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.
Star and planet formation is an intertwined process that starts with a cloud of molecules in space.
Bill Saxton, NRAO/AUI/NSF, CC BY
How planets are formed
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.
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.
Gas and dust can condense into clouds, like the Taurus Molecular Cloud, where collisions between hydrogen and oxygen can form water.
ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2, CC BY
Two theories for the source of water
There are two potential pathways that water in our solar system could have taken. The first, called chemical inheritance, is when the water molecules originally formed in the interstellar medium are delivered to proto-planetary disks and all the bodies they create without going through any changes.
The second theory is called chemical…
