How does water get to planets? Let's know the theories behind the source of water!

Water is the basic constituent of all living things, and it is impossible to imagine life without water on the planet. To understand how planets like Earth came into existence, it is crucial to know the history of water.

As per astronomers, the journey of water begins from its formation as individual molecules. When these molecules present in space reach the resting place on the planets, the trail concludes. The trail begins in the interstellar medium, having hydrogen and oxygen gas molecules. It ends with ice layers and oceans on the Earth and other planets. What still remains undiscovered is what comes in the middle of this trail.

Formation of planets

Planets and stars have an intertwined journey. The interstellar medium of space has substantial amounts of gaseous hydrogen and grains of dust, along with tiny amounts of various other gases. Because of the gravity, some sections of this interstellar medium become dense as compared to others due to particles attracting each other and creating clouds. When the density of such clouds increases, atoms start colliding more frequently and creating even larger molecules. This includes water that gets formed on dust grains.

When do the stars get formed?

Stars are formed when some sections of the collapsing cloud touch a certain density, and then heat up so much that they start fusing atoms of hydrogen together. It is important to note that only a small fraction of the gas collapses in the beginning into the newborn proto-star. The remaining dust and gas create a flattened disk of a material that circles around the newborn star. In the world of astronomy, this is known as a proto-planetary disk.

When, inside a proto-planetary disk, icy dust particles collide with one another, they start clumping together. This process goes on and on until eventually, it creates familiar objects like comets, asteroids, and planets.

Theories behind the source of water

How water exists in our solar system is still a mystery for many. There are many theories behind the source of water, one of them being the chemical inheritance theory. This theory says that chemical inheritance takes place when the water molecules formed in the interstellar medium of space are delivered to proto-planetary disks along with the bodies thus formed sans any other changes.

Another theory that tells us something about the source of water is the chemical reset theory. When the proto-planetary disk is formed, heat is released. This heat and the water molecules released when a newborn star breaks down are behind the formation of water. 

How are such theories tested?

The ratio is drawn between the water that we use normally and semi-heavy water (a special kind of water). We all are aware of the fact that water is made of two components; two hydrogen atoms and one oxygen atom. However, it is interesting to note that semi-heavy water is formed of one hydrogen atom, one oxygen atom, and one deuterium atom. Deuterium is actually a heavy isotope of hydrogen that has an extra neutron in the nucleus. 

The ratio of this semi-heavy water to the water we generally have actually thrown light on the water trail. This ratio speaks volumes about the source of water.

Talking about the chemical model experiments, the findings of such experiments state that around 1,000 times more semi-heavy water will be formed in the cold interstellar medium. It is not formed in the states of a protoplanetary disk. 

By means of deriving the ratio between semi-heavy to normal water, astronomers gauge whether chemical inheritance or chemical reset took place to form water. 

How much water is there at the time of the formation of a planet? 

Chemical inheritance is perhaps what must have happened in the case of comets, as they have a ratio of semi-heavy to normal water that actually fits the theory. This means that in such a case, water has not gone through any substantial chemical change since the very beginning, since it was formed in space for the very first time.

Talking about our very own blue planet, i.e. the Earth, the ratio of semi-heavy to normal water lies somewhere between the inheritance and reset ratio. This makes it a puzzle to trace where the water actually came from.

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