Free Astronomy Magazine May-June 2023

22 MAY-JUNE 2023 ASTRO PUBLISHING to be tricky. “Most of the water in planet-forming discs is frozen out as ice, so it’s usually hidden from our view,” says co-author Margot Leemker, a PhD student at Leiden Observatory in the Netherlands. Gaseous water can be detected thanks to the radiation emitted by molecules as they spin and vibrate, but this is more complicated when the water is frozen, where the mo- tion of molecules is more con- strained. Gaseous water can be found towards the centre of the discs, close to the star, where it’s warmer. However, these close-in re- gions are hidden by the dust disc it- self, and are also too small to be imaged with our telescopes. Fortunately, the V883 Orionis disc was shown in a recent study to be unusually hot. A dramatic outburst of energy from the star heats the disc, “up to a temperature where water is no longer in the form of ice, but gas, enabling us to detect it,” says Tobin. The team used ALMA, an array of radio telescopes in northern Chile, to observe the gaseous water in V883 Orionis. Thanks to its sensitivity and ability to discern small details they were able to both detect the water and determine its composi- tion, as well as map its distribution within the disc. From the observa- tions, they found this disc contains at least 1200 times the amount of water in all Earth’s oceans. In the future, they hope to use ESO’s upcoming Extremely Large Tele- scope and its first-generation instru- ment METIS. This mid-infrared instrument will be able to resolve the gas-phase of water in these types of discs, strengthening the link of water’s path all the way from star-forming clouds to solar systems. “This will give us a much more com- plete view of the ice and gas in planet-forming discs,” concludes Leemker. A LMA images of the disc around the star V883 Orionis, showing the spatial distribution of water (left, orange), dust (middle, green) and carbon monox- ide (blue, right). Because water freezes out at higher temperatures than carbon monoxide, it can only be detected in gaseous form closer to the star. The appar- ent gap in the the water and carbon monoxide images is actually due to the bright emission of the dust, which attenuates the emission of the gas. [ALMA (ESO/NAOJ/NRAO), J. Tobin, B. Saxton (NRAO/AUI/NSF)] heavy water form under different conditions, their ratio can be used to trace when and where the water was formed. For instance, this ratio in some Solar System comets has been shown to be similar to that in water on Earth, suggesting that comets might have delivered water to Earth. The journey of water from clouds to young stars, and then later from comets to planets has previously been observed, but until now the link between the young stars and comets was missing. “V883 Orionis is the missing link in this case,” says Tobin. “The composition of the water in the disc is very similar to that of comets in our own Solar Sys- tem. This is confirmation of the idea that the water in planetary systems formed billions of years ago, before the Sun, in interstellar space, and has been inherited by both comets and Earth, relatively unchanged.” But observing the water turned out !