A new study may help explain how the Solar System gained its final configuration. In addition, perhaps it clarifies how a hypothetical ninth planet orbiting the Sun tens of billions of kilometers away could have ended up there, far beyond the region through which Neptune travels, the farthest known.
Today everything seems well organized: four inner rocky planets, four more distant gas giants, an asteroid belt between the two and a second belt of icy objects after the eighth planet. But has everything been like this since it all began, 4.6 billion years ago? Back then, when the Sun was rising and found itself shrouded in a spinning disk of gas and dust – the cradle for the process of planet formation, things looked different. At least, that’s what the simulations made by scientists say.
They suggest, for example, that the giant planets formed at smaller distances from each other and only later spaced out in their current configuration. To explain how this happened, a group in 2005 proposed that an episode of dynamic instability between the planets ended, after considerable mess, leaving them in their current configuration. This model, known by the name of the city where it was conceived (Nice, France), assumed that this had happened several hundred million years after the dissipation of the protoplanetary disk. That is, it would be a late event, with an unknown trigger.
New work by Beibei Liu of the University of Zhejiang, China, Sean Raymond of the University of Bordeaux, France, and Seth Jacobson of Michigan State University, USA, appears to solve much of this mystery. It simulates the effect that the disk’s own dissipation would have on the planets, occurring from the inside out in the system, as the Sun “lit up” and the solar wind began to sweep away the surrounding gas.
Published in the latest issue of Nature, the study shows that this mechanism would naturally cause the spacing between Jupiter and Saturn to increase, in a kind of rebound effect, and would end up inducing instability in the two outermost planets, Uranus and Neptune. Even simulations with a fifth gaseous planet could have an outcome similar to that of the Solar System, with one of them being ejected out. And all this would happen not hundreds of millions of years after the birth of the Sun, but very fast, maybe 10 million years at most.
With this, what once seemed an inexplicable quirk in the evolution of the Solar System can now be seen as a typical and natural process. “The rebound effect may explain why dynamic instabilities appear to be nearly ubiquitous in exoplanetary systems,” the authors write. Finally, if the planet 9 (so far purely hypothetical) does exist, the new work makes it easier to explain how it got there, ejected during the period of instability.
This column is published on Mondays, in Folha Corrida.
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