In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object. 1 year later, its heavy water revealed clues to a cold alien planetary system

In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object. 1 year later, its heavy water revealed clues to a cold alien planetary system. The object entered the Solar System on a path that could not close around the Sun. As sunlight warmed its surface, gas escaped from its nucleus and allowed researchers to study material formed around another star. ALMA and the James Webb Space Telescope detected deuterium in its water at levels far above those measured in Solar System comets. The result points to ice that formed at temperatures below about 30 kelvin and retained that chemical record.

3I/ATLAS entered the Solar System from interstellar space

3I/ATLAS was reported by the NASA-funded ATLAS survey telescope at Rio Hurtado in Chile on July 1, 2025. Astronomers soon determined that its orbit was hyperbolic. This means the object was moving too fast to remain gravitationally bound to the Sun. It was therefore passing through the Solar System rather than becoming part of it.


3I/ATLAS became the third confirmed interstellar object known to have entered our Solar System. The first was 1I/‘Oumuamua, followed by 2I/Borisov. Unlike ‘Oumuamua, 3I/ATLAS developed a gaseous coma. Borisov had also shown outgassing. The coma allowed astronomers to study molecules released from the comet's ice.

The solid nucleus could not be resolved directly. However, the gases around it produced spectral signals. These signals provided information about material that formed in another planetary system.

The comet reached perihelion on October 29, 2025. It passed about 1.36 astronomical units from the Sun, which placed it inside the orbit of Mars. It did not pose a threat to Earth. After passing the Sun, 3I/ATLAS continued outward. It will eventually leave the Solar System and return to interstellar space.

In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object

The main clue came from water and the two forms of hydrogen found in it. Normal hydrogen has one proton. Deuterium has one proton and one neutron. Because of the additional neutron, deuterium is about twice as massive as ordinary hydrogen.

Water normally contains two hydrogen atoms and one oxygen atom, giving it the formula H2O. When one hydrogen atom is replaced by deuterium, the molecule becomes HDO. This is called semi-heavy water.

Astronomers measure the ratio of deuterium to hydrogen, known as D/H. The phrase “30 times more heavy water” needs some explanation. It does not mean 3I/ATLAS contains 30 times more total water. It refers to the proportion of deuterium relative to ordinary hydrogen in its water.

Solar System comets have an average water D/H ratio of about 0.029 percent, although individual comets differ. Earth's ocean water has a lower value of about 0.0156 percent. ALMA initially placed the D/H ratio of 3I/ATLAS above 0.66 percent under its conservative assumptions. Webb later measured 0.98 percent, with an uncertainty of plus or minus 0.06 percentage points.

The Webb measurement is about 34 times the Solar System comet average and about 63 times Earth's ocean value. Different reports use different comparisons. This is why some descriptions refer to 30 times and others to more than 40 times. The main finding remains that the D/H ratio is far above measurements for known Solar System comets.

ALMA detected HDO when ordinary water was not detected

On November 4, 2025, six days after perihelion, the Atacama Compact Array observed 3I/ATLAS using two ALMA radio bands. One spectral window targeted ordinary water at 183.310 gigahertz. Another targeted HDO at 241.561 gigahertz.

ALMA detected HDO and several methanol lines. The selected H2O line remained below its formal detection threshold. This was the first detection of deuterated water in an interstellar object.

The timing was important. The comet had just passed the Sun, and water outgassing was increasing. Radio telescopes could observe the object close to the Sun after perihelion, while many optical observatories could not use the same viewing geometry.

Water had already been detected through other methods. The Swift spacecraft had detected hydroxyl, which forms when sunlight breaks water molecules apart. The ALMA analysis had a problem. Researchers needed the ordinary-water production rate to calculate the D/H ratio, but the selected H2O line was not detected.

A non-detection helped establish a lower limit

Researchers used a radiative-transfer model called SUBLIME to analyse the HDO, H2O and methanol spectral regions together. Methanol transitions provided information about temperature and gas density in the coma. These measurements helped constrain the collision rate and total gas production.

The model produced an estimated water production rate of about 1.6 × 10²⁹ molecules per second. Under the main model, the D/H ratio was above 0.46 percent. Researchers also considered a conservative case. They used the highest water production rate allowed by the non-detection of the H2O line. A higher amount of ordinary water would reduce the calculated deuterium fraction.

