Super-Earths may form in two ways

Contrast in star composition hints at two categories for large rocky planets

Super Earth vs. Earth

BULKING UP   The formation of super-Earths like Gliese 832c, a potentially habitable planet roughly 5 times as massive as Earth (illustrated here), puzzles astronomers. New research suggests that not all super-Earths form the same way.

Planetary Habitability Laboratory/University of Puerto Rico at Arecibo

Super-Earths, rocky planets that are several times as massive as Earth, form in two different ways, a new study suggests.

Stars with super-Earths huddled up close are enriched in heavy elements such as iron, while stars where the super-Earths keep their distance are slightly deficient in those elements. Since planets form from the same reservoir of gas and dust as their stars, astronomers use the chemical makeup of a star to see what material was available to the growing planets. Wei Zhu, an astronomy graduate student at Ohio State University, suggests that the contrasting stellar environments are a clue that these planets formed in different ways. The research appeared online March 8 at

Super-Earths are super baffling, and astronomers struggle to understand how these rocky heavyweights formed. Zhu suggests that close-in super-Earths might have formed near where we see them today in disks brimming with planet-building material. The stellar enrichment reflects the bounty of heavy elements available, some of which rained down onto the star.

As for the more distant super-Earths, “honestly, I don’t know,” Zhu says. One idea is that some formed much farther from their stars, where ice grains could bulk up the planet, and then wandered in closer. But it’s not obvious that these planets can do that. “The formation of super-Earths is really unclear right now,” he says. “I don’t find any current theory trustable.”

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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