A ‘Goldilocks’ star reveals hidden step in how water gets to planets like Earth

The beginnings and ends of the water trail are easy to see, but the middle has remained a mystery

Without water, life on Earth could not exist as it does today. Understanding the history of water in the universe is critical to understanding how planets like Earth come to be.

Astronomers typically refer to the journey water takes from its formation as individual molecules in space to its resting place on the surfaces of planets as “the water trail.” The trail starts in the interstellar medium with hydrogen and oxygen gas and ends with oceans and ice caps on planets, with icy moons orbiting gas giants and icy comets and asteroids that orbit stars. The beginnings and ends of this trail are easy to see, but the middle has remained a mystery.

I am an astronomer who studies the formation of stars and planets using observations from radio and infrared telescopes. In a new paper, my colleagues and I describe the first measurements ever made of this previously hidden middle part of the water trail and what these findings mean for the water found on planets like Earth.

The formation of stars and planets is intertwined. The so-called “emptiness of space” – or the interstellar medium – in fact contains large amounts of gaseous hydrogen, smaller amounts of other gasses and grains of dust. Due to gravity, some pockets of the interstellar medium will become more dense as particles attract each other and form clouds. As the density of these clouds increases, atoms begin to collide more frequently and form larger molecules, including water that forms on dust grains and coats the dust in ice.

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Stars begin to form when parts of the collapsing cloud reach a certain density and heat up enough to start fusing hydrogen atoms together. Since only a small fraction of the gas initially collapses into the newborn protostar, the rest of the gas and dust forms a flattened disk of material circling around the spinning, newborn star. Astronomers call this a proto-planetary disk.

As icy dust particles collide with each other inside a proto-planetary disk, they begin to clump together. The process continues and eventually forms the familiar objects of space like asteroids, comets, rocky planets like Earth and gas giants like Jupiter or Saturn.