How long is a day on an exoplanet when the atmosphere forces an entire planet to spin faster

Even before exoplanets were discovered, astronomers thought that most planets with orbital periods less than a year would be spinning “synchronously”, meaning that, like the Moon around the Earth, they would always show the same side to their star. In our Solar System, Venus escaped this state, but it was believed that this peculiarity was due to the unusually massive atmosphere of that planet. Using a global climate model, researchers have shown that even a relatively thin atmosphere, like that of Earth (which is a hundred times less massive than that of Venus), could be capable of forcing a planet to spin rapidly and thus to escape synchronous rotation. By creating a night-day cycle, this faster rotation completely changes the climate of these planets.

The Moon always shows us the same side, because the tides raised by the Earth on its satellite create a friction that alters the spin of the moon. This process stops when the time taken by the Moon to rotate around its spin axis equals the time it takes the Moon to orbit around the Earth. The Moon is then said to be in a state of synchronous rotation. Most of the regular satellites in the Solar System are also in a state of “synchronous rotation”.

Many known exoplanets are close enough to their host star to experience similarly strong tidal friction. As a result, it has long been thought that exoplanets with orbital periods significantly shorter than a year would be synchronously rotating. It follows that they would not have any day/night cycle: one hemisphere would always face the Sun, as the other would be in eternal darkness! If this synchronization does occur, it would have far reaching consequences for the climate of the vast majority of potentially habitable planets in the galaxy, because the majority of stars are much dimmer than the Sun. To be able to temperatures appropriate for the presence of liquid water, planets around such stars must be much closer to their host stars than the Earth is from the Sun. The tidal friction such short period planets undergo is thus very intense.

But Venus, the planet with an atmosphere that is closest to the Sun, does not spin synchronously. (Neither does the Earth, but in our case, it is because we are too far away from the Sun for tidal friction to be important). In fact Venus rotates backward: the Sun rises in the West and sets in the East. There is evidence, however, that tidal friction has been at play because Venus spins very slowly: one rotation every 243 days.

Why does the tidal theory work for objects like our Moon and other satellites like Io and Europa, but not for Venus? The tidal theory neglects the effects of an atmosphere. Although the atmosphere of Venus is only one part in ten thousand of the mass of the planet, it has been able to accelerate Venus’ spin over geological timescales. How? By creating temperature differences at the surface, between day and night and between equator and poles, the solar heating drives winds that redistribute the mass of the atmosphere, so that it is not spherical. In fact the atmosphere is distorted in such a way that it tends to be globally spun-up by the gravitational attraction from the Sun.

Although one part in ten thousand does not seem like a lot, Venus remains the terrestrial planet with the most massive atmosphere in the Solar System! The second most massive terrestrial atmosphere, that of the Earth, is a hundred times smaller. This much lower mass may explain why it has long been thought that the effect of the atmosphere on the spin of exoplanets, believed to be proportional to the mass of the atmosphere, would remain weak and that Venus-like non-synchronous planets would be the exception.

However, researchers at the Canadian Institute for Theoretical Astrophysics have developed a three-dimensional climate model able to predict the effect of the atmosphere on the spin of a given planet for a wide diversity of atmospheres. The main surprise that comes out of their work is that if the Earth were in Venus’ current location, the effect of its atmosphere, while a hundred times less massive, would be almost ten times as strong as the effect of Venus’ atmosphere.

The reason for this surprising result is that Venus’ atmosphere is very opaque. Most of the light from the sun that strikes Venus is stopped by the very thick cloud deck that prevents us from seeing the surface and gives the Venus its peculiar appearance. In contrast, on Earth, most of the sunlight reaches the surface of the planet, where the effect of the subsequent heating on the redistribution of the atmosphere is maximized.

What about exoplanets? While astronomers are still awaiting observational evidence, theoretical arguments suggest that many exoplanets should be able to keep an atmosphere as massive as that of the Earth. In that case, this new study shows that a large number of known terrestrial exoplanets should not be in a state of synchronous rotation, as initially believed. Thus, they would have a diurnal or night/day cycle like on Earth. The duration of their days, however, could last between a few weeks and a few months.

Planets with potential oceans could thus have a climate that is much more similar to the Earth’s than previously expected. When a diurnal cycle is present, there is no permanent, cold night side where water can remain trapped in a gigantic ice sheet. Does this increase the ability of these planets to develop life as we know it? This is still an open question.

Légende

Source:Leconte, J.et al. “Asynchronous rotation of Earth-like planets in the habitable zone of lower-mass stars”. Science, in press, 2015.

Contact(s):

– Jérémy Leconte, Canadian Institute for Theoretical Astrophysics, jleconte@cita.utoronto.ca. Tel: +1 647 895 2100