Five Reasons Why Planet Earth Should Be Grateful for Planet X

The Moon is very odd: it's larger than Mercury, and it's so close to Earth that many astronomers have characterized the Moon and Earth as a two-planet system. The Moon also exerts tremendous influence on its planet, far more than is normal for a satellite (see below). 

As scientists have studied the Moon more closely, its gravitational tug and the rocks brought back by Apollo have yielded two other important clues: the Moon's iron core is unusually tiny for its size, and its rocks are surprisingly identical to the Earth's crust.  Meteors from anywhere else in the solar system have different chemical signatures. It appears as if the Moon "budded off" from Earth like a lobe of clay pinched off from a larger piece, except for one thing: it's missing the heavier elements of the Earth's interior.

Meanwhile, Earth itself is odd. It has an unusually large metal core for its size. In fact, its nickel-iron core is the size of the planet Mars.

Something bizarre happened in our neighborhood to cause all this. Scientists and physicists have run the tests, and can't come up with any physical way for the Moon to "pinch off" through the Earth's own motions... it doesn't work. 

Simulations show that the only physical way to get our Earth and Moon is if a Mars-sized planet struck Earth at an angle, peeling off a large chunk of crust and throwing it into space. That formed the moon. The remains of Theia sank into the Earth's depths, doubling its metallic core. 

This is the Giant Impact theory, the most accepted theory in science to explain the Moon's formation. We have five reasons to be thankful for that mega-impact.

1. The Moon's Gravitational Pull acts like the outrigger on a canoe, stabilizing the Earth so it doesn't tip and wobble. That gives us seasons and prevents climate chaos. If the Earth wobbled like Mars, we'd have very frequent ice ages and unimaginable hot spells. If the Earth tipped over like Uranus, it would have days and nights that lasted for months and months, cooking one end of the planet and freezing the other.
2. The Moon's Tides were a powerful force early in Earth's history, when the Moon was closer and its gravitational pull much stronger. Tides raced inland for miles and miles, mixing the water with nutrients from the land, creating marshes and tidewater estuaries where stromatolites could grow and create the Earth's first oxygen, a necessary ingredient for most life.
3. Earth's Magnetic Field is extremely strong for a planet its size, thanks to the doubled metallic core. It protects us from solar flares. It also protects Earth's atmosphere. Mars, lacking a strong magnetic field, has had most of its atmosphere stripped away by the solar wind. 
4. Earth's Atmosphere has not only been protected by its magnetic field, but by the planet's gravity, which holds it in place. Yes, even air is subject to gravity! The density of the Earth is due to Theia's mass deep within. Air not only gives us something to breathe, but shields us from meteorites -- most burn up -- and blocks solar radiation. Mars' surface is sterilized, since it has no such protection. Not only that, but atmospheric pressure is what allows water to exists on the surface in liquid form. On Mars, liquid water either freezes or boils away.
5. Earth's Plate Tectonics are another unique artifact of the extra heat and mass in the Earth's core. This is the process by which Earth's thin rocky shell replenishes itself, rising from volcanic ridges like the Mid-Atlantic Ridge and plunging back into the core at subduction zones like those off the coast of Indonesia and Japan. This process helped build continents; otherwise we'd have a water world dotted with islands. Longterm, plate tectonics have helped keep Earth from freezing, by continually releasing greenhouse gasses to trap the sun's heat.