This Interstellar Comet is Leaking Water “Like a Water Hose at Full Blast”

3I/ATLAS is shedding a whopping 40 kg of water per second.

The building blocks of life may be much more common than we previously thought. Observations of a comet coming from outside our solar system show a massive trail of water that suggests Earth may not be all that unique after all.

Finding Water in Space

3I/ATLAS is the third confirmed interstellar object to whoosh through our solar system, after 1I/ʻOumuamua (discovered in October 2017) and 2I/Borisov (discovered in August 2019). You wait a long time and nothing happens, and then three of them come within only a few years. This is certainly not what researchers were expecting.

“Every interstellar comet so far has been a surprise,” added Zexi Xing, postdoctoral researcher and lead author of the new study. “‘Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn’t expect it. Each one is rewriting what we thought we knew about how planets and comets form around stars.”

Researchers are rushing to study it as quickly as possible. In October, it will get too close to the Sun to study, though it will reappear in December. By March, it will get to Jupiter and then continue its trajectory outside the solar system. It wandered for millions or billions of years, and will continue to do so for many more to come.

When scientists pointed NASA’s Neil Gehrels Swift Observatory toward the object, they found signs of hydroxyl (OH). Because it’s in outer space, the telescope spotted the faint ultraviolet glow that ground observatories can’t see as they’re obstructed by the atmosphere. A hydroxyl group can be a part of water, but it can also be a part of other molecules like alcohols. In this case, however, researchers are sure it’s water, and it’s a pretty big deal.

Water is the chemical benchmark for studying comets. Comets are small, icy bodies that release gases when they pass close to the sun. In this context, water is the yardstick by which scientists measure the overall activity of comets and track how sunlight drives the release of other gases. Scientists have spotted water in plenty of comets from our solar system, but finding it in an interstellar object means that astronomers can compare the chemistry of our comets to others across the galaxy.

“When we detect water — or even its faint ultraviolet echo, OH — from an interstellar comet, we’re reading a note from another planetary system,” said Dennis Bodewits, professor of physics at Auburn. “It tells us that the ingredients for life’s chemistry are not unique to our own.”

What Does This Mean?

The discovery of 3I/ATLAS on July 1, 2025, sparked a worldwide campaign to characterize the object. Following the precedents set by the first two interstellar objects, astronomers raced to gather data on its brightness, shape, and spectral characteristics. Early observations from James Webb Space Telescope suggested that the comet was releasing both CO₂ and water, but further confirmation was necessary.

This new study provides that confirmation.

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Water is a common driver of cometary activity in our solar system, but its detection in 3I/ATLAS at such a great distance raises intriguing questions. For starters, how did it get the water in the first place? Secondly, why is it releasing so much of it?

Solar system comets usually release more CO₂ when they’re far from the sun and more water when close to the sun. But this comet was releasing water far away from the sun, and lots of it. Estimates put it at 40 kg of water per hour, or about as much as a fire hose at maximum output.

This unusual behavior suggests that our model for comets is probably very incomplete.

The ongoing monitoring of 3I/ATLAS as it continues its journey through our solar system will be essential for clarifying its thermal and compositional properties. It will provide an unprecedented opportunity to test existing hypotheses about how these cosmic wanderers are formed and what they’re made of. This study, and others like it, will ultimately improve our understanding of planetary systems beyond our own, offering a glimpse into the raw materials from which other worlds were built.

The study was published in Astrophysical Journal Letters.