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By Kartav Kesri, BS-MS 2020
Our solar system seems orderly and stable but something isn’t right, outer icy gas giants, Uranus and Neptune are in the wrong place.
REASON:- Our solar system grew from a disk of dust and gas but there wasn’t enough stuff to form these planets, where they both are currently orbiting.
So, here came the most accepted and possible theory for this explanation,
In this theory, it was considered that initially, both planets formed very close to the orbits of Saturn but the combined gravity of Saturn and Jupiter kicked both of these so much hard that it not only shifted their orbits but might have reversed their orbits. Yes! It is believed that the original order of gas giants was Jupiter, Saturn, Neptune, Uranus Because Neptune being the farthest planets is still not the coldest, it is because that Neptune is highly active than Uranus which leads to the conclusion that initially, Neptune must be closer to the sun to achieve its high activeness nature. Yes! Neptune is more active than Uranus which is surprising but the data sent back by the Voyager 2 about their measured heat flux, implies that Neptune is still cooling while Uranus is in equilibrium condition. The data strongly suggested that there is high volcanic activity inside Neptune, let’s discuss this topic some time else.
So coming back to the gravitational kicking, now everyone must be wondering why they are still in our solar system if they were pushed out of their orbits. It is because, during their journey, they collided with millions of leftover rocks, ice, and dust particles which acted as a brake for them( these particles were thrown even farther away due to the galaxy where they later contributed to the Kuiper belt).
Now, the only question which remains is that what is the basis of this theory?
In 2007, Steve Desch, an astrophysicist at Arizona state university, designed some models and a bunch of equations and stated that by the time both planets would have built up a solid core large enough to drag in helium and hydrogen for their atmospheres. So almost all of the gases would have drifted into interstellar space. If by luck those gas may not drift away, it will take billions of years to form both planets where they both are currently orbiting, but our solar system is not that much old. Therefore, he concluded that they must have created twice as close to the sun as they are now.
And soon after this, scientists started observing other solar systems. In a far-off solar system, WASP-47, they observed that a gas giant, WASP-47b, as big as Jupiter, is orbiting around its host star so close that it takes only days(4.2-5) to complete one revolution.
There is no way a gas giant could have formed this close in. the only explanation is that it must have formed far away from the star and was kicked in by the outer gas giants.
In 2014, Helled and Bodenheimer investigated the formation of Uranus and Neptune in the core accretion model accounting for different locations ranging from 12 to 30 AU (remember Uranus is at 20 AU and Neptune is at 30 AU from the sun) and with various disk solid surfaces densities and core accretion rates. They concluded that the formation of both planets is only possible through pebble accretion.
In 2015, Levinson et al. used all the Helled and Bodenheimer model results and calculated that for pebble accretion to occur, permissible distances from the sun are 5AU to 15AU. Now, this does prove that both of them must have formed closer than they are now.
Further simulations and using the data of Voyager 2, scientists claim that both planets must also have switched their positions during this process.
But still, there is not enough data to say that this exact thing must have happened back then. Therefore, no one can argue with any theory running in the market that suggests anything about the formation of these planets, but this is the most accepted version.
Well, our sun has its own version of this gravitational kicking and pulling. This theory says that our sun contains more lithium than a similar star should have, and the best-accepted explanation so far that a similar process may have occurred. But this time, the gas giant(rich in lithium) was thrown towards the sun and was engulfed by it. LOL!!
SOURCES/REFERENCES: How the universe works season 1, NBC News, https://link.springer.com/article/10.1007/s11214-020-00660-3
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