Using a computer simulation, a team of astronomers from Sweden's Lund University suggests the possibility that Planet 9 is an exoplanet that was stolen by our sun approximately 4.5 billion years ago. If the theory is true, it will mark the first time that an exoplanet has been discovered in our solar system.
Exoplanets are currently defined as planets located outside of our solar system, although the current study suggests that a revision of this definition is needed. The team says that its data points to Planet 9 being captured by the sun and remaining in our solar system undetected since this event.
"It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there's probably one hiding in our own backyard," said Alexander Mustill of Lund University and lead author of the study.
Star clusters often make their way by each other, and during these encounters they can potentially "steal" one or more planets that are orbiting other stars.
Using a computer-simulated model, Mustill and his team suggest that this is likely what happened when the sun came into proximity of Planet 9 as it orbited another star.
"Planet 9 may very well have been 'shoved' by other planets, and when it ended up in an orbit that was too wide around its own star, our sun may have taken the opportunity to steal and capture Planet 9 from its original star," he said. "When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun."
"There is still no image of Planet 9, not even a point of light," he added. "We don't know if it is made up of rock, ice, or gas. All we know is that its mass is probably around 10 times the mass of earth."
Although the implications of the new study are exciting, more research will need to be conducted in order to determine that Planet 9 was stolen into our solar system by the sun. However, if the theory is correct, it will give us a better understanding of the sun and the Earth and the nature of exoplanets.
"This is the only exoplanet that we, realistically, would be able to reach using a space probe," Mustill said.
The findings were published April 26 in the Monthly Notices of the Royal Astronomical Society: Letters.