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AN EXOPLANET remarkably similar to Mercury has been found, and it might shed light on why our diminutive neighbour has such a big heart.

Mercury has an immense iron core that makes up roughly 70 per cent of its volume. That鈥檚 massive compared with Earth鈥檚, which is just 30 per cent of its volume. But without another planet like Mercury in the solar system, astronomers have had a difficult time describing how such a wacky world formed.

Now, Alexandre Santerne at Aix-Marseille University in France and his colleagues have discovered an exoplanet that seems like a bigger version of Mercury. This larger cousin is called K2-229 b and lives in a planetary system 340 light years away (Nature Astronomy, ).

Santerne and his colleagues have studied the exoplanet with the La Silla Observatory in Chile. Although its radius is only fractionally larger than Earth鈥檚, it is 2.59 times the mass of our planet. The team modelled the planet to work out how it might have formed and found that it must have a massive iron core and a thin silicate mantle 鈥� making it more like Mercury than any planet discovered thus far.

One idea credits extreme heat for making planets turn out like Mercury. Consider K2-229 b, where it reaches a sizzling 2033 kelvin, or 1760掳C. 鈥淚magine Earth eight times hotter,鈥� says David Ciardi at the California Institute of Technology. 鈥淎ll of the atmosphere would evaporate; all the water would boil off and the surface rocks would probably melt and soften.鈥� Heat could easily vaporise the outer layers of silicate-rich rocks, leaving mostly metal behind.

Another hypothesis says a giant impact could shatter a planet, sending the mantle flying and leaving a world that was more core than not. Models can recreate this scenario for Mercury. If they work for K2-229 b, it will lend weight to this idea.

This article appeared in print under the headline 鈥淢ercury鈥檚 big cousin found in distant system鈥