Planet-forming disks hold water, and ALMA data link it to Earth’s water
ALMA has found at least 3.7 Earth oceans of water vapor in the HL Tauri disk and at least 1,200 oceans' worth in V883 Orionis, supporting the idea that water predates the Sun.
Water is common in the disks where planets form. ALMA measured at least 3.7 Earth oceans of water vapor in the inner disk of HL Tauri, and the European Southern Observatory (ESO) reports at least 1,200 oceans’ worth around V883 Orionis, supporting an origin older than the Sun.
At least 3.7 oceans inside 17 astronomical units
HL Tauri is a young star in Taurus, about 450 light-years away according to ESO, and its disk is one of the sharpest ever imaged by the Atacama Large Millimeter/submillimeter Array (ALMA). In a Nature Astronomy paper published on Feb. 29, 2024, a team led by Stefano Facchini of the University of Milan mapped three lines of water emission in that disk.
Most of the emission comes from within about 17 astronomical units of the star, and the authors place the water snowline at 17 au or closer. They describe the amount as “a stringent lower limit of 3.7 Earth oceans” of vapor. The emission sits inside a prominent gap in the dust continuum, and the paper gives the star’s age as under 1 million years.
The detection matters because the gas lies in the region where rocky planets could form. It does not show that any planet there will be wet. The water traces warm gas in the upper disk layers, and the authors note that dust hides deeper gas, which is why they treat the total as a minimum.
A comet-like signature in V883 Orionis
A separate result addresses where the water came from. In a release dated March 8, 2023, ESO described ALMA observations of the disk around V883 Orionis, about 1,300 light-years away, published in Nature. A burst of energy from the star had heated the disk enough to turn ice into gas that radio telescopes can read.
The team measured the water’s deuterium content, a heavy form of hydrogen whose ratio indicates where and when water formed. ESO says that this ratio in some Solar System comets resembles Earth’s water, and the V883 Orionis water matches the comets. “We can now trace the origins of water in our Solar System to before the formation of the Sun,” said lead author John J. Tobin of the U.S. National Radio Astronomy Observatory. “V883 Orionis is the missing link in this case,” he added.
The disk holds at least 1,200 times the water in all of Earth’s oceans, according to ESO. Its team plans to follow up with the Extremely Large Telescope’s METIS instrument, which should be able to observe water gas in such discs. Our report on the telescope’s main mirror covers its construction progress.
What the two studies do and do not show
Taken together, the results show that planet-forming disks around other stars contain large reservoirs of water, and that in one case the water looks inherited from the cloud that formed the star rather than made in the disk. That fits the view that water came with the raw material of the Solar System.
Several limits remain. The HL Tauri figure is a lower bound, and the authors say that more detailed modeling is needed to pin down the snowline. HL Tauri is also a different star from the Sun, and neither paper measures Earth’s own formation. V883 Orionis tells researchers what a disk can inherit, and the link to Earth rests on the comet comparison. The papers do not establish how much of Earth’s water arrived that way, or when, so the question of how Earth’s oceans formed is not closed.
Our coverage of rings and gaps in tiny disks shows ALMA resolving similar structures in much smaller systems.
Ocean worlds that hold water today
Water also persists on bodies in the Solar System, and NASA’s missions are examining it. According to NASA, Saturn’s moon Enceladus, about 500 km across, has a global saltwater ocean beneath its ice. Cassini found in 2005 that water vapor and ice particles erupt from fractures near the south pole, informally called the tiger stripes, at roughly 400 meters per second. The plume contained about 20 times more organic material than scientists expected, and silica grains suggest hot water interacts with rock deep below the ice.
For Jupiter’s moon Europa, NASA’s Europa Clipper launched on Oct. 14, 2024. The page, updated Sept. 10, 2026, says the spacecraft is scheduled to make a gravity-assist flyby of Earth on Dec. 3, 2026, and to reach Jupiter in April 2030. It will then make 49 close flybys of Europa to determine whether places beneath the surface could support life. NASA says scientists think Europa’s ocean may hold more than twice as much liquid water as all of Earth’s oceans combined.
