Mercury’s Tidally Locked Orbit

Mercury is weird

Mercury’s tidally locked orbit is a good example of how the universe always throws astronomers a few surprises.

The planet is tidally (or gravitationally) locked to our Sun, but this is not the typical “synchronous” tidal locking with a 1:1 ratio of rotation and orbit, such as the Moon and Earth, with the same face always presented to the larger partner. Mercury is locked into a what’s known as a 3:2 spin-orbit resonance, which is unique in our solar system.

The thing about the universe is that things look different from different places. Although Mercury’s orbital period is around 88 Earth days, from Earth it appears to move around its orbit in around 116 days (because we are moving too).

With Mercury’s 3:2 resonance it rotates exactly three times for every two revolutions the planet makes around the Sun. Yet the Sun is also turning. From the Sun’s frame of reference, Mercury appears to rotate only once every two Mercurian years. So the little yellow men who live in the caves there have to wait two years to see a single day go by, or about 176 Earth days. Birthdays must be complicated!

So how did astronomers get the idea that Mercury was synchronously locked to the Sun? This was because whenever Mercury was best placed for observation it was nearly always in the some point in its freaky 3:2 orbital resonance, so was showing the same face to observers on Earth. Since, by coincidence, Mercury’s rotatation (58.7 Earth days) is almost exactly half of its orbital period as observed from Earth (116 days). It was not until the radar observations of the planet in 1965 that astronomers learned the truth of its orbital antics.

Mercury's Tidally Locked Orbit

Thermal underwear a must on Mercury

Mercury has virtually no atmosphere, and is at the mercy of the Sun. Its surface temperature can rise on its equator to 427C (800F) during the day, and plummet to -173C (-280F) at night, while the poles are little more stable at around -93C (-136F). Although the planet has a small tilt, it has the highest orbital eccentricity of all the solar system planets, its orbital distance from closest (perihelion) to furtherest from the Sun (aphelion) varying by as much as 1.5 times.

Like our own Moon, the surface of Mercury is heavily cratered, indicating that the planet has been geologically inactive for billions of years.

My novel, The Tau Ceti Diversion, is a story about our search for new planets to colonise outside our solar system. Much of the action takes place on planet tidally locked to Tau Ceti that has some rather unique characteristics. The novel is due to be launched on September 1st 2016, and pre-order is now available on Amazon! Read more about what happens in the story here!

Stay tuned for a free chapter download, coming soon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)

Day Side & Night Side – Tidal Locking of Planets

So what is tidal locking? Our Moon is tidally locked to the Earth, always presenting the same face to us. That doesn’t mean that the Moon is stationary, far from it, it just means that it takes just as long to rotate around its own axis as it does to revolve around the Earth. The same thing can happen for planets, which can be tidally locked to their stars, always presenting the same side of the planet to their star, giving those planets a permanent ‘day’ side and ‘night’ side.

We could expect these planets to have some pretty unique characteristics, with a hot, dry side, and a frigid frozen night side. Some scientists have even dubbed them ‘eyeball’ Earth’s due to the likely combination of features that might develop.

one-side-planet eyeball earth


In my SF novel, the Tau Ceti Diversion, the action is set on a planet that is tidally locked to Tau Ceti, always divided into a hot day side and a cooler night side. This set up was crucial to the novel, and to the civilisation that the stranded crew of the starship Starburst find when they land on the planet’s night side. Actually they were aiming for the terminator – the dividing line between the day and night sides – expecting this to be a temperate zone. But I can’t say much more without spoilers:-)

Tau Ceti is a G-class sun, around 12 lightyears from Earth. One of our close stellar neighbours. Could we expect that one of its planets would be tidally locked to its star?

Well, it did not take me too much research to realise that this is one very complex question. It would indeed be surprising to find a tidally locked planet around Tau Ceti. Finding a tidally locked planet might be more likely around a smaller M class star. But there are many, many variables that might allow a planet to become tidally locked to its star within a reasonable fraction of that star’s lifetime. The variables that might increase the likelihood of a planet becoming tidally locked early in the star’s lifetime include the lack of a companion satellite (i.e. more likely if there is no moon), a low initial rate of planetary spin, a low dissipation function (the rate at which mechanical energy is converted into heat), a low rotational inertia . . . even the rigidity of the planet can be variable.

So, all these variables gave me enough wiggle room to allow my planet to be tidally locked. Plus I had a secret weapon – a key bit of backstory that affected the planet’s spin at a key point of its history. But I can’t say anything about that either, not without giving away the story!

Stars are classified based on their spectral characteristics. The M-class spectrum contains lines from oxide molecules, particularly TiO, with absorption lines of hydrogen typically absent. M-class are the most common of stars, representing over 76% of our stellar neighbours.  So we might expect more than a few tidally locked planets out there. Of course these will be the smaller bodies, Earth-sized and smaller, so will not be well represented in our current exoplanet catalogue, which features a lot of big, Jupiter-sized and heavier planets due to the methods used to identify exoplanets (so far). M-class stars are light orange red in colour, from 0.08-045 solar masses and low luminosity (less than 0.08 of our Sun’s). This class features rare and exotic creatures that can rarely be seen by the naked eye, mostly red dwarfs, although some are red giants, or even red supergiants. The class also includes the intruiging brown dwarfs, which are ‘late’ class M stars.

My novel, The Tau Ceti Diversion, a story about our search for new planets to colonise outside our solar system, is due to be launched on September 1st 2016, and pre-order is now available on Amazon! Read more about what happens in the story here!

Stay tuned for a free chapter download, coming soon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)