Astronomers detect exocomet evidence in young planetary system PDS 70

The research team analyzed observations from 2018 and identified sodium gas moving at several kilometres per second relative to the star. The gas was found to appear and disappear over the course of several nights, a pattern consistent with the behaviour expected when comets pass in front of a star. As a comet approaches its host star, it heats up and the ice on its surface undergoes sublimation — converting directly from solid to gas — leaving a measurable imprint in the star's light. The researchers concluded that these variations in sodium gas may indicate comets passing through the inner regions of the PDS 70 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 the same way as in the early Solar System," said Aline Novais, an astronomy researcher at Lund University. Novais also noted that this is the first time evidence of exocomets has been observed orbiting a star that is relatively cool and similar to the Sun, and that PDS 70 is the youngest system in which exocomet activity has been proposed.

The researchers additionally conducted orbital simulations, the results of which show that objects located far out in the planetary system can be affected by the gravitational pull of the gas giants and deflected toward the star. Because comets form in the cold outer regions of planetary systems, where water may exist as ice, this inward channelling could allow them to carry water and other volatile substances into the zone where planets are developing. "It is reminiscent of a possible process in the early Solar System, in which comets may have helped to deliver water to the young Earth," said Alexandra Stockwell Murphy, an astronomer at Lund University.

The origin of Earth's water remains an open scientific question, with water-rich asteroids and comets identified as two possible sources. The PDS 70 system offers researchers the opportunity to observe a comparable process while a planetary system is still in its early stages of formation. Jens Hoeijmakers, an astronomy researcher at Lund University, noted that when the Extremely Large Telescope — currently under construction in Chile — becomes operational in the coming years, it will enable further investigation into whether additional planets exist in the system, and provide a clearer picture of how water and other planetary building blocks are transported within young planetary systems.