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A test for creating a family tree for aliens

The Argentinian city of Cordoba held its Alien Festival on Feb. 13. Natacha Pisarenko/AP

A pair of researchers at Harvard is preparing for the day when humans encounter alien life.

“Not in the next 5 or 10 years, but maybe in the next 20 or 30,” says Henry Lin, co-creator of a statistical test that would allow the astronomical community to determine whether blooms of life on different planets are one-off events or related to each other.

“The natural questions is, how could we ever tell if life formed independently versus a scenario where somehow life is being transmitted between planets,” says Lin, an undergraduate at Harvard who posted a paper describing the process on the astrophysics archive over the summer. “Surprisingly, there’s a simple test you can do which had not been thought of yet.”


The idea that life on distinct planets might be related is known as “panspermia.” In this view, life arises on one planet and is transported to others, either through space exploration or, more likely, incidentally aboard a meteor. The test Lin and his coauthor, Harvard astronomer Avi Loeb, came up with is designed to analyze discoveries of life on exoplanets—planets outside our own solar system.

Hopes for such discoveries have largely been fantastical, but that might be changing. In 2018 NASA plans to launch the James Webb Space Telescope (JWST), which will have an unprecedented ability to analyze the atmospheres of exoplanets. If JWST — or its successor telescopes — detects certain chemical signatures, like the presence of molecular oxygen, chlorophyll, or industrial pollution, it could be telltale evidence that life exists on those planets.

That’s when Lin and Loeb’s work would spring into action. If we were to detect evidence of life on, say, 25 different planets, their test would allow astronomers to study how those planets are distributed and make a good guess about whether or not they tie back to a common source.


“When you look at the statistical probability of where life is found, you can try and infer whether panspermia was responsible for transferring life from one system to another,” says Loeb.

Lin makes a similar point, explaining that if life has the ability to spread from planet to planet, it’s reasonable to imagine it might do so in the way a virus spreads through a population of people.

“Viruses don’t just randomly appear amongst all people uniformly, they form in these sort of bubbles,” Lin says. “This is similar to what we might expect in space.”

It may be decades or longer before we’re able to detect alien life, if it exists at all, but other astronomers think Lin and Loeb’s test is well-designed to tell us something meaningful, should detection occur.

“It’s kind of exciting to step beyond [the challenge of finding life] and take a flight of imagination and think, what if we really did turn on the detectors, light up the galaxy, and see lots of planets with life,” says Gregory Laughlin, an astronomer at the University of California, Santa Cruz.

Laughlin also thinks that while Lin and Loeb’s test is useful, the first discoveries of extraterrestrial life are likely to be much closer to home and much easier to explain.

“I’m pretty confident we will find life on Mars and I’m also pretty confident it will turn out to be clearly the same genesis as life on Earth,” he says. “Either life started on Mars and was delivered to Earth, or vice-versa. Or it could have arisen on Venus.”


Kevin Hartnett is a writer in South Carolina. He can be reached at