Podcaster: Avivah Yamani

Title: Why Do Venus and Uranus Spin The Wrong Way?

Organization: 365 Days Of Astronomy ; langitselatan

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Mengapa Rotasi Venus dan Uranus Berbeda?:
Rotation Of Venus:
Why Venus Spins the Wrong Way :

Description: The question is why do Venus and Uranus rotate differently from all the others?

Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator from Indonesia.

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Venus, the second planet in Solar System and our neighbour, is also a twin of our blue planet. It has a similar size, mass, density and chemical composition. But of course just like any twin, there is always a difference between them. In this case, Venus’ high temperature and extreme pressure make it look very different from our world. How could you live in a such hot planet like that?

And not just that. The day on Venus is much longer than one on the Earth. It takes 243 Earth days to complete its rotation but it only takes 224.7 days to complete each orbit. It means you already are in another year once the planet finishes the day. On top of it, Venus rotation is opposite to the Earth. It rotates backwards compared to the Earth and the other planets.

Well not all, since Uranus also rotates in a different way. If you look at the solar system from its north pole, then you will see all the planets orbiting the Sun counter clockwise and rotating on their axis counterclockwise, except for Venus and Uranus. Venus rotates clockwise while Uranus rolls on its side as it orbits the Sun.

The question is why do they rotate differently from all the others?

Let’s take ourselves traveling into the past. Well not literally going to the past, but let’s explore the time when all the planets formed. At that time, all the planets rotated in the same direction. But something came along that changed the rotational direction of Venus and Uranus.

Lets start with Venus. There are a few possibilities that we can take into account to explain why Venus rotates backwards. Venus is initially rotating counterclockwise like the other planets and it still does. In other words, it spins in the same direction it always has, just upside down, so that looking at it from the other planets makes the spin look backward.  As I said before, there are a few explanations for this.

Scientists have argued that the sun’s gravitational pull on the planet’s very dense atmosphere could have caused strong atmospheric tides. Such strong tides could have caused the flip to happen.

Another explanation comes from the cratering evidence on each planet. Soon after the planets were formed, there still were many large and small objects or maybe we can classify them as mini planets that orbited the Sun.  So.. this interaction with all the planets eventually cleared each planet’s orbit as we know now.  This is what happened to the Earth/Moon system when a Mars-sized body collided with the Earth and the leftover material merged and formed the Moon.

And Venus may have experienced an encounter with one of these larger bodies in a gigantic impact in which, unlike the Earth, the extra material didn’t form a separate moon, but stopped and even reversed its rotation altogether.

Another simulation was made by Alex Alemi and David Stevenson from California Institute of Technology, to trace the oddness of Venus’s orbit. They proposed 2 large impacts in Venus’s history. The first impact’s result was a satellite for Venus but after 10 million years Venus encountered another gigantic impact on the opposite side of the planet from the first impact which reversed its rotation and the satellite it once had spiralled in and collided with the planet itself.

That’s Venus. As for Uranus, it’s quite unique since it’s not rotating in the regular clockwise or counterclockwise direction. Well if Venus was rotating upside down then.. someone must have kicked this planet and made it roll on its side. Most planetary axes are perpendicular to the orbital plane. But Uranus’s axis is extremely tilted to 97.7º and its pole is pointed toward the equatorial plane of the other planets.

This extreme tilt leads to the radical seasons that the planet experiences and makes the planet have unusual days at the poles. At the equator, Uranus experiences normal days and nights. But because it rotates on its side, at any given time one pole or the other is pointed more or less towards the Sun. This results in one pole experiencing 42 Earth years of day followed by 42 years of night. When the North Pole is toward the Sun, the South Pole is in darkness and vice-versa.

So how could this have happened? Same as with Venus, Uranus also had counterclockwise rotation until a gigantic impact changed everything.  The explanation for this is that in its formation history, Uranus collided with an Earth-sized object which lead to the change of its rotation.

Another theory and simulation without collision by Gwenael Boue and Jacques Laskar from Paris Observatory shows that Uranus had very large moon with 0,1 % of Uranus’s mass. The gravitational interaction between the two changed Uranus’s axis to tilt the way it is now. And for the moon itself, it was ejected from the system when they encountered other massive planets.

Thank you for listening. This is 365 Days Of Astronomy

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365 Days of Astronomy

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