'Planet Nine' may not exist: Study

Scientists have found that there is low probability of the existence of the mysterious ‘Planet Nine’, a Neptune-mass world that may circle our Sun at a distance of about 64 billion to 225 billion kilometres.

Earlier this year scientists presented evidence for Planet Nine, leaving theorists puzzled over how this planet could end up in such a distant orbit.

“The evidence points to Planet Nine existing, but we can’t explain for certain how it was produced,” said lead author Gongjie Li, Harvard-Smithsonian Centre for Astrophysics (CfA).

Planet Nine circles our Sun at a distance of about 64 billion to 225 billion km, or 400-1,500 astronomical units.

This places it far beyond all the other planets in our solar system.

Researchers conducted millions of computer simulations in order to consider three possibilities. The first and most likely involves a passing star that tugs Planet Nine outward.

Such an interaction would not only nudge the planet into a wider orbit but also make that orbit more elliptical.

Since the Sun formed in a star cluster with several thousand neighbours, such stellar encounters were more common in the early history of our solar system.

However, an interloping star is more likely to pull the planet away completely and eject it from the solar system.

Researchers find only a 10 per cent probability, at best, of Planet Nine landing in its current orbit. The planet would have had to start at an improbably large distance to begin with.

Using computer simulations, researchers studied plausible scenarios for the formation of Planet Nine in a wide orbit.

“The simplest solution is for the solar system to make an extra gas giant,” said CfA astronomer Scott Kenyon.

Researchers propose that Planet Nine formed much closer to Sun and interacted with gas giants like Jupiter and Saturn.

A series of gravitational kicks then could have boosted the planet into a larger and more elliptical orbit over time.

“Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they’ll go higher and higher,” said Kenyon.

“Then the challenge becomes not shoving the planet so much that you eject it from the solar system,” he said.

That could be avoided by interactions with the solar system’s gaseous disk, he suggests.

Researchers also examine the possibility that Planet Nine actually formed at a great distance to begin with.

They found that the right combination of initial disk mass and disk lifetime could potentially create Planet Nine in time for it to be nudged by a passing star.

Researchers looked at possibilities of Planet Nine being an exoplanet that was captured from a passing star system, or a free-floating planet that was captured when it drifted close by our solar system.

However, they conclude that the chances of either scenario are less than 2 per cent.

The study was published in the Astrophysical Journal Letters.