Out in solar system, Planet Nine or a stellar tryst?

It all began almost 200 years ago trying to explain the perturbations in the orbit of Uranus. The discovery of Neptune, which also marked a triumph of Newtonian mechanics, ushered in a new era. More and more minor planets were discovered, touching a million mark in all now. The fact that Pluto had so many siblings led to a new class of objects called the dwarf planets. The role of small bodies in the formation of the solar system is being seen in a new light now.

It is now generally believed that the solar system was formed from a protoplanetary disk consisting of gas and dust. Since the cumulative mass of all objects beyond Neptune is much smaller than expected, it should also be noted that these bodies have mostly inclined, eccentric orbits. The big question is, what is the process that restructured the outer solar system after its formation?

This has led to several interesting studies. One of them pointed to a massive planet beyond Neptune. Simulations with such a massive body in an elliptical orbit provided convincing answers to the orbits and so the argument favoured a ninth planet yet to be discovered. Quite interestingly, the researchers called this body as a perturber. The word “Planet Nine” was coined subsequently for this body as massive as Neptune or slightly less. Its orbit is believed to be highly eccentric. Thus its closest approach itself can be at a distance which is 20 times the orbit of Neptune. That makes it orbit the Sun once in about 10,000 years!

Another recent study involved a close fly-by of a neighbouring star. This can simultaneously lead to the observed lower mass density in the outer part of the solar system and excite the bodies there onto eccentric, inclined orbits.

Their numerical simulations show that many additional bodies at high inclinations still await discovery. This does not rule out a Planet X out there. This stellar encounter billions of years ago was effective in the outer parts of the solar system, not the inner regions.  

Our solar system was born out of a collapsing cloud of gas and dust. In the process that is well understood, a flat disk was formed creating large planets and smaller objects like the asteroids and dwarf planets. The flatness of the disk leads to the argument that the planets ought to orbit in a single plane unless something dramatic happened afterwards.

The dwarf planet Sedna, with an inclined, highly eccentric orbit and so far outside, posed the puzzle — Could it be because of a planet there? Or something more?

The clue was provided by the mass distribution. The cumulative mass of all the objects dramatically drops by almost three orders of magnitude at a distance just outside the orbit of Neptune.

The idea of a star approaching the Sun just when the planets were forming must have altered the composition and dynamics of the outer material.

This can also explain the inclined and eccentric orbits of the bodies there.

Computer simulations offer the perturbations by a star of the same mass as the Sun or half of it approaching at a distance three times the distance of Neptune. These simulations provide answers to several other questions on the mass ratio of Uranus and Neptune and two distinct populations of the bodies beyond Neptune. However, the unfavourable situation is the probability of such an encounter which is very rare. That takes us to another question — was the situation same four billion years ago? 

Thus we move from one puzzle to another. Is it the ninth planet? Or the stellar encounter? Or yet another brewing hypothesis? In the absence of any proof of the stellar fly-by, we have to revise our approach for a very simple explanation and the best fit for observable parameters.

(The writer is visiting scientist, Jawaharlal Nehru Planetarium, Bengaluru)

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