spaceSpace and Physics

Mystery Of How Gas Giants Formed Could Be Explained By “Planetary Pebbles”


Jonathan O'Callaghan

Senior Staff Writer

1903 Mystery Of How Gas Giants Formed Could Be Explained By “Planetary Pebbles”
How did gas giants form so quickly in the early Solar System (illustrated)? NASA/JPL-Caltech.

There’s a problem brewing in planetary science. The disk of dust and gas that surrounded our Sun at the start of its life 4.5 billion years ago, from which the planets formed, is thought to have existed for just 1 to 10 million years. Earth, and the other rocky planets, spent at least 30 million years forming. But the gas giants are thought to have come into existence much faster, before the disk disappeared. How were they able to?

Finding out how gas giants formed in our own Solar System and beyond has puzzled astronomers, but we may now be a step closer to the answer. Researchers at the Southwest Research Institute (SwRI) in Boulder, Colorado and Queen’s University in Ontario, Canada think that “planetary pebbles” were the key.


In their study, published in Nature, they speculate that the gradual accumulation of icy objects about 0.3 meters (1 foot) in diameter – called pebbles – led to the formation of the gas giants within the timeframe of 10 million years. This would have allowed them to form in the gas nebula surrounding our young star. Once the gas giants formed, other material was able to coalesce into the rocky planets we know today.

Recent discoveries, such as a young Jupiter 100 light-years away and planetary pebbles in a distant extrasolar system, could provide supporting evidence for this new theory.

Previous theories for planetary formation suggested that rocky cores – known as planetesimals – would gradually grow by swallowing other objects of a similar size; rocks joined rocks, mountains joined mountains, and so on. But while this can explain the rocky planets, it does not account for the expected rapid formation of the gas giants. The process was thought to be slow and gradual.

“The timescale problem has been sticking in our throats for some time,” said Dr Hal Levison of SwRI in a statement. “It wasn’t clear how objects like Jupiter and Saturn could exist at all.”


This latest model, though, provides a method of formation that is up to 1,000 times faster. This is because, rather than similarly-sized objects joining together, one dominant object would sweep up small material more quickly, and bully other objects out of the way. It could be the model we have been waiting for to explain gas giant formation.

What’s more, it is the first model that can account for the formation of not only Jupiter and Saturn, but Uranus, Neptune and the Kuiper Belt as well. “After many years of performing computer simulations of the [original] model without success, it is a relief to find a new model that is so successful,” said Dr Martin Duncan from Queen’s University in the statement.

Studying other planetary systems that are at different stages in their lives will help to reveal if it is correct or not.


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