Home >mint-lounge >features >Is there a Tatooine out there?

Perhaps the most iconic of all images in the Star Wars franchise is a double sunset that Luke Skywalker casually stares at on his home planet, Tatooine. It’s a wondrous sight that gently but persistently grips the imagination. Star Wars wasn’t the only—or even the first—science fiction story to hold habitable planets in binary star systems (a system where planets orbit two suns). Magrathea in The Hitchhiker’s Guide To The Galaxy and Gallifrey, the home planet in Doctor Who, are both habitable planets that orbit a binary star system. Wouldn’t it be absolutely delightful if such planets actually existed?

The idea is not new and has been around far earlier than popular science fiction. The very first binary stars were discovered as early as the 17th century, immediately after Galileo improved the telescope and first pointed it skyward. Since then, over 60% of all stars that we’ve observed are binary stars. Our nearest neighbouring star, Alpha Centauri, is one. It would seem almost arrogant to assume that these stellar systems would not contain a habitable planet.

Planets do exist in binary star systems. Unfortunately for us, all of the ones we have found so far have been gas giants. This isn’t a big surprise because larger planets are easier to detect. Tatooine is a habitable, rocky planet with multiple moons. For the longest time, it was considered unlikely that during the formation of binary systems, the two stars would allow enough “heavy" material such as iron and rock to coalesce into planets. Numerous observations of gas giants seemed to confirm this. However, a study conducted earlier this year announced that habitable rocky planets are not only likely to exist, but could actually be common.

Now, this raises some pertinent questions. First, how stable is the orbit of a planet if it is in the vicinity of two stars? To answer this, we need to understand some basics about binary star system configurations.

A binary star system itself has multiple variants, each with its own degree of stability. Stars that are too close to each other will eventually either merge or have the more massive star consuming the other, disrupting the orbit and temperature of planets around them. If the stars are too far apart, it would mean one star is gravitationally stronger, so the second star actually “revolves" around the first. A planet could orbit either star, but won’t be stable because of constant tidal forces from both stars.

The third variant is the most stable configuration for a planet in a binary system: It is when the two stars are sufficiently far apart to orbit each other or around a common centre of mass, and the planet orbiting both of them together is at a distance that’s greater than the distance between the two stars. These kind of planets, like Tatooine, are called circumbinary planets.

The first such planet was discovered in 1993 and was a gas giant, twice as big as Jupiter. Since then, every circumbinary planet discovered has turned out to be a gas giant, nearly five times bigger and more massive than Earth. This almost entirely negates chances of finding life on any of them.

But for argument’s sake, let’s consider that Tatooine exists. It would need to have very specific characteristics. Primarily, its mass and size should be very close to that of Earth’s. Any planet with a mass greater than 2.5 times that of the Earth’s will have gravitational forces too strong to allow life to develop. Anything with less than 0.8 times will not have enough gravity to hold on to an atmosphere. Tatooine not only enabled people to move about normally, it also let them breathe.

If indeed the planet can hold on to an atmosphere, temperature and weather are important. From the observational data we have so far, temperature varies between the two extremes. Planets orbiting stars that don’t give off enough heat are too cold. But most planets we’ve found have all been too close to the stars. Going by average, our hypothetical planet would be a hot world, just like Tatooine. It would most likely not be able to hold on to the critical liquid water needed for life. Since the two stars would most definitely remove sources of abundant water during formation, the system will not have icy bodies like comets that can go about irrigating planets. Since there wouldn’t be enough water vapour either, the planet wouldn’t be able to retain heat and would freeze at night. But the presence of water cannot be eliminated completely. Our planet could have hidden pockets of water at the polar caps or as mild water vapour in the atmosphere.

The planet could also have multiple satellites. As long as the moons are within a certain radius from the planet where the planet’s gravitational influence is much higher than the stars’—called the Hill Sphere—a sufficiently massive planet can hold on to proportionately smaller moons. And if the planet is in the habitable zone, these moons are prime candidates for astrobiology. The gas giant Kepler-47c is our strongest candidate for detecting habitable moons around a circumbinary planet, as it lies in the habitable zone.

If we consider that our real-world Tatooine can sustain life, what kind of life would it be? Exotic and unique for sure. Tatooine’s two stars, Tatoo I and Tatoo II, are G-type and K-type stars, much like the two stars of Alpha Centauri. A G-type star, like our Sun, gives off white light. A K-type star, an orange dwarf, gives off visible light, but also emits infrared radiation. Any plant life would adapt itself to absorb copious amounts of infrared radiation in addition to the entire visible wavelength. This would mean that all plant life would be darker and range from very dark green to brown to almost black in colour. Flora found on our Tatooine would be variations on our cactii, having popping-brown and magenta flowers and black leaves.

Would animals like banthas and krayt dragons survive? Why, yes! And this can be explained simply by observing animals in the deserts of Earth today. We have snakes, scorpions, thorny devils, pyxie frogs, and many more creatures that have adapted well to the heat and lack of water. Snakes and scorpions remain under the sand to keep themselves cool; thorny devils absorb water into their skin, pyxie frogs hibernate during the summer; squirrels have bushy tails that give them shade anywhere they go; desert beetles condense mist from the air and ingest water...but these are all tiny creatures. They can adapt quickly and hide from the heat easily. Tatooine has banthas and dewbacks. Can such megafauna even live in the desert?

The answer is, again, a resounding yes. In fact, the bigger and furrier the animals are, the better they are with the heat. It’s counter-intuitive, but easy to understand. Large bodies have greater thermal inertia, meaning it’s harder to heat them up compared to smaller ones. So while a bantha ambles about in the sun, its body heats up very slowly. All the fur the bantha has offers great protection. We usually associate fur with warmth and protection from cold, but fur is an insulator; it protects from both extreme heat and extreme cold. This is the reason why dogs like St Bernards and huskies do well even in hot countries. But being ectotherms, reptiles are probably best adapted to the intense heat. So krayt dragons, skettos and dewbacks are most suited for our Tatooine.

A more pressing problem for these animals would be the lack of water. Large desert animals on Earth today, such as the camel or the dorcas gazelle, have specialized body adaptations to counter these added conditions. Similarly, animals that would be found on our planet would have mechanisms to store water and fat that would enable them to survive a long duration without having to consume more.

Probably the most exciting question of all is whether an intelligent species like humans can survive on our hypothetical planet. Yes, if you talk about us moving there and colonizing the planet. Mostly no, if we’re talking about the existence and evolution of an intelligent species like us. We can safely say that humans have it easy on Earth, with ideal combinations of temperature and water. On a Tatooine-like planet, the conditions would most likely be too harsh, and diversity too little, for an intelligent human-like species to evolve from unicellular life.

Sandhya Ramesh is a professional programmer, amateur astronomer and science writer.

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