Life on Planets where 3 Spins = 2 Orbits


It’s been my pleasure to help on a recent paper to be published in the International Journal of Astrobiology, about how life might be on a planet with a peculiar spin.

Imagine a world where the planet’s spin was so slow, that one day took two thirds of a year.  Well, actually we don’t have to, as we can see a world in our Solar System that does this – Mercury:

MercuryNow, it’s immediately obvious that Mercury is extremely inhospitable, as it is so close to the Sun, and has no atmosphere to control its temperature (Mercurian days are 600 degrees C hotter than Mercurian nights!).  It is because Mercury is so close that it has this unusual relationship between its day and its year.  The Sun’s gravity causes tidal forces that twist and crush the planet, slowing down its rotation.  It just so happens that these tidal forces act in a rhythmic way, just as people do when they push each other on swings.  This rhythm allows the planet to enter what is known as a 3:2 spin-orbit resonance.  This means that there are 3 spins for every 2 orbits, 3 days for every two years!

Now imagine we take a planet like the Earth, and put it around a dim star.  For it to still be warm enough for liquid water, we have to put this pseudo-Earth closer to the star, close enough that it might fall into one of these 3:2 spin-orbit resonances.  What would it be like for life?

This is what we set out to discover.  Firstly, we had to think about how the sunlight would be distributed across our planet’s surface.  Now on a planet spinning quickly, it doesn’t matter whether you live in the West or the East, you get the same amount of sun.  Not on this 3:2 world:

2 years of sunlight on our planet's surface.  This planet's orbit is elliptical, like Mercury's.
2 years of sunlight on our planet’s surface. This planet’s orbit is elliptical, like Mercury’s.

When the planet’s orbit is elliptical, the sunlight tends to fall in hotspots.  This is because the star undergoes retrograde motion on the sky – this means that depending on where you stand on the planet’s surface, the star can rise in the east, change its mind, and set in the east! This happens because during an elliptical orbit, the orbital speed changes quite a bit, so sometimes the speed of spin outpaces the orbital speed, and sometimes it doesn’t.

Thanks to this (and the planet moving closer to and away from the star), the amount of light received at a point on the planet’s surface varies drastically, and according to a very unusual schedule.  Plants trying to use sunlight to carry out photosynthesis will need to take heed of this schedule, working frantically while the sun is up, and laying dormant for a very long time during prolonged periods of darkness.  The circadian rhythm for life on Earth is set to around 24 hours, and easily readjusted when it goes out of sync.  Imagine how complicated circadian rhythms would be on our imagined planet!

So what’s the point of all this? Well, we know that small dim stars are much more common than stars like our Sun, and we are getting closer to identifying Earth-sized worlds in the habitable zone of these stars.  So far, the only world we know of in a 3:2 resonance is Mercury, but that could soon change.  And when it does, we’ll continue our work, thinking carefully about how we might detect signs of life on these worlds.

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