That produced a lower limit of 0.66 percent. Compared with the error-weighted average for Solar System comets, these limits indicated enrichment of more than about 20 and 30 times. The researchers also listed assumptions in the calculation. The model treated ordinary water as the main collision partner in the coma and used an averaged description of molecular collision rates.

Carbon dioxide had been the main coma component farther from the Sun. Water production increased near perihelion. The result was therefore presented as a constraint rather than a direct measurement of every H2O molecule.

Webb measured the D/H ratio using infrared observations

On December 22 and 23, 2025, the James Webb Space Telescope observed 3I/ATLAS after the comet had moved to about 2.4 astronomical units from the Sun. Webb's NIRSpec instrument detected infrared signals from H2O, HDO, carbon dioxide and carbon monoxide.

A separate study published in Nature reported a D/H ratio of 0.98 plus or minus 0.06 percent. The study also found unusual carbon-12 to carbon-13 ratios. The Webb observation was important because it used a different method from the ALMA analysis. It involved a different instrument, wavelength range, observation date and coma model.

Both approaches found high deuterium enrichment. The agreement does not remove all uncertainty in the models. However, it provides another measurement supporting the conclusion that the water in 3I/ATLAS contains a high proportion of deuterium.

What the deuterium tells scientists about the comet?

The high D/H ratio points to chemical processing at low temperatures. Deuterium formed during the first minutes after the Big Bang. However, the large deuterium concentration in 3I/ATLAS water cannot be explained by the normal hydrogen isotope abundance of the Milky Way alone.

Chemical fractionation provides an explanation. At low temperatures, differences in reaction energies can favour processes that move deuterium into ions and molecules. Deuterium can then become part of water ice that forms on dust grains.

This process becomes effective at temperatures below about 20 to 30 kelvin. That corresponds to about minus 253 to minus 243 degrees Celsius. Later heating can change the chemical record. Ice can sublimate. Molecules can exchange isotopes. Water can also move through a planetary disk and mix with material formed at other temperatures.

The D/H ratio of 3I/ATLAS suggests that much of its water avoided this type of thermal and isotopic resetting. Scientists can therefore identify cold formation conditions, but the isotope ratio does not identify one exact location.

The water may have formed in a molecular cloud before the parent star existed. The ice could then have entered the planetary disk. Another possibility is that the water formed or was processed in a distant region of the planetary disk, possibly beyond the carbon-dioxide snowline.

Radiation and cosmic rays can also provide the ionization needed for deuterium chemistry. The D/H ratio does not establish one specific radiation environment, formation distance or chemical sequence.

The comet's age and journey remain different questions

Studies of the motion of 3I/ATLAS have placed its possible kinematic age between about three and 11 billion years. Carbon-isotope measurements have also been used to suggest that some of its material could have formed 11 to 12 billion years ago in an environment with lower metal content.

This does not mean the comet spent all that time travelling between stars. A planetesimal can remain in its original planetary system for a long period before being ejected. A giant planet or a passing star can disturb its orbit and send it into interstellar space.

Once outside its parent system, the object's Galactic path can change because of gravitational encounters. The Webb team noted that tracing the orbit backward beyond about 10 million years becomes difficult because Galactic gravitational encounters are chaotic and not fully known.

Scientists can therefore discuss a possible interstellar journey lasting millions or perhaps billions of years. They cannot assign a precise travel time or identify its parent star with confidence. The chemistry provides stronger information about its formation environment than its present orbit provides about its original home.

What 3I/ATLAS means for future interstellar studies?

3I/ATLAS shows that water ice from another planetary system can retain an isotope composition that differs from measurements of familiar Solar System comets. It also shows why interstellar objects can provide information about planets around other stars. When such an object enters the Solar System and begins releasing gas, observatories can analyse molecules from its original material.

However, 3I/ATLAS cannot establish how common deuterium-rich water is across the Galaxy. Only three interstellar objects have been confirmed so far. Comparable D/H measurements were not available for ‘Oumuamua or Borisov. The available sample is therefore too small to determine whether Solar System comets are unusual, whether 3I/ATLAS represents one type of extrasolar ice, or whether several types of interstellar comet exist.

Future surveys may identify more interstellar objects early enough for ALMA, Webb and other observatories to observe them together. Each new measurement can add information about the conditions under which planetary material forms. For now, the water in 3I/ATLAS has provided a chemical record of a cold environment outside our Solar System. Its orbit cannot reveal exactly where that material came from, but its isotopes preserve information about the temperatures under which its ice formed.