'Impossible' Alien World is Impossible. Yet There It Is


A giant extrasolar planet, or exoplanet, has been discovered orbiting a distant star. But this is no "ordinary" alien planet -- it shouldn't exist. To put it bluntly, it's an affront to current planet formation theories.

HD 106906b is a gas giant exoplanet with a mass 11 times that of Jupiter. So far, this may not seem too strange; hundreds of massive explanets have been spotted in our galaxy. But this one is peculiar in that it orbits its star 650 times the distance the Earth orbits the sun. It's this 650 AU (astronomical unit) distance that is causing some serious astronomical confusion.

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"This system is especially fascinating because no model of either planet or star formation fully explains what we see," said Vanessa Bailey, fifth-year graduate student in the University of Arizona's Department of Astronomy and lead researcher of this study.

The exoplanet was discovered using the Magellan Telescopes' Adaptive Optics (MagAO) system, based in Chile. Adaptive optics are sophisticated laser systems used by ground based observatories to remove the atmospheric "wobble" when observing the night sky. This is the same effect as removing the "twinkle" from stars when viewed with the naked eye -- upper atmospheric turbulence causes the twinkle, the same effect that obscures the view for ground based observatories.

The leading planetary formation theory posits that planets grow from the agglomeration of smaller bodies, such as asteroids. Over millions of years, the planet gains mass as its gravitational field starts to pull in more and more dust, asteroids and other junk. However, this mechanism cannot be applied to HD 106906b -- at that orbital distance, this process acts slowly, making a planet of 11 Jupiter masses an impossibility.

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A second theory could be invoked: Did HD 106906b rapidly form from the rapid gravitational collapse of a knot of material in the star's protoplanetary disk? Again, this mechanism is more likely to occur very close to the host star where plentiful material can be found; at 650 AU there would be little material to trigger the collapse.

So, according to Bailey, that leaves only one explanation. But there's a problem with that one, too.

"A binary star system can be formed when two 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," said Bailey in a UA news release. "It is possible that in the case of the HD 106906 system the 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."

However, binary pairs exhibit a mass ratio typically no more than 10-to-1, a ratio HD 106906b clearly violates. "In our case, the mass ratio is more than 100-to-1," she added.

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So how the heck did the 13 million years old HD 106906b form? For now we just don't know.

The nature of the HD 106906 star system may help to unravel the mystery of the errant exoplanet's nature. As it's a young system, large quantities of gas and dust are still present. These "leftovers" from planetary formation may help astronomers better understand the nature of the young world, an opportunity too good to miss. We are basically looking into an astronomical Petri dish where the actual planet-building mechanism is being laid out to see.

"Every new directly detected planet pushes our understanding of how and where planets can form," said Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands and co-investigator of this study. "This planet discovery is particularly exciting because it is in orbit so far from its parent star. This leads to many intriguing questions about its formation history and composition. Discoveries like HD 106906 b provide us with a deeper understanding of the diversity of other planetary systems."

Source: University of Arizona

Image credit: Bailey et al./UA/MagAO

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