Greetings from week 4 of planetary atmospheres! Last week, we discussed the proposed Titan Mare Explorer mission to the methane seas of Titan, and I’ll admit I’m a little sad to see this one go. There’s something so delightful about nautical expeditions on other planets. For the follow-up act, this week’s topic is world-building climate from first principles.
Habitable Zone – the region around a star where an Earth-like planet can maintain liquid water on its surface.” ~ Virtual Planetary Laboratory
It sounds so binary, doesn’t it? Either you’re in or you’re out.
To first order, planetary climates are defined by their orbits around the local star, and the luminosity of that star. The brighter the star, the farther the habitable zone is from it. However the effect of the insulating atmosphere is also hugely important. Good old Earth, for instance, would have a globally-averaged temperature of -18°C (0°F) without the greenhouse effect provided by gases in the atmosphere. Notice that’s below the freezing point of water. Slap an atmosphere on us, and you get a balmy 15°C (59°F), which (trivia alert!) is also the annually-averaged temperature of Rome and Melbourne. The greenhouse effect (not to be confused with global warming, please) gives us a planet I’m happy to call home. Without it, not so much.
In fact, I think I prefer this definition of habitable zone from Wikipedia, because it emphasizes the role of the atmosphere:
In astronomy and astrobiology, the habitable zone is the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface.” (emphasis mine)
The two largest controls on planet habitability are the orbit of the planet and the greenhouse effect of the atmosphere, which warms the surface and moderates the climate. Even with an atmosphere, things can get complicated for life–say if nighttime temperatures are 60°C colder than daytime, or if the planet is tidally-locked such that only one side is the sunny side. To explore some of these nuances, let’s start by assuming a planet (with an atmosphere) that can sustain liquid water on the surface. What other factors should we consider for world building our science fictional setting?
Eccentricity
Eccentricity refers to how circular the planet’s elliptical orbit is around the sun. Earth’s orbit is pretty circular, and the variation in distance from the sun over the year only leads to a 1% change in atmospheric (not surface) temperature. On Mars, on the other hand, the eccentricity of orbit produces a 10% variation in atmospheric temperature. This variation strongly affects seasons on Mars.
Tilt
Aha! But seasons on Earth are not due to eccentricity, they are caused by the tilt, or obliquity, of the Earth’s rotation axis. In other words, summer happens NOT when the Earth as a whole is closest to the sun, but when your hemisphere is pointed at the sun. Likewise winter occurs when your hemisphere is pointed away away from the sun.
Precession
Today the Earth’s rotation axis (i.e. North Pole) points at Polaris. But in 3000 BC Thuban in the constellation Draco was the north star, in 320 BC Greek navigator Pytheas described the celestial pole as star-free, and in AD 3000 Alrai will be the north star. Vega, the second brightest star in the northern hemisphere was the north star in 12,000 BC and will be again in 14,000 BC.
All of which is to say, planets can behave as wobbly tops. In the case of Earth, this orbital parameter only affects climate on a 20,000 year cycle (so, Ice Ages), but I like to include it because it’s neat to think of the north star as something that’s not fixed. Taken together, changes in eccentricity, tilt, and precession define the Milankovich Theory of the Ice Ages. Milutin Milankovich was a really humble guy, as epitomized in this quote: “I do not consider it my duty to give an elementary education to the ignorant, and I have also never tried to force others to apply my theory, with which no one could find fault.” (It’s a pretty good theory but it’s not perfect, and it has been improved upon.)
Daily temperature range
Here’s where it starts to get interesting. The daily temperature range of a planet depends on the following: solar flux of energy impinging on the planet; planetary albedo, or reflectivity; gravitational acceleration; atmospheric pressure; and specific heat, or how much energy it takes to change the temperature. The latter two depend on atmospheric composition, and bring about a temperature lag as the planet spins from sunlight to darkness. If the heating is sluggish, then the climate of the entire spheroid is fairly temperate. But if the heating is rapid, then so to is the nighttime cooling. Brrr. Plugging in standard numbers for Earth and Mars (with its thin atmosphere), we see a daily temperature range of 2°C on Earth and 80°C on Mars. Now remember these exact numbers are valid for a level pretty high up in the atmosphere, not the surface–but they control the surface temperature, and clearly there’s huge swing between daytime highs and nighttime lows on Mars, in addition to its strong seasonality. The atmosphere of Mars is a crummy blanket.
Fortunately, writers get to play god and set things up just so. Though for all the prevalence of terraforming stories, I don’t see as much focus on atmos-forming, with the exception of climate-controlled domes, I suppose. And of course, I’ve skipped over the issue of atmospheric make-up entirely, but it’s important to keep in mind. For instance Mars is 95% carbon dioxide; humans and animals can’t breathe it without a feat of engineering … Of course, biology produces/requires oxygen, so these aren’t mutually exclusive. Though I do wonder about the timescales of terraforming a breathable atmosphere, and I hope to return to this topic in a future post.
Something I find really interesting, but it’s not my area of expertise, is imagining how photosynthesis would operate on planets with crazy orbital mechanisms. For instance, what kind of plant life would evolve on a planet that rotates very slowly, with long periods of night and long periods of day. Any ideas?
Nicole, these posts have been awesome. Thank you for the great write-up. I hope the class was fun, too. BTW, Extreme Planets anthology is coming up, with the deadline of June 30. I hope you submit.
Relatedly, I recall reading a few years ago that a planet’s orbit is defined by the foci of its ellipse. One focus apparently corresponds to the main gravitational body about which the planet orbits. I’ve never been able to determine the location of the second focus. Does it change over time as a result of other gravitational forces acting on the orbiting planet? For example, one focus for earth is the sun. Is the second defined as the sum of the solar system’s gravitational forces on earth as it spins?
Thanks for your comment, Wayne! I let this blog series slide a bit because I wasn’t getting much feedback on it. I’m delighted to hear you’re enjoying the posts. I actually have a lot more I could talk about, like violet skies and red plants … and I’ve been reviewing the academic literature the atmospheric circulation of tidally-locked planets around M dwarf stars, so could totally geek out over wind patterns and storm tracks on terrestrial exoplanets.
To answer your question, there’s nothing at the second focus of a planetary ellipse: It’s just a point in space. That’s actually why I prefer the definition of eccentricity, which just tells you how circular the orbit is. That’s more intuitive to me. (And it’s directly related to the location of the focus by a simple geometry equation.) If I’m understanding where you’re going with your example, it’s not quite so simple. Mercury and Pluto have similar eccentricities–their orbits are the most elliptical–but obviously exist at quite different distances from the sun. And Jupiter, Saturn, and Uranus all have fairly comparable eccentricities, despite being of vastly different mass. I suspect the shape of the orbit has more to do with vagaries of past collisions and/or solar system formation than any single factor at present.
Yes, the eccentricity of a planet can change over time. On Earth these perturbations occur on a 100,000 year cycle that, combined with other orbital variations, controls the Ice Ages. As I understand it, this cycle is due to gravitational attractions with other planets in the solar system.
Thanks again for your questions, and best of luck if you submit to the anthology!