Back in June, I asked astrophysicist Neil deGrasse Tyson whether life could exist in the Aenya-Infinity system, where Aenya (the moon) is tidally locked to Infinity (a gas giant planet). Here’s my original letter:
Dear Neil deGrasse Tyson,
I am a huge fan of your work. You not only have a brilliant mind for physics, but you’ve managed to bridge the light-year sized gap between human knowledge and those ignorant to it. My question relates to the novel I have been working on for the past decade, “Ages of Aenya.” The story takes place on the planet (or rather, the moon) of Aenya, which orbits a Jupiter like gas giant. Aenya is tidally locked, so one hemisphere perpetually faces the gas giant, while the other, at intervals, faces the sun. When Aenya moves into the dark side of the planet it is orbiting, the sunny side is also dark. In this way, one hemisphere remains dark while the other undergoes a kind of day/night cycle. So my question is this: On this type of planet, is it possible for humans, or beings with human-like anatomies, to survive? What happens to weather patterns if a planet doesn’t rotate? Is there any wind? What effect does a Jupiter sized planet have on the tides of that world? Am I wrong in any of my assumptions? Making a stab at feasibility, I have tried to wrap my head around these issues, but as I am not a scientist, the whole thing is beyond me. Any input on your behalf would be greatly appreciated.
Neil was too busy to answer me directly, but astronomer Alejandro Núñez was kind enough to offer this lengthy and in-depth response:
Your novel sounds like a very interesting story.
First, there is a mistake in how you are picturing the day/night cycles of Aenya. If you think of Aenya and its gas giant as the moon-earth system, you will realize that the side of Aenya that perpetually faces the gas giant also gets daylight. If it did not, then we would never be able to see the moon shine from Earth.
Second, whether human-like anatomies could be possible in such a place would depend more on the atmospheric conditions of Aenya than on day/night patterns.
An exception could be the visual organs: assuming that the gas giant is at a similar distant to its host star as Jupiter is to the sun, then the type of light that would mostly shine on Aenya would be the one reflected by the gas giant itself, and not the sun. In such situation, human-like creatures would have eyes more sensitive to the peak light frequency of the gas giant reflection, the same way that our eyes here on Earth are the most sensitive to the peak frequency of light coming from the Sun. Check this blog post to get a better idea of what I am getting at:
Third, Aenya IS rotating, only at a pace that matches its orbital time around the gas giant. Again, think of the moon-earth system. If the moon did not rotate, then we would see a different moon surface as it orbited around us.
Keep in mind that the main driver for wind is temperature differences: cool air sinks, hot air rises. Even if Aenya were not rotating, different areas on its surface would be exposed to more heat (either from the star or the gas giant) than others as it orbits, thus creating temperature gradients.
Finally, the gas giant will most definitely create tides on Aenya, assuming large bodies of liquid on its surface. The tides would occur because of the change in distance to the gas giant, from its closest to its farthest point along its orbit. This blog post explains these tides as they occur in Titan, Saturn’s biggest moon:
As a matter of fact, if Aenya is close enough to the gas giant, even its rocky body could get deformed by tidal forces. This, indeed, is what happens to some moons of Jupiter and Saturn, including Titan. This article on Sky & Telescope describes a recent discovery on Titan that you may find interesting:
Good luck with your novel.
Alejandro Núñez, Astronomer
American Museum of Natural History
As suspected, real life is much more complicated than fiction. I was afraid the response would be, “There’s no way humans could exist on such a world!” but fortunately that wasn’t the case. In fact, it seems the biggest problem is minor, light, and who’s to say Aenyan-human eyeballs aren’t more sensitive than Earth-human ones? I was also happy to learn that things like wind and tides can exist on Aenya, since those things are mentioned somewhat frequently in my book. As a fantasy writer, it’s difficult to create a sense of atmosphere without using any of the things we, as Earthlings, are familiar with. And how could I write a dramatic piece about Thelana without having the wind frolic in her hair? I suppose if we all lived on Mars, I would make references to things unique to Martian life/geology. Of course, I was dead-wrong on several occasions. I didn’t realize that all astral bodies rotate, even our moon, at least a little bit. Perhaps Aenya’s rotation is so slow as to seem negligible to its people, so that day and night has more to do with the eclipse of the moon, which happens more frequently. The biggest DUH! moment for me, however, was the fact that the moon is bright! So maybe the dark hemisphere of Aenya is not quite as dark as previously thought . . .
Anyway, I have a lot of reading to do with all these links. A good dosage of science should help add credibility to my fictional universe, and I have Alejandro Núñez and the Hayden Planetarium to thank. So if you’re out there reading this, Alejandro, here’s a big THANKS!