How Did Planets Get Their Axial Tilts?

There are seasons on the Earth because Earth’s rotation is tilted by 23 degrees relative to the Sun. But the situation for other planets is different. Some planets have significant axial tilts (causing more extreme seasons), some spin retrograde or on their side, while some planets do not have significant axial tilts at all. What is causing this variation in the axial tilts (a.k.a. obliquities) of the planets? Let’s find out.

To answer this variation, we need to ask ourselves, “Why do planets rotate in the first place?” Well, this comes down to the basic planetary formation process. Before a star forms, large amounts of material collapse to one spot in a nebula due to gravitational forces. As material starts accreting around the protostar and orbiting it in all directions, collisions between particles dampen the orbits and flatten them into a disk, rotating in a specific direction.

As the gases in the system get pulled into the center to form a star, gravitational collapses also occur in other places of the protoplanetary disk. These disks of materials are the precursors of planets, and they are spinning in the same direction as their orbital direction around the star. And as the disk materials coalesce to form a planet, it retains the direction of rotation of its precursor disk. In ideal cases, this would mean that all planets orbit on the same plane and rotate in the same direction. But it’s not the case as we see today. Why does this happen?

One of the ways that planets get tilted in different ways is through collisions with another planet. That might sound absurd and unlikely at the present stage, but it could be feasible in the early Solar System. As the leftover materials coalesce, multiple planets are formed, which could often undergo an instability stage where they are pushed into intersecting orbits. After some time, the planets could collide, generating a huge amount of mess and consequences that are almost inexplicable without these planetary-scale collisions.

For example, the spins of Venus and Uranus are strange. Venus is rotating retrograde and slowly, and Uranus is rotating on its side. While we don’t know exactly how that happened, one of the leading theories is that something collided with these planets at an early stage of the Solar System. The pure force of the impact generates enough disruption to affect its obliquity and direction of spin. After the impact, the planet could fragment, resulting in a new wave of satellite formation and accretion. Resulting tidal forces from the new moons could also act to change its rotation speed and axis.

Another prime example of such a planetary impact is the formation of the Moon. Scientists believed that the Moon formed from an impact of a Mars-sized planet with Earth. That impact might have also given rise to Earth’s axial tilt, and the Moon has even played a role in stabilizing Earth’s obliquity. The obliquity of the Earth isn’t always 23.5 degrees (as it is now), because it oscillates between about 22.1 to 24.5 degrees. Without the Moon, this variation could be on the order of tens of degrees. That change would destabilize our climate in relatively short periods of time.

Even though the early Solar System could be violent, these impact events might be too rare to explain the whole picture of planet obliquities. That’s where the complicated relationships between planets’ orbits kick in. Gravitational pulls from other planets can subtly and slowly affect the obliquity of the Earth, among other factors. In fact, the reason that the Earth’s obliquity oscillates is the interactions with the other planets in the Solar System. That oscillation would have been so significant, but the Moon is close enough to Earth that its gravitational pull becomes the dominant factor in the oscillation, playing a stabilizing role.

And there’s more. During the formation of planetary systems, planets can undergo instabilities that change their orbit eccentricities and inclinations in many different ways (such as in the Kozai-Lidov mechanism), tilting orbits in many different ways. Remember that the obliquity is actually the angle between its rotational equator and the orbital plane of the planet. And so, if the rotation does not change accordingly, these orbital tilts can cause axial tilts to change as well. It could also happen the other way around — a resonance with another planet could shift its axis gradually such that it becomes substantially tilted compared to its orbital plane.

Such instabilities and interactions could explain Saturn’s axial tilt, for example. To explain Saturn’s 26.7-degree obliquity, it has been proposed that Saturn’s axial tilt was actually in resonance with that of Neptune. And that kind of interaction might even occur in relatively tame planetary systems. “Low-level dynamical excitations”, as the author of a 2023 study states, could explain the relatively high obliquity of the exoplanet TOI-2022 b.

In this article, we’ve discussed the two main reasons why planets get different axial tilts. One involves collisions with other planets, while the other involves slower perturbations. If a planet didn’t get affected by these two factors, its axial tilt would be small, and it would not have distinct seasons. If a planet gets affected severely, like in the case of Venus and Uranus, weird axial tilts might result. And for Earth, whose impact with a Mars-sized planet likely formed the Moon and caused Earth’s axial tilt, this means that Earth gets moderate and stable seasons that we all enjoy today.

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