Oliver Sanderson of Waikuku School asks :-
How did all the water get here on earth?
William Tobin, an astrophysicist at the University of Canterbury, responded.
Water is made of hydrogen and oxygen atoms. The hydrogen was made in the Big Bang when the Universe began. The oxygen and lots of other chemical elements were made inside stars and later thrown out in gigantic explosions called supernovae.
The oxygen and hydrogen combined to form water in very cold, giant clouds of gas and dust in the space between stars. The water and lots of other complex molecules froze onto the dust. The dust grains stuck together into lumps which grew into comet heads 1 to 10 km in size. The cloud of gas and comets collapsed to form our Sun and planets.
Close to the newborn Sun a mighty wind blew away the easily-evaporated water leaving dry rocks which clumped together to form initially dry, rocky planets, which counting outward from the Sun are Mercury, Venus, Earth and Mars. Further out there was no evaporation of ice and the giant planets Jupiter, Saturn, Uranus and Neptune retained the full complement of material as well as lots of hydrogen and helium.
However quite a lot of comets remained in the young solar system and continued to bombard the planets, just like the Comet Shoemaker-Levy impact on Jupiter in 1994. These impacts brought water and other more complex materials essential for life.
The intense sunlight on Mercury rapidly broke up the water and the gases escaped. On Venus the enormous greenhouse effect raised the temperature and the water boiled off. Space-probe photographs of Mars show dry river beds which indicate that Mars used to have running water, but it has probably evaporated away because of that planet's low gravity. However Mars probably still has a lot of water locked up in permafrost.
This leaves Earth, where, because the temperatures and gravity are right, much of the water has been retained in the oceans and atmosphere. There also is probably as much water again locked up in the rocks of the Earth's crust. So that's why Earth is such a watery planet!