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We know that a planet is formed from the nebula of gas and dust. Gravity pulls the particles together in a core. So we all know that the gas cloud is a cloud of hydrogen.

But how did water form? From where did oxygen come? How did it combine to make $\text{H}_2\text{O}$?

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    $\begingroup$ Related: Origin of water on Earth $\endgroup$ – Klaus Warzecha Mar 13 '17 at 15:31
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    $\begingroup$ Old dying stars are able to fuse elements up to iron in the stellar core. The "gas and dust" you refer to contains oxygen. Hydrogen is very plentiful. Mix and combine. $\endgroup$ – Zhe Mar 13 '17 at 15:49
  • $\begingroup$ This might get you started: space.com/16943-supernova-explosion-solar-system-formation.html Elements like Oxygen, Silicon, Iron are all formed in a supernova. As I understand it, chemistry happens too, on a limited scale, within the nebula, so you have basic building blocks. Silicates like SO2, Ices/gases like H20 and CO2, but I'll let someone smarter than me give a more complete answer. Solarsystems don't form out of hydrogen easily because it's too light, it takes a certain amount of heavier elements. $\endgroup$ – userLTK Mar 14 '17 at 0:24
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    $\begingroup$ @userLTK You're wrong about the first part. Oxygen, Silicon, Iron, are all formed during a star's normal lifecycle, not a supernova. Any element iron or below can be formed during a star's normal lifecycle. A supernova causes the formation of any element Uranium or below (so basically all of them). $\endgroup$ – Aaron Franke Mar 14 '17 at 1:53
  • $\begingroup$ @AaronFranke You're right about that, though those elements spread across the galaxy and became part of new solar system formation from the supernova explosion. My bad. $\endgroup$ – userLTK Mar 14 '17 at 2:22
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Stars, especially larger stars, are hot enough at their core to produce significant amounts of heavier elements: Carbon, Nitrogen and Oxygen. When the stars die, some of these elements are returned to the interstellar medium.

The nebula from which stars form contains not only Hydrogen, but also all the other elements found on Earth. The nebula contains the Carbon that is the basis of life, the Nitrogen that forms the atmosphere, the Silicon that makes rocks, and the oxygen that reacts with hydrogen to form water.

Most of the water on Earth was actually brought to the Earth on asteroids after the Earth had formed, see How did water get on Earth

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  • $\begingroup$ Just to add something here. The reason it is thought that water was brought to Earth rather than always being here is because ice would have condensed considerably farther out than the Earth's orbit in our protoplanetary disk. (past the "snowline" 150 - 170K) ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks . It is thought to have been brought to earth by asteroids rather than comets because of the D/H ratios shown in the chart referenced by James above. $\endgroup$ – Jack R. Woods Mar 21 '17 at 3:56
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Usually, when a star dies, it will usually form a planetary nebula or supernova and most of this matter will reach the interstellar medium.

When it collapses due to some disturbance, a protoplanetary disk forms, it will cool off eventually. Once it is low enough temperature to form compounds, usually molecular hydrogen (H2) will form first. Here is something that may help you: No, H2O cannot exist in stars, but H and O separately can. Hydrogen is the basic building material of the universe, created in the Big Bang. Oxygen is created by nuclear reactions in stars. If you put H and O together in the cold of space, you get H2O. There are enormous amounts of water in space. In fact, nearly all of the oxygen in space is in the form of water or carbon monoxide. Similarly, most the carbon and nitrogen in space are also in their most hydrogenated forms: methane (CH4) and ammonia (NH3). (Source) Since water is H2O, it is likely to form because nebulae contain so much hydrogen and some oxygen. The Earth was most likely hit by comets and asteroids that contained water, and when our planet cooled off, we got our oceans.

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