A surprise that throws a wrench in planet formation theories : an international team of astronomers in the University of Arizona discovered the most distantly orbiting planet found to date, HD 106906 b, orbiting at ... 654 AU around its star.

We know a few things about this Super-Jupiter. Detected by imaging, HD 106906 b mass is 11 times Jupiter's mass. Its star has approximatively the same size as the Sun but has 1.5 times its mass. Both (planet and star) are only 13 million years old.

Now the problem is how the planet can be so far from its star. 654 AU equals 98 billion kilometers. In our solar system, the distance Sun-Neptune is 30.11 AU (4,5 billion km) and Sun-Pluto 39.48 UA (5.9 billion km). How can the planet be so far of HD 160906 ? The answer is in the moment the star was born, but what is it ?

According to Vanessa Bailey, 5th-year graduate student in the University of Arizona's Department of Astronomy, no model of planet or star formation can explain it completely. However a theory to explain it is about the formation of a binary star system. "A binary star system can be formed when 2 adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit. It's possible that in the case of the HD 106906 system, star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."
The problem in this stuation is that the mass ratio of the 2 stars in a binary system is normally no more than 10-to-1. And here, the mass ratio is more than 100-1, rejecting the binary system theory.

Other strange thing. You probably think that, because it is so far from the star, HD 106906 b is cold (maybe -100 or -150°C). False. Because the planet is very young, it glows from the residual heat of its formation, and, indeed, the planet is very hot, approximatively 1500°C, but the host star is much hotter. By the way, the planet emits most of its energy as infrared rather than visible light. We could think that the planet is a hot Jupiter, except that it doesn't orbit very close to its star.

We see a new time that discovering new exoplanets with incredible particuliarities leads us to rethink planet formation theories, and to see what universe is able to do.

Source : phys.org

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