Iron World Appears to Be Lightest Exoplanet Ever Detected

The sport of hunting for exoplanets, or planets around other star systems, has a new featherweight champion — a tiny world designated GJ 367 b with about half the mass of the Earth. The lightest exoplanet found to date, GJ 367 b zips around its parent star in a speedy 7.7 days and is unusually dense, appearing to be made of almost pure iron.

Called a “super-Mercury” for its location and composition, the lightweight entity’s existence challenges aspects of planetary formation theories. It could be hinting that miniature worlds come in a much wider diversity than previously believed.

“It’s absolutely great to have found a planet like this,” said Diana Valencia, an astrophysicist at the University of Toronto Scarborough who was not involved in the discovery. “It’s mind-blowing.”

GJ 367 b orbits a cool red star located roughly 31 light years from Earth. Researchers initially spotted it using NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope that finds nearby worlds by measuring how much their stellar hosts dim as they move in front of the stars’ faces. Because this eclipse produces a little trough in the light received from a star, astronomers can use it to estimate a planet’s size.

The newly discovered world is around 5,700 miles across, about three-quarters that of our planet. Follow-up observations with the High Accuracy Radial Velocity Planet Searcher (HARPS), an instrument on a telescope at the La Silla Observatory in Chile, allowed researchers to figure out its mass.

The HARPS detector looks for the subtle wobble exoplanets induce on their parent stars, which gets stronger the heavier a planet is.

With its mass and diameter in hand, scientists could calculate GJ 367 b’s density, showing it is an outlier compared to most exoplanets. It is closer in size to Earth or Venus but with a composition more like Mercury, which is mainly iron.

“It’s a weird ball,” said Kristine W.F. Lam, an astronomer at the German Aerospace Center in Berlin and lead author of a paper out Thursday in Science.

Because it sits so close to its parent, one side of GJ 367 b likely always faces the blazing star. Its dayside temperatures should soar toward 2,700 degrees Fahrenheit, hot enough to melt rock and metal, making it a potential lava world, Dr. Lam added.

GJ 367 b offers scientists a way to study how worlds closer to stars form. The leading theory for how a place like Mercury was created is that, early in the solar system’s history, a rocky planet similar to Earth formed near the sun. Colossal space rocks subsequently pummeled this entity, stripping the world of its crust and mantle. This means Mercury is essentially a gigantic planetary core sitting next to the fiery sun.

But the problem with such a scenario is that it doesn’t entirely work, Dr. Valencia said. Collisions may have bashed proto-Mercury’s outer layers away, but the material wouldn’t get very far. Trapped by the sun’s gravity, the rocks and metals would stay in a close in orbit and eventually find their way back to the object’s surface.

It’s possible to invoke special circumstances for why this didn’t happen, but the existence of GJ 367 b and similar objects means such planets aren’t exactly rare, Dr. Valencia said.

“We have to think about how can you reliably, not sporadically, produce a super-Mercury,” she added.

One possibility is that there’s still something missing from models for planetary creation. Perhaps dense elements like iron somehow end up closer to a star during its younger days, Dr. Valencia said. For now, such an idea remains speculative, she added, though worlds like GJ 367 b might start nudging scientists in such a direction.

The team that discovered the petite planet is already planning more observations of the system. They would also like to use a giant telescope to pick up light from GJ 367 b, potentially uncovering whether it has an atmosphere or if its surface truly is molten.

Larger worlds have always been easier to detect than smaller bodies, and researchers have found an array of giant Jupiter-like entities with all manner of composition and orbital characteristics. As improved telescope techniques have opened up exploration at the other end of the size spectrum, odd little objects like GJ 367 b keep turning up.

“It jars you awake to remember that these planets have their own stories,” said Jonathan Fortney, a planetary scientist at the University of California, Santa Cruz, who also was not involved in the study. “It’s yet another way of showing that final planetary outcomes can be tremendously diverse.”

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