A PLANET THAT DOESN’T MAKE SENSE
Everything about exoplanet HD 106906 b is unusual. Not only is it super-heavy – 11-times the mass of Jupiter, our Solar System’s heaviest planet – it is also marooned in the dark reaches of its stellar neighbourhood. It orbits its star 650-times the distance Earth is from the Sun. Said another way, that’s over 16-times the average Sun-Pluto distance!
Our conventional models of planetary formation can’t square the presence of such a big planet that far from its host star. Planets are thought to form from a flat disc around a young star. The further out you travel in the disc, the fewer building blocks there are from which to fashion orbiting worlds.
One idea is that it is less of a planet and more of a little brother. Perhaps a tiny star formed at the same time as its bigger sibling. The trouble is that in binary systems the bigger partner is normally no more than ten-times the mass of its companion. In the case of HD 106906 b, the size difference is a factor of one hundred. How it came to be there remains a mystery.