Astronomers have found intriguing evidence that comets may play an important role in delivering water to planets as planetary systems develop. By studying a young star system called PDS 70, researchers at Lund University in Sweden have detected signs that comets may be moving from the cold outer regions of the system toward the inner region, where planets can form.
The discovery offers scientists a rare opportunity to observe a process that may have also occurred billions of years ago in our own Solar System. It could help explain one of astronomy's biggest questions: How did Earth get its water?
A Young Planetary System
PDS 70 is located approximately 370 light-years from Earth and is only a little over 5 million years old. In astronomical terms, that makes it extremely young compared with our Solar System, which formed about 4.6 billion years ago.
The star at the center of PDS 70 is slightly cooler than our Sun. Scientists already know that the system contains at least two large gas-giant planets orbiting the young star.
Because the system is still in the early stages of development, it provides astronomers with a valuable window into the processes that may have shaped planets long before Earth existed in its current form.
In 2023, researchers detected water vapor close to the star. The new study adds another important clue: changing amounts of sodium gas appear to be associated with objects moving through the inner part of the system.
These objects could be exocomets—comets orbiting stars beyond our Sun.
A Strange Signal in Starlight
The researchers examined observations collected in 2018 and noticed changes in sodium gas in front of PDS 70.
The sodium appeared to move at speeds of several kilometers per second relative to the star. More importantly, the signal was not constant. It appeared and disappeared over several nights.
This behavior caught the researchers' attention because it resembles what scientists might expect when a comet passes between an observer and its star.
When a comet travels through the cold outer regions of a planetary system, much of its water and other volatile material can remain frozen as ice. But as the comet approaches its star, temperatures rise dramatically.
The ice on its surface can then change directly from a solid into gas. This process is called sublimation.
The released gas forms part of the comet's surrounding atmosphere, known as a coma. As the comet moves across the star's line of sight, the gases can absorb particular wavelengths of the star's light.
Astronomers can study these changes in starlight to learn about material moving around distant stars.
According to first author Aline Novais, the study provides the first evidence of exocomets orbiting a relatively cool star similar to the Sun. It is also the youngest planetary system in which exocomet activity has been proposed.
Comets Could Be Nature's Water Delivery System
The discovery becomes even more interesting when scientists consider where comets are likely to originate.
In a developing planetary system, the outer regions are extremely cold. Water and other volatile substances can remain frozen there, becoming incorporated into icy bodies such as comets.
But these objects do not necessarily stay in the same orbit forever.
PDS 70 contains massive gas-giant planets. Their enormous gravitational forces can strongly influence smaller objects traveling through the system.
Researchers simulated possible cometary orbits and found that gravitational interactions with the gas giants could push objects from the outer regions toward the central star.
In some cases, these gravitational encounters can change a comet's path dramatically, sending it inward.
This could create a natural transportation system.
Comets form far away, where water can exist as ice. Gravitational interactions move them inward. As they approach the star, their ice turns into gas and releases water and other volatile materials.
If some of these objects eventually interact with forming planets, they could potentially contribute water and other important ingredients needed for planetary development.
Could This Explain Earth's Water?
The origin of Earth's water remains an important scientific mystery.
Scientists know that Earth contains enormous quantities of water, but exactly how that water arrived during the planet's formation is still debated.
One possibility is that some of Earth's water was delivered by water-rich asteroids. Another possibility is that comets contributed at least part of it.
The new observations of PDS 70 do not prove that comets were responsible for Earth's oceans. However, they provide scientists with an opportunity to observe a similar type of process while another planetary system is still young.
Astronomer Alexandra Stockwell Murphy of Lund University notes that the process resembles a possible mechanism in the early Solar System, where comets may have helped transport water toward the young Earth.
Studying a system like PDS 70 is particularly valuable because astronomers cannot travel back in time to observe the formation of our own Solar System.
Instead, they can study young planetary systems elsewhere in the galaxy and use them as natural laboratories.
A Glimpse Into Planet Formation
Planet formation is a complex process.
Young stars are surrounded by disks of gas and dust. Over time, particles collide and stick together, gradually producing larger bodies. Eventually, these objects can develop into planets, moons, asteroids and comets.
During this process, material is constantly moving around the system.
Some objects remain in the outer regions, while others can migrate inward because of gravitational interactions. Giant planets can be particularly powerful drivers of this movement.
The PDS 70 observations suggest that this movement may involve more than just rocks and dust. Icy comets could transport water and other volatile compounds from distant regions toward the areas where terrestrial planets may eventually develop.
That makes exocomets important pieces of the planetary-formation puzzle.
The Future Could Reveal Even More
Astronomers are now looking forward to observations with the Extremely Large Telescope (ELT), currently being built in Chile.
The telescope will provide unprecedented capabilities for studying distant planetary systems. Researchers hope it will help determine whether PDS 70 contains additional planets that have not yet been detected.
Finding more planets would be particularly important because their gravitational influence could help scientists better understand the possible paths of comets through the system.
Future observations may also reveal more about how water and other volatile materials travel through young planetary systems.
What It Means for Earth
The discovery does not mean scientists have solved the mystery of Earth's water. Instead, it provides an exciting piece of evidence showing that comets could act as natural carriers of water during planet formation.
PDS 70 gives astronomers a rare look at a planetary system only a few million years into its evolution. The changing sodium signal could be a fingerprint of exocomets traveling through its inner regions, potentially after being redirected by giant planets.
If this interpretation is confirmed by future observations, it could strengthen the idea that planetary systems have their own version of a cosmic delivery network—one capable of moving water from cold, distant regions toward developing worlds.
And perhaps billions of years ago, something similar happened here.
Earth's oceans may be the legacy of material that began its journey in the cold outer reaches of the young Solar System.
Reference: Novais, A., Hoeijmakers, H.J., Stockwell Murphy, A. et al. Potential sublimating exocomets around the young star PDS 70. Nat Commun 17, 8894 (2026). https://doi.org/10.1038/s41467-026-76880-y
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