New Planet Migration Model and Planet Nine
Andy Lloyd
New Planet Migration Model and Planet Nine
Readers of my 2019 book 'Darker Stars' will be familiar with the currently accepted model of solar system formation, known as the Nice Model. The Nice Model involves migration of the gas and ice giants - Jupiter inwards and Saturn, Uranus and Neptune outwards. In the book, I highlighted some of the unresolved issues this complex theory faces. One of them is what triggered this colossal migration of the giant planets. A new hypothesis (published in Nature this month) has been proposed to address this problem:
"...the giant planets’ instability was probably triggered by the dispersal of the gaseous disk. As the disk evaporated from the inside out, its inner edge swept successively across and dynamically perturbed each planet’s orbit in turn...a few to ten million years after the birth of the Solar System." (1)
Computer simulations suggest this early event might have laid out the solar system's larger planets into their current positions before the terrestrial worlds, including Earth, even formed. There is an additional feature of this early assortment: An additional ice giant beyond Saturn required by the Nice Model.
The instability and the subsequent planetary migration involves the ejection of this third ice giant from the solar system:
"In this simulation, the inner disk edge pushes Saturn toward the ice giants, squishing the planets so close that they go unstable. The instability ejects one ice giant and leaves the planets close to their present-day orbits." (2)
Astrophysicist Sean Raymond, one of the authors of this new paper, has previously considered how the purported Planet Nine might have formed (in fact Sean considered all kinds of weird and wonderful stuff!). He suggests three scenarios - orbital instability, capture from a passing star, or ejection of planet-forming remnants by Jupiter and Saturn.
"Let’s rewind the clock to when the Solar System’s planets were forming in the protoplanetary disk. Gas giants Jupiter and Saturn formed quickly. Ice giants Uranus and Neptune grew from a population of large ice-rock bodies that were blocked by Jupiter and Saturn. Uranus and Neptune each underwent at least one humongous collision with another large body; we know this because their spin axes are tilted with respect to their orbital planets (they have significant obliquities, especially Uranus).
"The growth of the ice giants is not perfectly efficient. About half of the icy building blocks don’t end up in a stable planet but instead get kicked out, usually by Saturn or Jupiter. Planet Nine could be a large leftover that was kicked out. After being kicked onto a wide enough orbit, external gravitational kicks from the Sun’s birth cluster could shift Planet Nine onto its current orbit, like we saw above." (3)
So, such an ejection could have ended up producing a Planet X body - one of about 5 Earth masses according to Raymond's blog. If his new hypothesis is correct, then this mechanism could also account for a Planet X body through the ejection of an ice giant planet during the early dispersal of the gas disk. When I asked him about this on Twitter, Sean said that the presence of an extra ice giant wasn't as necessary in the gas disk dispersal hypothesis as it was in the original Nice model and if it was ejected it would likely end up in interstellar space (4). I pressed him a little further, quoting his 2016 speculation about the origin of Planet Nine. Tantalisingly, he replied with:
"Indeed -- there is more to come on that front in the coming months.... ;) " (4)
Written by Andy Lloyd, 28th April 2022
Links at https://www.andylloyd.org/darkstarblog88.htm
Image credit: Beibei Liu, Sean Raymond & Seth Jacobson (2):
