Did Earthlings Come from Outer Space?

By Tom Hartsfield

The fossil record shows that life has evolved on Earth for something like 3.8 billion years. How did it start though? Aside from some half-convincing experiments, the answer is unclear.

Panspermia is one possible answer: Frozen microorganisms in an animated state journey vast distances across the void of deep space and land on a foreign world. Once they arrive, these microbes reawaken and populate the new world. This sounds far-fetched, but some physics calculations show that it’s plausible.

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We’ve long looked for extraterrestrial life hitching a ride on an asteroid or other small piece of space debris, likely blasted off the surface of a life-bearing planet. This discovery (called lithopanspermia) is occasionally claimed by well-meaning, but over-enthusiastic, scientists. No space rock containing life or its fossilized remains has ever been found.

A second, less explored possibility exists: travel by riding the radiation pressure of a star (radiopanspermia). This is the same way that solar-sail spacecraft pick up speed through space.

Each photon of light streaming out from the sun carries some momentum. (Relativity gives photons momentum despite the fact that the photons possess no mass.) If the photon strikes a particle in space, the momentum is transferred. To facilitate the chances of this happening, the space particle needs to present as much surface area as possible for the sunlight to strike. At the same time, however, the more massive the particle, the less effect each photon’s momentum can accelerate it.

What’s the best compromise for speed? As the size of a roughly spherical chunk of matter increases, its mass goes up by roughly the cube of increase in diameter. The surface area, however only increases proportional to the square of the diameter. Thus, the smaller the speck of life is, the more quickly it will be accelerated by starlight.

A radiation pressure calculation can give hard numbers. A star the size of our sun could accelerate a tiny clump of bacteria from earth’s orbit to interstellar space at a velocity of about 0.01% of the speed of light, faster than any spacecraft we’ve built to date. This clump could reach nearby stars in tens of thousands of years and cross the entire Milky Way galaxy in less than a billion years.

Still, to make this journey, a life form would need to be tough enough to survive millennia or more in the frozen vacuum of space. Life we’ve seen on earth is closer to being able to do this than you might expect. Bacterial spores can survive millions of years. Tiny, tiny little animals such as tardigrades (“waterbears”) can survive freezing to near absolute zero and vacuum conditions. They would die long before reaching any nearby stars, however. Further, the DNA in our life forms would likely be cooked by the same UV solar photons.

The chances of radiopanspermia are greatly multiplied by the vast numbers of stars and planets in the universe. If such a life form evolved on only one planet in a billion, the existence of a trillion trillion or more stars, many orbited by planets, would make for a decent chance of light-born life blasting through the universe. It’s fascinating to contemplate, but a long shot that is nearly impossible to predict or test scientifically.

The problem, however, with any version of panspermia is obvious: It fundamentally answers no questions because it simply places the origin of life somewhere else in the universe. That’s why biologists continue mostly to assume that life probably originated on this planet.

Tom Hartsfield is a physics PhD student at the University of Texas.

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