There are two super-Earths in the K2-106 star system, one of which is a metal ball

Artist's concept of the star K2-106 and its planets (Image courtesy / Vincenzo Guido, Emilio Molinari)
Artist’s concept of the star K2-106 and its planets (Image courtesy / Vincenzo Guido, Emilio Molinari)

An article published in the journal “Astronomy & Astrophysics” describes the study of two super-Earths in the system of the star K2-106. A team of astronomers led by Eike W. Guenther of the Thuringian State Observatory in Tautenburg, Germany used various telescopes to collect the data needed to determine the characteristics of the two exoplanets. The conclusion is that they’re both larger than the Earth but one is unusually dense while the other has a significantly lower density.

About 825 light years away from the Earth, the star K2-106 is a little smaller than the Sun and has an estimated age very close to it of about 5 billion years. In 2016 around it two exoplanets were discovered thanks to the transit method, meaning thanks to the fact that they periodically pass in front of their star. The team led by Eike W. Guenther used a total of five spectrographs in various parts of the world to obtain more detailed information about them.

The result was surprising but not because the two exoplanets are super-Earths, a type of discovery that is becoming almost common, so much so that at the beginning of this month a similar one was described in another research concerning the star K2-18 again in the journal “Astronomy & Astrophysics”. The surprise came from their characteristics, concerning both their orbital periods and their very different density.

The exoplanet K2-106b is the innermost one and has a year that lasts only 14 hours. Very few planets have been discovered so far with what is called an ultra-short period, meaning a year that lasts less than an Earth’s day. Its diameter is about 1.5 times the Earth’s but its mass is 7.7 times the Earth’s and this means that its density is truly remarkable, one of the highest calculated so far in an exoplanet.

The exoplanet K2-106c is also close to its star with a year that lasts 13.3 Earth’s days, very short compared to Mercury’s but long when compared to its neighbor’s. It’s larger than K2-106b with a diameter about 2.6 times the Earth’s while its mass is 6.8 times the Earth’s. This means that its mass is just lower than its neighbor’s but is much larger than it with the consequence that its density is much lower.

Between the two exoplanets, K2-106b is the most curious due to its proximity to its star and its density. Eike W. Guenther stated that more than a rocky planet it’s a metal planet calling it “heavy metal” because he and his colleagues estimated that 80% of its mass must be made of metals.

A system with two planets so different in terms of composition and distance from their star is interesting because it can provide new data useful to improve our models about the formation and evolution of planets. It’s a type of research that’s becoming more and more sophisticated thanks to the continuous discovery of new exoplanets and the availability of new instruments to study them more in-depth.

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