Suggestions that the hypothesized Planet 9 formed outside our solar system, or was even stolen from a rival solar system, have been dealt a blow by investigations into the likelihood of planetary capture by young stars.

The presence of a new planet on the outskirts of our solar system was famously proposed by Brown and Batygin in 2016 to explain unusual clustering in the Kuiper belt, a collection of small icy bodies that couldn’t quite form planets themselves. However, the idea raised questions as well as eyebrows.

“It’s very surprising there is a planet 10 times the size of the Earth out that far,“ says Richard Parker, an astronomer at the University of Sheffield.

Two theories emerged to explain the existence of something so big orbiting at these sparsely populated distances from our Sun.

The first suggested Planet 9 formed much closer to the Sun, perhaps near Mars, before the movements of Jupiter and Saturn threw it out.

A rival school of thought suggested Planet 9 could be a captured exoplanet, scooped up from interstellar space or even stolen directly by our young Sun from an early galactic neighbor. Whilst our Sun currently resides in a fairly empty patch of space where interactions with other large bodies are rare, we know stars often form in clusters containing anything from a few 10s to millions of celestial siblings.

In these clusters tightly packed young stars interact regularly, which is thought to synchronize their velocity and direction of motion, making it more than conceivable for one to capture another’s planet.

“It’s like having two people walking down the street in the same direction at the same time. It is very easy for them to hold hands.”

Even so Parker and his colleagues thought some of the assumptions made regarding the likelihood of exoplanet capture might have been overplayed in view of Planet 9’s very particular and unusual orbital properties.

To investigate, the team made use of software developed to simulate how stars capture others stars to create binaries.

“Like the binary problem, planetary capture is a purely gravitational process.”

For this new challenge Parker and his colleagues added in a population of ready-stripped, free floating Planet 9s. One was added for each of the simulations 1,000 stars, a ratio that Richard admits is probably an overestimate.

They then ‘evolved’ their model star-forming region for 10 million years, a process they repeated 10 times, each time randomly mixing up the stellar masses, and positions and velocities of all the objects.

Their results, published in the Monthly Notices of the Astronomical Society Letters, showed only 1 – 6 per cent of their free floating planets were ensnared by stars, even with the most optimal initial conditions for capture. Even less, just 10 of the total 10,000 simulated stars capture planets onto orbits that lie within the constraints for Planet 9.

“Planet capture happens reasonably often but if you are throwing away exoplanets with orbital inclinations of 60s degrees or more that gets rid of most of them.”

However the team weren’t finished. They then added in a dramatic additional constraining factor: a giant supernova.

There is a lot of evidence that a nearby supernova played a significant role in the formation and chemical makeup of our solar system. Models have shown that highly radioactive material ejected from these giant explosions can easily be captured by young suns and their newly forming planets. This source of additional heat could even be supporting ongoing plate tectonics on Earth.

By incorporating a supernova into their simulation Parker and his colleagues were now looking not just for successful planetary captures, but examples of capture by star systems that also absorb the levels of radioactive material required to give us the conditions of our present solar system. They found the chances of the two happening for the same system was virtually zero.

“Its 1-2 per 10,000 that have the conditions our solar system would require,” says Parker.

However, Alexander Mustill believes Parker and his colleagues’ focus on free floating planets ignores the potential for direct capture straight from another star.

“In this case the encounter required is less of a coincidence. The Sun encounters the star hosting Planet 9, and because Planet 9 is in orbit fairly close to that star you effectively only need a random meeting between two bodies and not three. So while I would agree that they’ve shown the capture of a free-floating planet is an unlikely origin for Planet 9, capture from another star is still a real possibility.”

Parker disputes whether incorporating direct capture would make much difference to the end result. He even suggests his own simulations should make capture easier than in Alex’s previous work due to his own efforts to give the new model planets similar velocities to the stars.

“I think the differences between mine and Alex’s simulations are because he deals with the encounters on a star-by-star basis, whilst we do the brute force approach and model the entire star-forming region, so the two methods do not overlap entirely. This is something Alex and I should discuss over a beer next time we meet!”

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Richard J. Parker et al. Was Planet 9 captured in the Sun’s natal star-forming region? MNRASL, published online September 6, 2017; doi: 10.1093/mnrasl/slx141