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Updated January 6, 2024

Obliquities of the planets

Obliquity of the planets

Image: All the movements of the planets are irregular and vary over time under the gravitational influences of all the objects in the solar system. The axes of rotation of the planets are also modified.
The end of the axis of rotation of a planet slowly describes a circle in a horizontal plane, like a top, this is the movement of precession.
In the case of the Earth, the axis of rotation is inclined by 23° 26' 10.210" relative to its orbital plane. The tilt of the Earth's axis loses 0.4686" per year, or approximately 8' per millennium.
Credit: NASA Planetary Fact Sheet

Why do the planets have different obliquities?

The obliquity of the planets, or inclination, refers to the angle between the axis of rotation of a planet on itself and its orbital plane around its star. Obliquity is therefore the inclination of the planet's equator in relation to its orbital plane called the ecliptic.

All planets are subject to the vagaries of the gravitational forces of objects in the solar system. The permanent deformation of space-time, that is to say its curvature, creates the chaotic flows of gravity.
The axis of rotation of the planets, sensitive to all these disturbances, varies over time, very slowly and in a different way for each planet.

The planets have orbits which are all located approximately in the same orbital plane, but their obliquity is different today. The variation of the axis of rotation of the planets is a complex phenomenon which is caused by several factors (gravitational interactions, tidal forces, collisions with cosmic objects, redistribution of mass inside the planet, etc.). Thus, this axis of rotation is never perpendicular to the orbital plane of the planet. Indeed, the main reason is due to the distribution of mass inside a planet which is never a perfect sphere and of equal density, that is to say with an equal distribution of mass. This is what forces it to spin like a top.

Obliquities of the planets of the solar system

In the protoplanetary disk, planets should form with obliquities close to zero. However, the planets of the solar system today present a wide variety of obliquities which now vary between minimums and maximums. The case of Mercury is special because the strong tidal dissipation due to the proximity of the Sun keeps Mercury's obliquity tightly at a value close to zero.
Large-scale collisions in the solar system also probably contributed to the tilt of most obliquities. In addition, the overlapping resonances produce a large number of chaos which have a great influence on the rotation axis of the planets. In astronomy, orbital resonance is observed when the ratio of orbital periods are in relation to simple ratios (1:2, 2:3, 3:4, etc.). This affects orbits and movements in the planetary system.
In reality, the axes of most planets are still tilting, and today we are only observing a transitional stage of this evolution.

Mercury (≈ 0°): Due to its synchronous rotation, Mercury's rotation axis does not undergo significant variation in obliquity over time. Due to the interaction between tidal forces and Mercury's eccentric orbit, an orbital resonance develops. At a certain point in the orbit, Mercury's orbital period comes into 3:2 resonance with its rotation period, meaning that Mercury makes 3 rotations on its axis for every 2 revolutions around the Sun. When orbital resonance occurs, the combined effect of tidal braking and orbital resonance locks Mercury's rotation axis in a stable position relative to the Sun.

Venus (≈ 177°): The rotation of Venus is retrograde, which means that its axis of rotation is tilted in the opposite direction to most other planets in the solar system. Thus, the obliquity of Venus is approximately 177.36° relative to the normal of its orbit around the Sun. Compared to the vertical, its obliquity is relatively low. We can say that its axis is tilted by approximately 2.64 degrees. Its axis of rotation does not undergo significant variations over time.

Earth (≈ 23.5°): The obliquity of the Earth varies between ≈ 22.1° and ≈ 24.5° over a period of approximately 25,765 years. During this period called "precession of the equinoxes", the Earth's axis makes a complete rotation, passing through different inclinations. This tilt is responsible for the existence of seasons on Earth. The Earth's axis of rotation may have been stabilized by the capture of the Moon.

Mars (≈ 25.19°): Over a period of approximately 1.2 million years, the obliquity of Mars varies between 14.9 and 35.5°. However, too far from the Sun and lacking a large satellite, Mars would have a chaotic obliquity, ranging from 0° to 60°.

Jupiter (≈ 3.1°): The obliquity of Jupiter can vary from 3° to 30° over a period of 5 million years. Despite its particularly low current value, Jupiter's obliquity is currently increasing, due to the migration of Galilean satellites. The Galilean satellites (Io, Europa, Ganymede and Callisto) are currently moving away from Jupiter at a rate of about 1 cm per year. Models show that Jupiter's obliquity will gradually increase over the next few million years. Even though the force of the satellites is weak, it acts consistently over a long period of time. It will take approximately 2.7 million years for Jupiter's axis of rotation to advance by one degree because of the Galilean satellites.
However, although this gravitational influence is present, it is generally not sufficient to cause drastic changes in Jupiter's rotation axis. Other factors, such as the distribution of atmospheric mass or the resonance with the orbit of Uranus, play a role in the rotation dynamics of the planet. Air currents, storms, and weather disturbances can cause vertical and horizontal movements of atmospheric mass, thereby redistributing mass at different altitudes and latitudes.

Saturn (≈ 26.7°): The great obliquity of Saturn allows observers located on Earth to observe its magnificent Rings.
Saturn's satellites, and more particularly Titan (larger than Mercury), are partly responsible for the current obliquity of the giant planet. According to work published on January 18, 2021 by scientists from CNRS, Sorbonne University and the University of Pisa, the planet tilts more and more as its satellites move away.

Uranus (≈ 97.8°): Unlike all the other planets in the Solar System, Uranus is very strongly tilted on its axis, almost parallel to its orbital plane. It is so inclined that it gives the impression of rolling in its orbit alternately exposing its north pole, then its south pole to the Sun. Which means that its north and south poles are located where the other planets have their equators.
Two hypotheses have been put forward to explain this “extreme obliquity”.
Uranus may have suffered one or more violent collisions with primordial protoplanets early in the history of the solar system. Alternatively, gravitational disturbances, or resonances caused by Jupiter and Saturn, could have affected the tilt of Uranus over time. But it is likely that a combination of several factors contributed to Uranus' current tilt.

Neptune (≈ 28.3°): The axial inclination of Neptune is today approximately similar to the inclinations of the Earth (≈ 23°), of Mars (≈ 25°) and Saturn (≈ 26°).
Jupiter and Saturn crossed the 1:2 orbital resonance about 4.5 billion years ago, during the migration of the giant planets. This passage would have caused significant gravitational disturbances in the Solar System.

What you must remember

The obliquity of the planets observed today is the complex result, over time, of gravitational interactions, collisions between primordial protoplanets, disturbances linked to resonances and other unknown dynamic phenomena.
We can remember that all the planets must have formed with an obliquity close to zero.
But very early on, the planets' obliquities may have undergone large-scale chaotic behavior. And over time, a combination of dynamic factors in the solar system have influenced the obliquities of the planets.
Mercury and Venus, close to the Sun, have been stabilized by dissipative effects, the Earth may have been stabilized by the capture of the Moon, and Mars is still in a large chaotic zone. Concerning the obliquities of the outer planets (Jupiter, Saturn, Uranus and Neptune), the simulations consider them as primordial, that is to say with approximately the same inclination that they had originally after the great collisions with protoplanets.

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