Astronomers just found a planetary system that blows up the simple version of how planets “should” line up.
The star is called LHS 1903, a red dwarf. Researchers already knew it had three planets. The innermost one looks rocky, followed by two gas-heavy planets, which fits the basic expectation. Then new observations with ESA’s CHEOPS space telescope, though, revealed a fourth planet farther out, and that one also looks rocky. So the order goes rocky, gas, gas, rocky. That ain’t right.
That last rocky planet is the problem child, because distance usually helps a planet keep gas. Close to a star, intense radiation can strip lighter gases away, leaving smaller rocky worlds. Farther out, the environment tends to support thicker atmospheres, which is one reason big gassy planets often live at larger orbits. A rocky planet sitting outside two gaseous planets forces the researchers to explain how it ended up there without turning into a mini gas giant.
Thomas Wilson, a planetary astrophysicist at the University of Warwick and lead author on the study published in Science, put it plainly. “That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-and then rocky again,” he said in a press release, noting, “Rocky planets don’t usually form so far away from their home star.”
An Inside-Out Solar System Makes No Sense
The team worked through a few standard explanations and didn’t find any of them satisfying. Their favored scenario is more awkward and more interesting. The planets might have formed one at a time, instead of forming together in a shared rush. In that version of events, earlier planets form while the system still has plenty of gas available. Later on, the gas supply drops. If the outer planet formed after that depletion, it could end up small and rocky even at a distance.
Wilson described it as possible “first evidence” for a planet forming in a “gas-depleted environment,” meaning the material needed to build a thick atmosphere might not have been available anymore.
Isabel Rebollido, a planetary disc researcher at the European Space Agency, summed up why astronomers care. “Historically, our planet formation theories are based on what we see and know about our Solar System,” she said, adding that the growing catalog of weird exoplanet systems keeps forcing researchers back to the whiteboard.
This is how space works. Every time humans get comfortable with a clear explanation, a distant star throws a new curveball and makes everyone write another paper.
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