Because it's getting hit and the water just flies up into the universe and stuff? What would happen to it there?
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It depends on many factors: The speed of the incoming planet, the angle of the incoming planet, the size of the planet, the surface of it and where it hits.
If it manages to hit an ocean, say the Pacific, (Actually there is a higher chance of hitting water since Earth's surface is about 70% water) the immediately-adjacent water would be instantly vapourised into steam from the heat (and possibly pressure) of the collision between the two planets. Hundreds of kilometers away, the some of the water may be flung off into the air, along with rocky debris from the planets. The most of the water should return back to Earth because of Earth's gravitational field.
If the planet is extremely large, maybe 10 times larger (assume it is a rocky planet), its strong gravity may be able to pull the water forward before the Earth, but the resulting planetary collision will still vapourise most of the water.
If the planet is a gas giant, it would have no hard surface, because gas giants are mostly comprised of gas. Gas giants are usually huge -- more than 1000 Earths can be fitted inside the gas giant Jupiter. Hence if the planet crashed into Earth (or rather Earth would crash into the planet), Earth would simply sink into the gassy atmosphere. The water would go along with Earth.
If it manages to hit an ocean, say the Pacific, (Actually there is a higher chance of hitting water since Earth's surface is about 70% water) the immediately-adjacent water would be instantly vapourised into steam from the heat (and possibly pressure) of the collision between the two planets. Hundreds of kilometers away, the some of the water may be flung off into the air, along with rocky debris from the planets. The most of the water should return back to Earth because of Earth's gravitational field.
If the planet is extremely large, maybe 10 times larger (assume it is a rocky planet), its strong gravity may be able to pull the water forward before the Earth, but the resulting planetary collision will still vapourise most of the water.
If the planet is a gas giant, it would have no hard surface, because gas giants are mostly comprised of gas. Gas giants are usually huge -- more than 1000 Earths can be fitted inside the gas giant Jupiter. Hence if the planet crashed into Earth (or rather Earth would crash into the planet), Earth would simply sink into the gassy atmosphere. The water would go along with Earth.
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The giant asteroid that ended the dinosaurs was just a large rock the size of a mountain. Water from the Yucatan Peninsular would have escaped into space. The whole planet became like hell setting vegetation a blaze and then going into a winter that lasted for a very long time. With the result the dinosaurs starved to death. Earlier on in the earth's history the earth indeed had a collision with and earth size object (Venus?) from which the moon was created. Such an impact would destroy the earth today vaporizing the oceans and melting the surface of the earth truly an Armageddon