The formation of the Solar System is often seen as a messy and chaotic affair, with the seeds of the future planets in a gravitational tug of war, which might have even thrown out a few objects. Amidst all the chaos, it appears that Neptune had a more relaxed upbringing.
According to a paper published in Nature Astronomy, the deep blue giant got to its current location 30 AU away from the Sun after smoothly transitioning from 20 AU (AU means Astronomical Units, and 1 AU is the distance between Earth and the Sun).
The crucial clue for this discovery is related to a curious type of object found in the Kuiper Belt, a region of icy rocks at the edge of the Solar System with 1,700 known objects. Almost all of these are red in color, but an international team of astronomers led by Dr Wes Fraser from Queen’s University Belfast instead looked at a handful of odd blue planetoid pairs.
It was thought that blue binaries formed in the heart of the Kuiper Belt, but the latest observations suggest that they actually formed much closer to the Sun, being slowly pushed out by Neptune's gravitational nudges. Neptune's movement must have been smooth otherwise the binary planetoids would have been disrupted into two separate objects.
“This research has opened the window to new aspects of understanding the early stages of planet growth. We now have a solid handle on how and where these blue binaries originated,” Dr Fraser said in a statement. “There has been some evidence around how Neptune moved outwards to 30 AU. Our hypothesis about how these blue binaries came to be where they are requires that Neptune’s migration was largely a smooth and calm movement."
Artist impression of a blue planetoid pair in the Kuiper Belt. Gemini Observatory/AURA, artwork by Joy Pollard.
In the past, the gas giant planet's migration has been associated with the late heavy bombardment, the deluge of space rocks that rained down on the inner planets 4.1 to 3.8 billion years ago. One explanation for how it occurred is the Nice model, which suggests that Neptune was pushed into the outer Solar System by Jupiter and Saturn, disrupting comets and asteroids and sending them inwards.
The Nice model requires a bumpy migration of Neptune to have happened around 900 million years after the formation of the Solar System. But the newer model would put it much earlier, at 50 million years after the Sun's birth, with a much smoother transition.
There are many possibilities for this. The late heavy bombardment might not be associated with the migration of Neptune, planets might have had later instabilities, or maybe we don't fully understand the intricate mechanisms that drive outwards migration yet.
"It seems we are still missing some of the details of how our gas giants got to where they were, and what scattering the planetesimals suffered as a result," Dr Fraser told IFLScience. "Even after more than a decade since the Nice model was proposed, we are still in the early days of studying it!"
This research is part of the Colours of the Outer Solar Systems Origins Survey, Col-OSSOS, which aims to understand the surface properties of the many hundreds of known Kuiper Belt objects (KBOs).
Achieving this goal was not easy, KBOs are small and faint, but the team used two world-class telescopes, the Gemini North Telescope and the Canada-France-Hawaii Telescope (CFHT), both on Mauna Kea in Hawaii, at the same time. This allowed them to observe the KBOs in visible, ultraviolet, and infrared simultaneously, providing the data to understand where objects like the blue planetoid pairs came from.