Around PDS 70, not everything is in place yet. This star is only 5.4 million years old and is still embedded in its disk of gas and dust.
Two giant planets, PDS 70 b and PDS 70 c, have already been identified there. They orbit within a vast region depleted of material, between the inner and outer disks.
Wide and close-up views of the disk surrounding PDS 70 c, observed with ALMA.
Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.
It was in this still-young setting that, in 2023, James Webb detected water vapor heated to around 600 K, very close to the star. It is located in the inner disk, near the regions where rocky planets could form.
Where did this water come from? Several scenarios remain possible. It may have formed in place, or traveled on tiny icy dust grains from colder regions.
A study published in Nature Communications adds another possibility. Exocomets could also transport volatile material toward the inner part of the system.
The researchers revisited 52 spectra of PDS 70 recorded with HARPS between 2018 and 2020. In the 2018 data, some sodium lines appear, disappear, and rapidly change velocity.
This behavior does not suggest a uniformly distributed gas cloud. It more closely resembles signatures already associated with exocomets around other stars, particularly Beta Pictoris.
The idea is fairly simple. When a small body passes very close to its star, the heat can vaporize some of its material. The sodium released in this way absorbs certain wavelengths of starlight, leaving a trace in the spectrum.
Astronomers therefore do not see the exocomets directly. Instead, they track the gas these objects could release as they pass by. This distinction is important, because other phenomena linked to the disk can also produce gas.
The authors specifically tested the hypothesis of winds arising from the disk. It accounts less well for the rapid variations observed in 2018. Then, in 2026, UVES again detected variable sodium lines.
Some were redshifted, whereas those from 2018 were mostly blue. This diversity provides additional support for the exocomet scenario.
One problem remains: how can small bodies formed far from the star get so close to it? Simulations provide one possible answer. The gravity of the two giant planets can strongly perturb planetesimals in the outer disk.
Some are then thrown inward onto highly elongated orbits. The process can occur within a few million years, during the system’s very youth.
This is where the connection with water becomes interesting. These small bodies may contain ice and other volatile compounds. As they approach the star, they could release some of this material into the inner regions.
The detected sodium does not, however, demonstrate that these objects brought the water observed by James Webb. It mainly shows that the transport of material by small bodies is consistent with the observations and the system’s dynamics.
If this interpretation is confirmed, PDS 70 would become the youngest known system exhibiting exocomet activity. Longer-term monitoring could measure the acceleration of these signatures and better constrain the orbits. Searching for dusty tails would provide a much more direct clue.