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Saturn's birth place discovered

Saturn's birth Saturn's position in our solar system hasn't changed since the ringed-planet was first formed, according to a study of the chemical composition of its biggest moon Titan.

The findings reported on the pre-press website ArXiv.org, will affect existing theories about the extent of planetary migration during the early evolution of the solar system.

The study by astronomers at the University of New South Wales, examined ratios of hydrogen and its isotope deuterium in Titan's atmosphere.

Previous research has found the deuterium to hydrogen ratio in ices condensed out of the primordial cloud that formed our solar system, changes with distance from the early Sun.

By studying these ratios, astronomers can determine where planetary bodies would have formed and how far they've migrated since.

Determining this ratio on Titan allows researchers to pin-point where Saturn was formed.

"The results were a surprise," says lead author Dr Lucyna Kedziora-Chudczer.

"It's quite unexpected to see that our solar system giants didn't move a lot.

"It means the very violent migration seen in other solar systems probably didn't take place in our solar system."

Hot Jupiters

Planetary migration has become key to understanding a type of exoplanet known as a hot Jupiter, which orbits extremely close to its host star.

These planets couldn't have formed where they are, and so must have migrated inwards from more distant orbits.

Planetary migration is also needed to explain many features of our own solar system including the existence of the ice giants Uranus and Neptune.

"Both these planets must have formed closer to the Sun as there wouldn't have been enough material to form them at their current locations, says co-author Dr Jonti Horner.

"Some theories indicate Uranus and Neptune may have swapped positions before migrating further out, while another says they formed between the orbits of Jupiter and Saturn.

"These theories also explain the orbits of small icy Kuiper Belt objects called Plutinos, which were swept into their current positions as Neptune migrated outwards."

According to Horner, the answer may be that some planets migrated more than others.

"If Jupiter migrated inwards, there would have been less material to form Saturn," says Horner.

"And it's this material which the planets interact with, that drives migration, so Saturn wouldn't have moved much."

Not out of this world

"The idea that Saturn formed where it did, migrated and somehow returned to the same location isn't as ridiculous as it seems," says Dr Simon O'Toole from the Australian Astronomical Observatory, who was not involved in the study.

"It sounds far fetched, but a lot of our models have the planets oscillating in and out a bit.

"For planets around other stars, we have models showing an exoplanet moving out and in and out again as it interacts gravitationally with the other planets in the system, so it's not completely crazy."