Formation of the Earth by accretion:
Initial solar nebula consists of mixtures of grains (rock) and ices. The initial ratio is about 90% ices and 10% grains
The sun is on so there is a temperature gradient in this mixture:
In the inner part of the solar system, only things which exist as a solid at high temperature are available (so how come there is so much water on the earth? -- answer later)
So in the inner part of the solar system you can only make a rocky planet via acretion of grains.
In the outer part of the solar system, ices can exist so you can make larger planets out of the more abundant ices
Jupiter (mostly H and He) formed in a manner similar to the Sun, that is not by accretion.
Jupiter has a large mass and perturbs orbits of objects near them. There were lots of these objects scattered between Jupiter and Pluto.
Jupiter redirected some of this cometary material into the inner solar system and most of the earth's water was delivered through comet bombardment (therefore would we be here without Jupiter?)
Steps in the accretion process:
How did the Earth Get to Look Like This?
Two significant questions to now ask:
1. Where did the water on the earth come from?
2. This nearby object has a density of 3.3 grams per cubic centimeter. Is this a problem?
The Origin of the Moon:
Until recently, this was not understood. The moon has long been an anamoly because its mass compared to the earth is 1/80 and there is very large for a planetary satellite
Possibilities for Lunar Origin:
The above 3 scenarios for lunar formation are all physically implausible so why is the moon there?
Let's imaging the following sequence:
Outgassing Early Formation of the Earth's Atmosphere
Present day composition of volcanoe effluents:
It is likely that there was NOT enough water released via outgassing to account for the present day oceans
Most of the water was likely delivered to the earth after it formed via collisions with left over planetisimals and cometisimals. The influence of Jupiter on the orbits of leftover cometisimals may well be key.
Tidal forces from a newly formed moon may also be key in promoting first stages of development of life as well.
Hence the earth may need both Jupiter and the Moon if it is to ask a question sometime later.
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