June 2022
The Backyard Astronomer
Adam England

Planetary Collisions and Seasons

The early solar system was a violent place. We know this courtesy of modern telescopes and models generated by today’s supercomputers, allowing us to peer at clouds of gas around distant stars, observe their planets forming, and hypothesize how our own system could have formed under similar conditions.

A young star usually forms in a cloud of tenuous gas, slowly pushing inward and becoming more and more dense, with the star at the center hosting the majority of the mass — and therefore gravity — and the outer gas coalescing into rocks, planets, moons, asteroids and comets. Over time, these objects run into each other, as we observed with objects hitting Jupiter over the past few decades, as well as large craters on the surface of Earth and elsewhere around the system, preserved as monuments to these cataclysmic events.

It is generally believed that the sun absorbed enough material to initiate nuclear fusion around 4.6 billion years ago. Over the following 100 million years the disk of gas around it formed all the planets we now know, as well as many others long since forgotten.

One of those protoplanets, about the size of Mars, was Theia, which formed much farther out in the solar system with the comets, composed of much more water ice than the inner planets. Its orbit was erratic, or possibly disturbed by the passing of a wayward star, and after countless close encounters, Earth and Theia collided in what must have been an awesome event.

The violent impact knocked Earth sideways, merged the heavy planetary cores into a much larger core and mantle and ejected debris for hundreds of thousands of miles into orbit. The slow process of cooling formed our continents and oceans, even our moon.

Many other bodies experienced this same type of event, which we can see in the different tilts and rotations of every planet in our system. The largest, Jupiter, was able to better withstand impacts than the smaller, rocky planets, and has a tilt of only 3°, while Venus was somehow knocked completely upside down to a 177° tilt, rotating in the direction opposite that of the other planets. Uranus lies at a 98° tilt relative to the sun, essentially orbiting on its side, while Earth, Mars, Saturn and Neptune all have relatively stable seasons due to their axial tilts, Earth at 23.5°, Mars at 25°, Saturn at 27°, and Neptune at 30°.

Earth’s 23.5° tilt leans different parts of the planet toward or away from the sun at different times of year. As we orbit the sun, the northern hemisphere gets closer and more direct sunlight in spring, culminating on June 21 this year, which we celebrate as the summer solstice. The sun will appear to stand still in the sky for three days as it reaches its northernmost view of Earth, and then will begin to move southward again for six months. This change in the seasons affects all planetary weather patterns and the evolutionary habits of plants and animals alike.

And it’s all thanks to Theia.

If you would like to learn more about the sky, telescopes, or socialize with other amateur astronomers, visit us at prescottastronomyclub.org or Facebook @PrescottAstronomyClub to find the next star party, Star Talk, or event.

Adam England is the owner of Manzanita Financial and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow his rants and exploits on Twitter @AZSalesman or at Facebook.com/insuredbyadam.