Scientists have long suspected that our solar system was once home to a mysterious planet -- similar to the four giants Jupiter, Saturn, Uranus and Neptune -- but something happened about four billion years ago that caused the planet to disappear.
Now, a team of astrophysicists in Toronto have made significant steps towards solving the mystery.
"This is consistent with our expectation that if you want to eject a planet from the solar system, then you likely need a massive planet," Ryan Cloutier, a Ph.D candidate in the University of Toronto's department of astronomy and astrophysics and lead author of the new research, told The Huffington Post in an email. "Although our results may not have been that surprising, I was very excited."
The researchers created computer simulations of our solar system's four giant planets and their moons, including Jupiter's moon Callisto and Saturn's moon Iapetus.
In a process of elimination, the researchers then measured whether each moon in the solar system would have still followed its current orbit if its host planet was responsible for ejecting the ancient lost planet some four billion years ago.
The researchers found that Jupiter, the largest world in our solar system, was the only one capable of ejecting the lost planet while retaining the current orbits of its moons.
For instance, in order for Saturn to have ejected the long-lost planet, the collision of the two planets would have been so violent that Iapetus's orbit around Saturn would have been thrown off course.
"Conversely," Cloutier said, "if you run the same experiment with Jupiter you find that Jupiter is capable of ejecting the fifth giant planet whilst retaining an orbit of its moon Callisto."
The researchers concluded that a planetary collision with Jupiter was sufficient to eject the lost planet -- which they believe was an ice giant like Uranus and Neptune -- out of our solar system. Their simulations also provide evidence that the long-lost ice giant indeed was once part of our solar system.
"I was happy because this meant that we don't refute the existence of the fifth giant planet," Cloutier said. "Having had a fifth giant planet that got ejected from our solar system billions of years ago is cool."
It turns out that the planet may have had a bumpy ride before it was booted from our solar system. In August, a separate team of researchers created computer simulations that suggested the ice planet may have knocked into Neptune before it was ejected.
Dr. David Nesvorny, an astronomer at the Southwest Research Institute in Boulder, Colorado, who led the Neptune study, told HuffPost that his simulations seem to align with the findings in the new research.
"In fact, it is expected from [our] modeling ... that Jupiter, and not Saturn, should eject the hypothetical third ice giant," Nesvorny said.
The new findings not only hold clues to how our own solar system evolved, but also the researchers noted that they could shed light on how other planetary systems may behave.
"Developing an intimate understanding of our own solar system, especially at times long before we were even around, has important implications for our understanding of exoplanetary systems and how they compare to us," Cloutier said.
"Is our solar system unique," he added, "or does our evolution and current state fit nicely within the framework set by the thousands of other solar systems that we know about today?"
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