

The team not only measured the mass of the galaxies, but also used the NIRSpec to determine their chemical makeup. As would be expected for galaxies so early in their development phase, the principle components were hydrogen and helium, without enough time for the early stars within them to have developed heavier elements. “They are relatively metal-poor,” says Robertson, “with fewer heavy elements compared to the sun.”

The findings as a whole, promise still bigger discoveries—and still older galaxies—to come in the 20-some years Webb is expected to remain operational. “For us, this really was a paradigm shift in the way we were thinking about the high-red shift universe,” says Robertson. “Because we know for certain that there are some galaxies to be studied only a couple of hundred million years after the Big Bang.”

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Write to Jeffrey Kluger at jeffrey.kluger@time.com.

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