The mystery of the distant planet

The young astronomer Vanessa Bailey from the University of Arizona and her team have discovered the most distant exoplanet from its parent star. This is the result that undermines the current theories on star and planet formation. Crucial to its identification have been the Magellan telescope shooting in the Chilean Andes equipped with the adaptive optics system Magao developed with the help of INAF- Arcetri Astrophysical Observatory.



A newcomer to the Guinness Book of Records among extrasolar planets is HD 106 906 b, a giant 11 times more massive than our Jupiter, but far from its parent star 650 times the distance between the Earth and the Sun. For comparison, the Pluto’s orbit goes ‘just’ 50 times the Earth-Sun distance (or 50 astronomical Units, AU). The discovery of this planet so far from its star was made by Vanessa Bailey, a young graduate of the University of Arizona with the help of her team, using the images collected by the Magellan Telescope equipped with adaptive optics system Magao and the instrument for observations in infrared Clio2.

“The system we have discovered is particularly fascinating because no theoretical model on both the stellar planetary formation that is able to fully explain what we observe,” says Vanessa Bailey. In fact, with the present knowledge it is very difficult for astronomers to justify the presence of a massive planet at a distance from the star so far around which it is orbiting. It is believed that the planets close to their stars, such as ours, are formed from material increasing from small rock structures present in the protoplanetary disk of gas and dust surrounding the star in the making. However, this process is accomplished in a short period of time to create the giant planets so far as HD 106 906 b. An alternative hypothesis is that the giant planets are produced by very rapid and localized collapse of material from the protoplanetary disk. But even so, these structures should possess a considerable amount of gas at distances so far to justify the process that would produce HD 106 906 b. And also that such a scenario is unlikely.

When cornered by the observational evidence of this exotic planetary system, astronomers propose an additional hypothesis to justify the existence of the unusual cosmic couple: the observed planet may have formed by the same process of a mini binary star system. “It is possible that in the case of the system HD 106 906 b of the star and the planet, they are formed by the collapse of two distinct clusters of gas, but for some reason the progenitor of the planet has been deprived of a portion of that material and has no longer reached sufficient mass to become a star in his time, ” says Bailey. We still have to justify the relationship between the masses of the planet and the parent star, which appears to be from the observations of one percent (i.e., the HD 106 906 is a hundred times more massive than its planet). The current models of star formation in binary systems require that the mass of a star is not more than ten times that of its companion.

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