December 5, 2013
Astronomers Puzzled By Discovery Of Giant Planet Orbiting Distant Star
[ Watch the Video: What Planet Has Astronomers Puzzled? ]
Lee Rannals for redOrbit.com - Your Universe Online
Astronomers have found the most distant orbiting planet to date, and the puzzling discovery is effectively throwing a wrench in planet formation theories.
Scientists reported in The Astrophysical Journal Letters that exoplanet HD 106906 b is orbiting its star at 650 times the average Earth-Sun distance.
"This system is especially fascinating because no model of either planet or star formation fully explains what we see," stated Vanessa Bailey, from the University of Arizona’s Department of Astronomy and leader of the research.
Scientists believe planets close to their stars form from small asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star. However, this process acts too slowly to grow giant planets far from their star. Another theory is that giant planets form from a fast, direct collapse of disk material. However, primordial disks rarely contain enough mass in their outer reaches to allow a planet like the one the researchers found to form.
"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," Bailey said. "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."
Bailey said one problem with this scenario is that the mass ratio of the two stars in a binary system is no more than 10 to one.
"In our case, the mass ratio is more than 100-to-1," she said. "This extreme mass ratio is not predicted from binary star formation theories – just like planet formation theory predicts that we cannot form planets so far from the host star."
The planet weighs 11 times Jupiter’s mass and is only 13 million years old. The astronomers said the planet is still glowing from the residual heat of its formation, emitting most of its energy as infrared rather than visible light.
"Every new directly detected planet pushes our understanding of how and where planets can form," stated co-investigator Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands. "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."