In one of the Milky Way’s most interesting planetary systems, the presence of water vapor was recently discovered, and not just anywhere, but in the extended disk of dust and gas surrounding the star PDS 70. This discovery, made by the James Webb Space Telescope, is significant because it provides insight into how the Earth formed and where water came from. It also provides interesting insights into the formation of other potentially habitable worlds in our Galaxy.
Astronomer Giulia Perotti of the Max Planck Institute for Astronomy in Germany explains, “We have seen water in other disks, but not this close and not in a system where planet formation is currently taking place. Before Webb, we were not able to make similar measurements.”
The presence of water is essential for the existence of life, but scientists are still unsure how the Earth got water. One popular theory suggests that water was brought by water-bearing asteroids that collided with the young planet. However, recent evidence from the study of meteorites and lunar dust suggests that water was already present in the solar system during Earth’s formation.
Understanding the origin of water on Earth-like worlds is necessary to assess the potential habitability of other planets in the Galaxy. The detection of water vapor in PDS 70 also indicates that water may be present early in the formation of planets.
“Now we may have found evidence that water may also serve as one of the initial ingredients of rocky planets and be available at their birth,” says Perotti.
PDS 70 is a star and planetary system located about 370 light-years away from Earth. It is surrounded by a disk of matter in which exoplanets are currently forming. PDS 70 is about 5.4 million years old, which is relatively old for a protoplanetary disk. Scientists have already detected two gas giants orbiting the star, as well as a moon-forming disk and a co-orbiting dust cloud using direct images.
Due to the intriguing nature of the system, Perotti and her team used the James Webb Space Telescope to study the molecular composition of the disk on a regional scale. It was originally thought that the inner part of the disk, where rocky planets form, would be relatively dry due to intense ultraviolet irradiation and stellar winds. However, the spectrum obtained by JWST showed a perfect match with water vapor.
The origin of the water vapor remains unclear, but two possibilities are possible. It could have formed in situ with the star itself from a water-rich nebula or accumulated in the outer regions of the disk. The abundance of dust in the inner disk could serve as a shield protecting the water vapor from the influence of the star.
Perotti suggests that the truth likely lies in a combination of these options. However, it is a future task to determine which mechanism plays a crucial role in maintaining the water reservoir of the PDS 70 disk.
A study on this discovery has been published in the journal Nature.