Water plays a central role in the formation of planets: as ice, it attaches itself to dust particles, thus helping the smaller particles to form small planets and eventually fully grown planets. Water is found in the interstellar medium, in comets and rocky asteroids, and of course on advanced planets like Earth. A team led by John Tobin of the National Radio Astronomy Observatory (NRAO) in the US has now made another important discovery: water molecules in protoplanetary disks around distant stars have the same isotopic ratio as those found in most comets in our solar system. “We can follow the trail of water through the universe,” says Tobin. “We already know the endpoints, that is, water on planets and comets. Now we also wanted to trace our starting points: “Hydrogen (H) is the most common chemical element in the universe. Water is also a common substance. There is a fair amount of hydrogen because the heavier isotope of deuterium (D) – the nucleus of a hydrogen atom (an electrically positively charged proton) contains an extra uncharged neutron. Water molecules can also contain deuterium: like heavy water HDO, it has a higher molecular weight, but is chemically identical to regular water (H2O). H2O emits radio waves at 203 GHz, HDO at 225 and 241 GHz.
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