Researchers at the Max Planck Institute for Astronomy have discovered an extremely young super-Jupiter and measured its mass and radius. This is the youngest exoplanet of its kind for which astronomers have been able to determine these properties. And those properties are very puzzling, because we can’t easily explain them with our current understanding of planetary formation.
Planet HD 114082 b is located 310 light-years away in the constellation of Centaurus. It orbits a Sun-like star at a distance of about half that of the Earth from the Sun. It is the size of Jupiter, but weighs eight times as much. That makes it twice as dense as Earth, something that is not just remarkable for a gas giant planet – it simply doesn’t fit with the current best explanations.
“HD 114082 b is currently the youngest known gas giant planet with an established mass and radius,” Olga Zakhozhay, the principal author of the study, said in a statement. “Compared to currently accepted models, HD 114082 b is about two to three times too dense for a young gas giant with only 15 million years of age.”
The most popular model for planet formation is known as core accretion. Bits of material in the protoplanetary disk surrounding a young star collide and merge, accumulating into a solid core of rocky material that attracts gas leading to the formation of a gas giant. This process generates heat and is known as a “hot start” for a planet.
This scenario fits with a lot of the observations, but there are others that would better fit a “cold start” scenario. In this model, gas from material around the star cools down and contracts. Once it reaches a critical density, it collapses into a planet. A cool young planet would be denser than a hot one.
“It’s much too early to abandon the notion of a hot start,” co-author Ralf Launhardt explained. “All we can say is that we still don’t understand the formation of giant planets very well.”
For the core accretion model, planet HD 114082 b is too small. So, it either cooled much faster than expected or it has a much larger and denser core, or both. But two other very young Jupiter-like planets also seem to be better explained by a cold start.
“While more such planets are needed to confirm this trend, we believe that theorists should begin re-evaluating their calculations,” Zakhozhay stated. “It’s exciting how our observational results feed back into planet formation theory. They help improve our knowledge about how these giant planets grow and tell us where the gaps of our understanding lie.”
The results appear as a Letter to the Editor in the journal Astronomy & Astrophysics.