02 Sep—Earth and Planets

The planets of our solar system began to form about 4,540 million years ago from dust and gas orbiting the newly formed Sun.

Solar System—2 September, 1 AM

As the dust particles collided with each other, they stuck together to form larger and larger clumps, like a snowball gathering snow.

Our young Sun produced a strong stellar wind (the solar wind), a steady stream of particles that it sent outward into interstellar space, the space between the stars.¹ The solar wind heated the gas and dust and pushed it into interstellar space. After 3–10 million years, the solar wind had pushed all of the excess gas and dust out of the region where the planets were taking shape, and the planets stopped growing by accretion by about 100,000 years after the formation of the Sun.

Now, the planets could only grow by merging together the surviving planetesimals which had grown as big as one twentieth the size of the present-day Earth.

The solar wind made the inner solar system near the Sun too hot for ice. It scoured the inner solar system of its volatile chemicals that would easily vaporize, like water and methane. Because of the intensity of the young Sun's solar wind, only rocky, metallic planetesimals with no atmospheres or water remained in the inner solar system where we find the rocky planets today.

As the volatile chemicals traveled farther from the Sun, they got colder. Eventually, they reached a point where it was cold enough to become ice again, the frost line. As they collected outside this line, a planetesimal scooped them up. The abundance of volatile materials at the frost line allowed that planetesimal to become the largest planet in our solar system, Jupiter.

Because of its large mass, Jupiter was capable of capturing hydrogen and helium, the lightest elements and the most easily blown away the solar wind. Hydrogen and helium currently make up at least 87% of the mass of Jupiter. Because of its large atmosphere, it is known as a gas giant.

Saturn, another gas giant, formed farther from the Sun, and therefore had to take second helpings of the volatile chemicals caught up in the solar wind. It consequently never reached the same size as Jupiter, but it still became large enough to capture hydrogen and helium and become a gas giant.

Farther out still, two icy gas giants formed: Neptune and Uranus.

In the inner solar system, 50-100 planetesimals orbited the Sun. Each was between the size of the Moon and of Mars. Because the solar wind had swept the smaller particles of dust out of the solar system, the only way for these planetesimals to grow was by colliding and merging together. Over the next 100 million years, these violent collisions lead to the four rocky planets we know today: Mercury, Venus, Earth, and Mars.

Earth—2 September, 1 AM

The young Earth grew as planetesimals collided with it and they merged together. Gravitational compression, radioactive decay, and meteorite bombardment heated the Earth enough to melt its iron and nickel. These dense liquid metals sunk toward the core and lighter rocky materials floated to the surface.

Because of the intense pressure at the center of the Earth, a solid iron and nickel core formed. This core was surrounded by iron and nickel that were still liquid, then a mantle of liquid iron and magnesium silicates, and finally the lighter potassium and sodium silicates that later formed the crust.

This flowing core of liquid metal created Earth's magnetic field. The Earth's magnetic field makes compasses work, creates the aurorae, and protects us from the solar wind. Without the magnetic field, the solar wind would have dispersed the Earth's atmosphere long ago.

Earth's first atmosphere contained hydrogen and helium from the cloud that formed the Sun and planets. The atmosphere also contained gaseous silica, the mineral that we find in sand. The Earth lost most of this first atmosphere because it was not large enough like the gas giants to retain the light elements hydrogen and helium through the force of its gravity.

The Earth's birth signals the beginning of the Hadean Eon, an informal division of the Geological Time Scale.

The Earth was a forbidding place during this early eon. The surface was still a molten sea of lava. The continents had yet to form.

We would have suffocated because the atmosphere contained none of the oxygen essential to our kind of life. As the Earth grew large enough to retain a stable atmosphere, it captured the gases escaping from the molten sea of lava. This first stable atmosphere mas mostly water vapor with some carbon dioxide and hydrogen sulfide² and small amounts of nitrogen, carbon monoxide, hydrogen, methane, and inert gases. This thick blanket of gas kept the Earth warm at a time when he Sun was not as bright as it is now.

Moon—2 September, 9 AM

The Moon formed about 13 million years after the solar system, 4,527 million years ago.

The leading hypothesis for how Earth's natural satellite formed involves a collision with a Mars-sized planet named Theia, which may have formed near the Earth. When it grew too large, Theia set out on a collision course with Earth.

Because Theia was so close to the Earth and couldn't pick up much momentum, the collision was a moderate one. Theia's iron core merged with Earth's, but some of each planet's rocky mantle was ejected into orbit around the Earth. This ejected material quickly formed into the Moon which began its life as an ocean of molten magma at least 500 km deep. The Moon slowly cooled, crystallized, and became solid.

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1. The Sun still has a stellar wind, but it's not as strong as it was back then.

2. Hydrogen sulfide is the gas that gives rotten eggs their distinctive odor and contributes to the unpleasant odor of flatulence