Mercury’s small size is deceptive. Unlike the other rocky planets in our solar system, it has an immense iron core that makes up roughly 70 per cent of the planet’s volume. That’s massive compared to Earth, whose core makes up only 30 percent of its volume.
It’s also unexplained. Without another example like it in the solar system, astronomers have had a difficult time describing how such a wacky world formed.
That might be about to change. at the Aix-Marseille University in France, and his colleagues have discovered Mercury’s twin – an exoplanet called K2-229 b – in a planetary system 340 light-years away.
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Metallic Mystery
At first, they had no reason to think it didn’t resemble Earth. Its radius, after all, is just 1.165 times larger. But when Santerne and his colleagues later observed the world with the La Silla Observatory in Chile, they realised it might be rather different. That’s because it weighs 2.59 times the mass of the Earth.
To explain the diminutive yet overweight exoplanet, the team built a model to replicate its inner structure. From this, it was clear that the planet had to be composed of a massive iron core and a thin silicate mantle — making it more like Mercury than any planet discovered thus far. That makes it a unique laboratory to better understand how these oddities form, Santerne says.
Evaporation or impact
One theory blames superhot temperatures. When a planet snuggles up close to their host star, the intense radiation could easily transform even an Earth-like world into a Mercury-like one.
Consider K2-229 b, where temperatures reach a sizzling 2033 kelvin, or 1760°C. “Imagine the Earth eight times hotter,” says at the California Institute of Technology in Pasadena. “All of the atmosphere would evaporate; all the water would boil off and the surface rocks would probably melt and soften. And then eventually some of that would evaporate.”
The heat could easily vaporize the outer layers of silicate-rich rocks, leaving mostly metal behind. But another hypothesis suggests that a giant impact could shatter a planet. This would send the mantle flying and leave behind a planet that was more core than not. Though models can recreate this scenario for the solar system’s Mercury, Santerne is curious if those models will still hold for K2-229 b, which is much larger. If they do, it will lend weight to this hypothesis for the formation of Mercury-like worlds.
For now it remains an open question, but Mercury’s long-lost cousin may shed light on how extremely dense worlds come to be.
Nature Astronomy
Read more: 2018 preview: Epic mission to Mercury will unravel its mysteries
Article amended on 3 April 2018
Correction:Â We have corrected a temperature conversion in this article.
