Astronomers Discover A New Class of Super-Earths

Illustration of one of the exotic super-Earth candidates, 55 Cnc e, which are rich in sapphires and rubies and might shimmer in blue and red colors.
(Illustration: Thibaut Roger)

Researchers at the Universities of Zurich and Cambridge have discovered a new, exotic class of planets outside our solar system. These super-Earths were formed at high temperatures close to their host star and contain high quantities of calcium, aluminum and their oxides – including sapphire and ruby.

Twenty-one light years away, in the constellation Cassiopeia, a planet by the name of HD219134 b orbits its star with a year that is just three days long. With a mass almost five times that of Earth, it is what is known as a super-Earth. Unlike our planet, however, it most likely does not have a massive core of iron, but is rich in calcium and aluminum instead. “Perhaps it shimmers red to blue like rubies and sapphires, because these gemstones are aluminum oxides, which are common on the exoplanet,” says Caroline Dorn, astrophysicist at the Institute for Computational Science of the University of Zurich. HD219134 b is one of three candidates likely to belong to a new, exotic class of exoplanets, as Caroline Dorn and her colleagues at the Universities of Zurich and Cambridge now report in the British journal MNRAS.

The researchers use theoretical models to study the formation of planets and compare their results with data from observations. It is known that during their formation, stars such as the Sun were surrounded by a disc of gas and dust in which planets were born. Rocky planets such as Earth were formed out of the solid bodies left over when the proto-planetary gas disc dispersed. These building blocks condensed out of the nebula gas as the disc cooled. “Normally these building blocks are formed in regions where rock-forming elements such as iron, magnesium and silicon have condensed,” explains Dorn. The resulting planets have an Earth-like composition with an iron core. Most of the super-Earths known so far have been formed in such regions.

More diverse than expected

But there are also regions close to the star where it is much hotter. “Many elements are still in the gas phase there and the planetary building blocks have a completely different composition,” says the astrophysicist. With their models, the research team calculated what a planet being formed in such a hot region would look like. They found that calcium and aluminum are the main constituents alongside magnesium and silicon, and that there is hardly any iron. “This is why such planets cannot have a magnetic field like Earth,” explains Dorn. And since the inner structure is so different, their cooling behavior and atmospheres will also differ from those of normal super-Earths. The team therefore speak of a new, exotic class of super-Earths formed from high-temperature condensates.

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