With the release of Disclosure Day, Steven Spielberg’s new film about aliens, a question as old as science fiction itself resurfaces: if aliens were to arrive on Earth, would they come to conquer us, to study us… or perhaps to eat?

We must begin here with a word of caution. There is no scientific evidence that extraterrestrial beings have visited Earth, let alone any information about what they eat. NASA states that there is no data to support the claim that unidentified anomalous phenomena (UAPs) are alien technology, and the US Department of Defence has also found no verifiable evidence of extraterrestrial technology or activity.

But the literal meaning of “extraterrestrial” is simply “from outside Earth”. In that broad sense, the only extraterrestrials whose diet we know of are, oddly enough, humans – specifically the astronauts who spend weeks or months in space. While their experiences do not tell us what an alien would eat, they do show us that leaving Earth changes the way we eat.

In microgravity, appetite, taste perception, muscle mass, bone health, hydration and energy expenditure all change. Even for our own species, eating off-planet means adapting diets, nutrients, textures, food preservation and metabolic control.

Read more: Astronauts don’t eat enough because food tastes bland in space. We’re trying to work out why

This article is therefore a purely speculative exercise, though not an absurd one. Biology allows us to ask reasonable questions, even about imaginary organisms. How much do they weigh? Do they move around a lot? Do they maintain a constant body temperature? Do they breathe oxygen? Do they have a large brain? Do they live in gravity similar to that on Earth?

Even with these pieces of information, we cannot guess at what “the alien diet” consists of. But we can figure out the minimum energy needs of a hypothetical living being.

The word “alien” is not a biological category. In popular culture, they range from “little green men” to reptilian beings, tall humanoids, luminous creatures, and mechanical, non-organic intelligence. But these classifications belong in UFO folklore and fiction, not zoology.

Science does, however, have tools for estimating a creature’s metabolism. In land animals, one guideline is that basal energy expenditure (the minimum energy an organism needs at rest to maintain vital functions) increases with body mass, though not proportionally.

This means a mouse consumes a lot of energy per gram of body weight, while an elephant, despite expending a great deal more energy in total, uses much less per gram of its mass. In other words, the larger an organism is, the more energy it needs, but each kilogram of its body is usually “cheaper” in terms of energy use.

Applied to a hypothetical........

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