Why isn’t Venus “locked” with the Sun?

The closer a planet is to its star, the more it undergoes tidal effect important. An effect that can slow down its rotation. And sometimes even go as far as “lock”. Until it synchronizes with sound orbit. This is what happens for the Moon. As a result, our satellite shows us always the same face.

On paper, this is also what should happen between Venus and the Sun. But in reality, this is not the case. Well, not exactly. Because the rotation of Venus is still slowed down. Indeed, it takes 243 Earth days for Venus to turn on itself. While it barely takes 225 to go around the Sun. And, for researchers at the University of California at Riverside (UCR, USA), what seems to make the difference between Venus and the Moon is their atmosphere. Thick and choppy in one case. Thin and insignificant in the other.

Learning from Venus to find habitable exoplanets

“The example of Venus teaches us that the atmosphere of a planet matters. She even affects her rotation speed»comments Stephen Kane, astrophysicist in a press release from the UCR. Recall that the winds powerful that blow on Venus make turn its atmospherearound the planet in just 4 days. And these movements influence the rotation of Venus, loosening the gravitational grip that the Sun may have.

The question that remains is that of the repercussions this may have on the temperatures prevailing on the surface of the planet. Unbearable temperatures, over 480°C, due to what scientists call a runaway greenhouse effect . And this question appears important, not only to better understand Venus, but also to better target the exoplanets potentially habitable.

The atmospheres of exoterres would save them from synchronous rotation

A team of researchers from the Canadian Institute for Theoretical Astrophysics, in collaboration with the Laboratoire de meteorology dynamic, simulated the effect of the atmosphere on the rotation of the planets. According to them, even a relatively thin atmosphere, such as that of the Earth, can modify the durationday until the day-night alternation disappears. This slowing down is only effective for planets very close to their star, but this is the case for most potentially habitable exoplanets in the galaxy . Contrary to what we thought, the majority of habitable exoplanets could therefore experience a day-night alternation like the Earth.

Insu article published on 01/19/2015

If the Moon always shows us the same side, it is because it feels the tides exerted by the Earth which play the role of a friction which slows down its rotation on itself. This phenomenon stops when the time taken by the satellite to make a turn on itself is equal to the time taken to travel its orbit around the planet. Like the Moon, most natural satellites of the Solar system are in this state called “rotation synchronous “. More generally, the same applies to the planets around their star, when they are close to it. However, the majority of stars are much less luminous than the Sun and in fact, to be habitable, the exoplanets which revolve around must be closer to it (which is not the case with the Earth). They therefore undergo this significant tidal effect which makes their rotation synchronous.

Thus it was thought that the majority of exoplanets could not experience day-night alternation like the Earth. the climate of a large number of exoplanets potentially possessing oceans should therefore be allocated accordingly. Indeed, there would be a side of permanent night where the water present on the planet could remain trapped in the form of ice. The present study shows that in the case of telluric planets, there is a very important effect of the atmosphere on the rotation of these, which counteracts the tidal effect and therefore induces the possibility of a cycle. diurnalLike on Earth .

Venus, a planet that escaped synchronous rotation

An indication of this effect was given to us with the example of Venus which, like most habitable exoplanets, is close to its star, and therefore undergoes vigorous tidal effects, but yet does not experience synchronous rotation. However, scientists believed that this effect was explained by the particularly massive atmosphere of this planet. By causing temperature differences (between day and night, betweenequator and the poles), solar heating creates winds that redistribute the massof the atmosphere so that the gravitational pull of the Sun can accelerate its rotation around the globe. Thanks to this and in the case of Venus the atmosphere has been able, over geological time, to accelerate the rotation of the entire planet!

Asynchronous rotations for exoplanets with atmosphere

It was therefore thought that such effects were reserved for planets whose atmosphere is particularly massive, but the study by the Franco-Canadian team actually shows the opposite: contrary to popular belief, a more tenuous atmosphere acts more strongly on the rotation of a planet. The reason for this difference is that the atmosphere of Venus is very opaque and that much of the insolation is stopped by a thick cloud ceiling, which prevents us from seeing its surface. On the contrary, on Earth, the majority of the sun’s rays reach the surface where their effect on atmospheric redistribution is greatest.

The big surprise is that these results show that if the Earth were at the current position of Venus, the effect of its atmosphere, although a hundred times less massive, would be almost ten times greater. To achieve this, the researchers devised a climate model three-dimensional system capable of predicting the effect of the atmosphere on the rotation of a planet, and this for very different atmospheres.

Thus, a large number of habitable exoplanets could well follow a day-night alternation cycle like the Earth, although undergoing significant tidal effects due to their proximity to their star. These “exodays” would however have a duration closer to the terrestrial month than to our 24 hours. But that would be enough to impose a climate closer to that experienced by the Earth. This sheds new light on the question of the potential of these planets to host the alive as we know it.

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See the animations illustrating the two situations, synchronous and asynchronous. Note that the meridiansin gray give the rotation of the planet. In superposition of these, we see in the case of asynchronous planets the fluxes of temperatures moving on the surface of the globe.