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Did our solar system lose a planet?

Artist's concept of the hypothetical Planet Nine Caltech/R. Hurt [IPAC]

Although Pluto lost its status as "Planet Nine" when it was downgraded to dwarf planet, there is ample evidence that our solar system either had or currently has a large planet far beyond Pluto that may one day claim Pluto's former mantle and become the rightful ninth planet. Unusually regular orbital patterns observed in the Kuiper belt hint that some celestial body more massive than Pluto lurks beyond the distant band of icy debris at the edge of the solar system where Pluto, Eris and other dwarf planets live.

The hypothetical existence of a distant Planet Nine or "Planet X" remains contentious, but evidence continues to mount in its favor. Certainly, it would not be the first time a hypothetical planet was found. Neptune was the first planet found through studying orbits of other bodies in the solar system; intriguingly, its location was discovered with predictions derived from pen-and-paper calculations about telescope observations.

Inadvertently, a recent astronomy paper in Nature found a high likelihood that a gas giant, akin to those in the outer solar system, may have been rapidly ejected from its orbit around the sun early in the evolution of a solar system. The existence of a "lost" Planet Nine early in the formation of the solar system's history would go far in explaining a lot of how and why the solar system looks as it does today. 

RELATED: What scientists know so far about Planet Nine

Modeling the birth and evolution of feasible star systems, the team of scientists collaborating from China, France, and the United States, ran approximately 14,000 simulations of the early solar system to figure out how it got to looking as it does today, with four terrestrial planets and an asteroid belt orbiting near the sun, four gaseous planets orbiting further out, and a scattering of cold rocky bodies beyond the gas giants.

"What's really cool is exoplanet astronomers have already confirmed that a very high percentage of both gas giant systems as well as super-earth systems have gone through planetary system instabilities, and we think the solar system is similar," Jacobson continued.

Intriguingly, simulations strongly suggest that there was an early instability in the orbits of the giant planets — Jupiter, Saturn, Uranus, Neptune, and possibly Planet Nine. Such bodies would have been much closer to the proto-Sun at one point, before gas coalesced into the sun and it really triggered strong fusion reactions that expelled gas and dust outwards, including said planets. This, scientists think, triggered a rapid and chaotic displacement to their current orbits.

The simulations suggest that in the early days, the gas giants had very circular and regular orbits at regular intervals from the sun; after the nascent star began pressuring them outwards, they experienced an unstable transition from compacted, even orbits in line with the plane of the disk to current orbits.

Professor Seth Jacobson of Michigan State University, who was involved in the study, called this a "universal source of planetary instability in the galaxy." 


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"We think all disks go through this, what astronomers call a transition disk phase, where the disk is being photo-evaporated from the inside out," Jacobson told Salon, referring to the proto-planetary disk of gas and dust that prefigured our (and all) solar systems. We can see nascent solar systems forming around the galaxy in the similar way, which suggests there's a similar pattern to how all solar systems form. 

"What's really cool is exoplanet astronomers have already confirmed that a very high percentage of both gas giant systems as well as super-earth systems have gone through planetary system instabilities, and we think the solar system is similar," Jacobson continued.

Within a collapsed cloud of stellar debris — a gaseous solar nebula and likely the remnants of a dead supernova —our proto-sun started to turn on the heat. Heating and ionizing gaseous elements in the disk, energetic photons emissions from our young sun eventually expelled the gas from the protoplanetary disk via evaporation.

The inner edge of this gaseous disk would theoretically "drag" the planets with it as it expanded outward. The initial position of the gas giants in the inner solar system would have been "a very robust trigger for instability," Jacobson said. That could have swung a Planet Nine-type world out of the solar system — forever.

Indeed, in 90% of simulated scenarios, this instability was triggered. Planetary orbits have been stable for billions of years in our solar system. The mystery of our solar system's early evolution, however, is still unclear. Location of the Trojan asteroids of Jupiter and the irregular satellites of the giant planets point to a chaotic reshuffling as does the varied composition of the Earth and its moon, which would require a great deal of mixing of different bodies. (It is widely believed that a Mars-size body called Theia collided with the early Earth, and the sloughed-off material formed the Moon.) 

Experts now realize timing in the migration of giant planets was a problem. Geological evidence has also radically outdated the timescale of this model, known as the "Nice" model (as in Nice, France): specifically, a series of three papers appeared in a single issue of Nature laid out a solution, originally suggesting the giant planet instability event occurred roughly half a billion years after the solar system formed, and would have relied on a gravitational encounter between two planets to set off a chain of destabilizing reactions. 

"Instability would always occur very early in solar system history a few million years after the start," Jacobson added. "The sun would still be in its stellar cluster at that time. If there was an ejected ice giant, then that ejected ice giant might not have truly been ejected. It might have been caught on this elliptical orbit."

If the ejection was too late, it likely would become a rogue planet. In this scenario of movement, starting within 10 million years of formation rather than 500 million years into the life of the solar system, the nursery stellar cluster the system is born in can intercept the runaway planet. The result is an extended elliptical orbit.

"During the lifetime of a nebular protoplanetary disk, the amount of gas in the disk is decreasing with time," Jacobson emphasized. "It's only when the disk has already gotten the amount of gas in the disk has already gotten quite low that the photoevaporation effect can take place. The photoevaporation effect then moves pretty rapidly. The transition disk phase actually is quite short and it clears out the disk from the inside out." The effect is similar to that of a puddle of water around a fireplace, where the water closest to the fire evaporates quickly and that further out takes a bit longer.

Jacobson said the moving-around of planets was a surprise result of the simulation. "What I think even we didn't completely appreciate until after we had started these simulations is that there's still enough gas in the disk and this process still takes enough time that it can significantly affect the orbits of the planet as the process takes place," he noted. 

Why the solar system looks like it does: