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How does water travel through our solar system?

By using the Atacama Large Millimeter/submillimeter Array (ALMA) – an array of radio telescopes in northern Chile – an international team of astronomers has detected gaseous water in the planet-forming disc around the star V883 Orionis, located about 1,300 light-years away from Earth. This water carries a chemical signature which helps scientists explain the journey of water from star-forming gas clouds to planets, supporting the hypothesis that water on Earth is even older than our Sun.

Scientists have long known that, when a cloud of gas and dust collapses, it forms a star at its center, while material from the cloud also forms a disc around the star. Over a few million of years, the matter in the disc clumps together to form asteroids, comets, and eventually planets. Although the journey of water from clouds to young stars – and later on from comets to planets – has previously been observed, the link between the young stars and comets remained a mystery.

“V883 Orionis is the missing link in this case. The composition of the water in the disc is very similar to that of comets in our own Solar System. 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,” explained study lead author John Tobin, an astronomer at the National Radio Astronomy Observatory.

Although water usually consists of two hydrogen atoms linked to one oxygen atom, the water examined in this study was a slightly heavier version, in which one hydrogen atom is replaced with deuterium (a heavy isotope of hydrogen). Since simple and heavy water form under different conditions, their ratio can be used to find out when and where the water was formed. For example, in some comets from our Solar System, this ratio is similar to that in water on Earth, suggesting that comets may have delivered water on our planet.

However, since most of the water in planet-forming discs is frozen, observing it from Earth is challenging. While gaseous water can be easily detected due to the radiation emitted by molecules as they spin and vibrate, this is more complicated when the water is frozen and the movement of molecules is more constrained. Although gaseous water can usually be found toward the center of the discs, closer to the star where it is warmer, these regions are often hidden by the dust disc itself, and they are also too small to be reliably observed with telescopes.

Fortunately, since the V883 Orionis disc is unusually hot, it contains vast amounts of gaseous water – at least 1,200 times the amount of water in all the Earth’s oceans – that scientists managed to detect, examine its composition, and map its distribution within the disc. In future studies, by employing even more performant technology, the researchers hope to provide a clearer picture of how water travels all the way from star-forming clouds to solar systems.

The study is published in the journal Nature.

By Andrei Ionescu, Staff Writer

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