Blueberry Earth: The Delicious Thought Experiment That's Roiling Planetary Scientists

Updated on August 3, 2018

Hey, it’s summer! It’s blueberry season! Can I offer you a thought experiment on what would happen if the Earth were replaced by “an equal volume of closely packed but uncompressed blueberries”?

When Anders Sandberg saw this question, he could not let it go. The asker was one “billybodega,” who posted the scenario on Physics Stack Exchange. (Though the question was originally posed on Twitter by writer Sandra Newman.) A moderator of the usually staid forum closed the discussion before Sandberg could reply. That didn’t matter. Sandberg, a researcher at Oxford’s Future of Humanity Institute, wrote a lengthy answer on his blog and then an even lengthier paper that he posted to arxiv.org, a repository for physics preprints that have not yet been peer reviewed.

The result is a brilliant explanation of how planets form.

To begin: The 1.5 x 10²⁵ pounds of “closely packed but uncompressed” berries will start to collapse onto themselves and crush the berries deeper than 11.4 meters—or 37 feet—into a pulp. “Enormous amounts of air will be pushing out from the pulp as bubbles and jets, producing spectacular geysers,” writes Sandberg. What’s more, this rapid shrinking will release a huge amount of gravitational energy—equal to, according to Sandberg’s calculations, the energy output of the sun over 20 minutes. It’s enough to make the pulp boil. Behold:

The result is that blueberry earth will turn into a roaring ocean of boiling jam, with the geysers of released air and steam likely ejecting at least a few berries into orbit. As the planet evolves a thick atmosphere of released steam will add to the already considerable air from the berries. It is not inconceivable that the planet may heat up further due to a water vapour greenhouse effect, turning into a very odd Venusian world.

Deep under the roiling jam waves, the pressure is high enough that even the warm jam will turn to ice. Blueberry Earth will have an ice core 4,000 miles wide, by Sandberg’s calculations. “The end result is a world that has a steam atmosphere covering an ocean of jam on top of warm blueberry granita,” he writes.

The process is not so different from the birth of a planet out of a disc of rotating debris. The coalescing, the emergence of an atmosphere, the formation of a dense core—all of these happened at one point to the real Earth. And it is currently happening elsewhere in the universe, as exoplanets are forming around other stars in other galaxies.

Sandberg has been thinking about exoplanets at Oxford’s Future of Humanity Institute, where he takes an interdisciplinary approach to considering the long-term future. He’s also written about the Fermi paradox, which gets at the question of why we haven’t seen evidence of extraterrestrial life. He originally got into Physics Stack Exchange to ask his own questions about physics; he quickly stayed and started answering others. Now, he says, “I’m kind of addicted.”

The blueberry Earth question jumped out to Sandberg because it reminded him of the hypothetical questions answered in xkcd creator Randall Munroe’s “What If?” series. “The important thing is getting enough weird-enough questions,” says Sandberg. And blueberry Earth, he realized, was a great weird question—absurd enough to amuse non-physicists while actually teaching a great deal of physics in the answer. He’s now planning to submit the paper to a physics education journal.

“It cracked me up,” Catherine Cooper, a planetary scientist at Washington State University, says about why she agreed to read a paper on Earth turning to a giant clump of blueberries. “This would be a fun assignment for a student,” she adds, perhaps by assigning them to sub in another fruit.

“Of course you can think, ‘What about watermelons?’” Sandberg later told me over the phone. “Watermelons are much more resistant to pressure than blueberries. But not much. Now you get this mixture of pulp and shells. It’s probably not going to be as tasty.”

Sandberg’s descriptions are all unusually vivid and sensory. He credits it to playing sci-fi role-playing games. He would imagine, for example, being on Titan and smelling the atmosphere leaking into the space habitat. When he and his friends played the Terraforming Mars board game, they would get into discussions about how Martian gravity would change the design of chimneys, and how Martian Santa Claus might get down those Martian chimneys on Christmas Eve.

Hunter Waite, who has been an investigator on multiple NASA missions including Juno and Cassini, calls the paper a “cute illustration of some of the physics involved in planetary studies.” But, he says, it was lacking in chemistry—as in, what new molecules could form out of the boiling-jam ocean and atmosphere? So much of life depends on the specifics of Earth’s chemistry. Sandberg does gesture at the possibility of fermentation that turns sugars in the blueberry pulp into alcohol. (He did not say whether this would lead to swimmable blueberry wine oceans, or whether it would all evaporate into an alcohol fog.) Enzymes in the blueberries could also catalyze chemical reactions. But there’s a lot more chemistry to consider.

To consider blueberry Earth at all, though, is to begin with a an inert clump of berries and end up with a roiling, dynamic planet with geysers, convection currents, boiling oceans and more. Planetary processes are usually so slow, they are practically invisible to us. But, as blueberry Earth shows, there is so much going on beneath our feet and in those faint dots in the sky. “That’s what gets people like myself interested in this as a career,” says Waite.