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Tidally-Locked Planets

This article originally appeared in the September/October 2019 issue.

As many of us know, we always see the same side of the moon as it circles the Earth. The term used for this phenomena is “tidal lock”. By definition, this means the moon is in synchronous rotation around our planet, where the gravity pull between both bodies affects the moon to the degree where its rotation matches the amount of time it takes to complete a single orbit around Earth. The gravitational effect on the moon also causes it to be stretched so that the moon’s diameter measured along a line connecting the center of the moon with the center of Earth is slightly larger than any other measurement.

As we look further into our solar system, many of the moons of the other planets are tidally locked with their host planet. In one such case, the dwarf planet Pluto is mutually tidally locked with its moon Charon where both bodies face each other in the same way all the time. Mars, Jupiter, Saturn, Uranus and Neptune all have multiple moons that are tidally locked to them. So if this is a natural and commonly seen astronomical phenomena, why wouldn’t a planet in a habitable zone be tidal locked to their host star? What would this planet look like?

There have been several artist’s renderings that address this very question (unfortunately, none have sufficiently permissive availability to use with this article—ed). The native flora and fauna would be concentrated in and around the twilight/dawn threshold. The back side of the planet, locked in a permanent winter of darkness, would likely be covered in ice and snow. Wherever there would be geothermal activity on this back side of the planet there would be geysers and immense mounts of fog, clouds and snow, much of the water would shoot high into the atmosphere and be carried aloft to condense and fall back to the planet miles away. An example of this phenomena would be Saturn’s moon Enceladus, whose ice geysers supply the ice materials for Saturn’s E-ring (and which is tidal locked to Saturn).

The front side of the planet, baked in intense daylight, would be too hot to support life as the constant bombardment of solar energy would vaporize any water. Heat and radiation both would make this land sterile in the extreme. Near the dead center of the uninhabitable zone one would expect more elevated land masses as the gravitational pull would cause a bulge in the area closest to the sun. I would envision barren land with an abundance of geothermal and volcanic activity. If the technology is sufficient, this land should be loaded with all kinds of minerals and valuable raw materials for the ambitious belter to harvest.

Along the twilight/dawn periphery, the land would be just warm enough to support liquid water. Much of this water would be runoff from the glaciers melting on the darker side of the planet. The interaction between the hot and cold zones of the planet would cause the water to flow in currents between the islands that lie off shore of the main continent of sand and heat. Even though there are no winds being caused by the planet’s rotation, the temperature difference would also generate a constant “trade wind.” This permanent sirocco would also be a means to melt the edges of the glaciers, perpetually churning the water vapor cycle.

So if we go with the assumption that there is a fully operational water evaporation/condensation cycle, we can see where, with a little help through terraforming, life could be sustained here. Rain would be possible, but moisture will likely come from the glacial melting creating fog or mists. It would, however, have to compete with the dry winds from the deserts to hold moisture. Where there is abundant and constant moisture, plants would grow – and quite well due to the constant sunlight. I would expect huge flora sizes and excellent crop yields where arable land could be found and worked.

Now again, we have been working under the assumption that this planet is located in the habitable zone of a solar system. What if that planet was located further away, let’s say by another whole AU (Astronomical Unit, the average distance from Earth to the Sun). Then the heat would only melt an ice planet in one area (assuming that there is moisture there). Where most of the planet would be encased in hard ice, there would be a small sea or ocean that should contain enough gravity pull from the host stellar body to create land masses. This land of perpetual sunlight would be bathed, I would think, in near-constant fog or clouds with the moisture cycle on constant overdrive. Again, with a little terraforming, this planet could also produce large amounts of foods and agricultural products in a smaller amount of space.

Now there have been arguments made that a tidal locked planet would be rendered inhospitable due to a runaway greenhouse effect. I believe that any planet can be rendered inhospitable by a runaway greenhouse effect; tidal locking isn’t necessary—for example, the Earth was dangerously close to undergoing a runaway greenhouse change during the Permian-Triassic Extinction Event some 252 million years ago. This event was likely triggered by extreme volcanic events in the Siberian Traps area where massive amounts of volcanic gases started to create this condition. This being the case, I can see where a sufficiently advanced technological society, could do well with planets such as these with just a little effort.

So for those of us who enjoy a planet with unique and different features from our own, I humbly submit these thoughts for your consideration. In your mind, how would the plant life be different? Would they lean one way towards the sunlight or be bent another way due to the constant winds? What animals would thrive in such an environment? Would they have moisture retaining features to allow them to live in the hotter regions, or thicker fur with larger eyes for the colder regions? What would the weather be like? With all the added gravitation stress, how would your planet’s tectonics be impacted? I see a lot of possibilities for many different types of adventures under challenging conditions.


“‘Eyeball Earth’ Planets Could Harbor Extraterrestrial Life, Scientists Say”, Huffington Post, 30 Apr 2013,

“Tidal Locking”, Wikipedia

“Permian—Triassic Extinction Event”, Wikipedia

“Tidal Locking Could Render Habitable Planets Inhospitable”, Astrobiology Magazine, 8 Dec 2011