The planetary system PDS 70 is just over five million years old and lies around 370 light-years from Earth. The system contains at least two gas giants and orbits a star that is slightly cooler than the Sun. Water vapour was already detected close to the star in 2023. Researchers in Lund can now show that variations in sodium gas observed in front of the star may be a sign of comets passing through the inner regions of the system.
“Our study suggests that comets may be responsible for transporting water to the inner parts of the planetary system, where planets can form, in much the same way as in the early Solar System,” says Aline Novais, astronomy researcher at Lund University.
The researchers analysed observations from 2018 and detected sodium gas moving at several kilometres per second relative to the star. The gas appears and disappears between different nights, a pattern consistent with what would be expected when comets pass in front of a star. As a comet approaches its star, it heats up and ice on its surface changes directly into gas through a process known as sublimation. The gas can then leave a measurable signature in the starlight.
“This is the first time we have seen evidence of exocomets around a relatively cool star, somewhat similar to our Sun. It is also the youngest system in which exocomet activity has been proposed,” says Aline Novais.
The researchers also simulated the comets’ orbits. The results show that objects located far out in the planetary system can be influenced by the gravity of the gas giants and be scattered inwards towards the star. This is particularly interesting because comets form in the cold outer regions of planetary systems, where water can exist as ice. If they are then sent inwards, they may transport water and other volatile substances to the region where planets are forming.
“This resembles a possible process in the early Solar System, where comets may have contributed to delivering water to the young Earth,” says Alexandra Stockwell Murphy, astronomer at Lund University.
The origin of Earth’s water remains an open question. Water-rich asteroids and comets are two possible sources. PDS 70 provides researchers with an opportunity to study a similar process while a planetary system is still in its early stages of development.
“When the Extremely Large Telescope, currently under construction in Chile, begins operations in the coming years, we will be able to determine whether there are additional planets in the system and gain an even clearer picture of how water and other planetary building blocks are transported,” says Jens Hoeijmakers, astronomy researcher at Lund University.
The study is published in the scientific journal Nature Communications:
”Potential sublimating exocomets around the young star PDS 70”
Facts: How comets form
Comets form from the same material as stars and planets. A star forms from a cloud of gas and dust, and as it forms it begins to rotate, creating a disc of material around it. Planets, moons, asteroids and comets then form from the remaining material in the disc.
The composition of this material generally depends on its distance from the star. Refractory substances are found closer to the star, where temperatures are higher, while volatile substances such as water are found farther away in colder regions. Comets, which typically form in the outer reaches of planetary systems, are therefore rich in volatile materials such as water. The fact that they can be placed on trajectories that bring them close to their star means that they may transport these materials to the inner regions of a planetary system.
Source: Aline Novais
Aline Novais profile in the Lund University Research Portal.