One planet is missing from the solar system, blame 'Jumping Jupiter'

Our solar system today has eight planets. Till a few years back, Pluto was our ninth planet, until it was demoted to a dwarf planet. But some studies suggest that the ancient solar system once still had nine planets, minus Pluto. A little-known theory states that once upon a time, there were five gas giants - Jupiter, Saturn, Uranus, Neptune and another planet. But, it is no longer here.

What happened to it? Where did it go? It might be deep in interstellar space, wandering like a rogue planet, thanks to Jupiter. Or, it could be the elusive Planet Nine everyone has been talking about these days.

It is believed that Jupiter kicked a planet out of the solar system billions of years ago. It was also a gas giant, but an icy world like Uranus and Neptune. The first time scientists proposed the existence of this other planet was in 2011. In the bid to explain the orbits of Earth and Mars, they reached the conclusion that things weren't always like this in the solar system. The current picture did not make sense.

This led them to propose that another planet once resided in the solar system, and was kicked out around four billion years ago. But, they could not pinpoint the culprit - it could either be Jupiter or Saturn.

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A study carried out by Canadian scientists in 2015 found that it was Jupiter after all that kicked out this mystery planet. "Our evidence points to Jupiter," said lead researcher Ryan Cloutier from the University of Toronto. It's like an "interplanetary chess game," he said.

Jupiter hasn't always been where it is today in the solar system. The "Jumping-Jupiter" scenario proposes that the largest planet of the solar system migrated inwards towards the Sun, and in the process kicked out a planet. It did not move smoothly, but made several jumps, leading to the term "Jumping Jupiter."

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Its gravitational influence was so strong because of its massive size that it likely disrupted the orbits of other gas giants. One of them got so close that it was ultimately slingshot out of the solar system.

This migration is further explained by the Grand tack hypothesis. According to it, Jupiter formed at a distance of 3.5 AU from the Sun, then migrated inward to 1.5 AU. It captured Saturn in an orbital resonance, ultimately stabilising at 5.2 AU.

Proof of the ninth planet

The Oort cloud at the edge of the solar system contains tonnes of icy bodies. Scientists think that if a planet was ejected from the solar system, it could have scattered planetesimals, thus creating the Oort cloud. Besides, the current structure of the solar system also proves that there was a fifth gas giant.

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All the comets, asteroids and other material in the solar system in the Kuiper Belt also likely occurred because of the planet's ejection.

How was the criminal planet nailed down?

Scientists at the University of Toronto decided to look at the moons of Jupiter and Saturn - Callisto and Iapetus, respectively. They investigated the probability of the moons having their current orbit if one of them had ejected a planet.

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"Ultimately, we found that Jupiter is capable of ejecting the fifth giant planet while retaining a moon with the orbit of Callisto," Cloutier wrote in the study published in The Astrophysical Journal.

Where is the ninth planet?

Scientists think that it could be wandering in the Milky Way, or maybe somewhere in deep space. However, for some years, scientists have been talking about a ninth planet, far beyond Neptune. They noted a change in the orbits of long-period objects in the Kuiper Belt, and think that Planet Nine could explain this strange phenomenon. Some think that this planet that was kicked out did not leave us after all, and in fact continues to orbit far off from the Sun, at about 400 and 800 AU (Astronomical Units) from our star. In comparison, Pluto is only 30 to 50 AU from our Sun.