Researchers say it's possible the solar system's biggest planets, the mighty gas giants Jupiter and Saturn, started slowly from the most humble of ingredients: tiny pebbles.
By "pebbles," they mean anything from fractions of an inch up to three feet across, still "pebbles" in the cosmic scheme of things.
Most current theories of how the large gas giant planets formed assume the cores of the solar system's largest worlds formed from collisions between much larger objects, 10 or even hundreds of kilometers in diameter.
However, if that were the case, the cores would not have formed quickly enough to capture gases in the early solar system that constitute their atmospheres today, a new study by scientists at Queen's University in Canada and the Southwest Research Institute in Texas suggests.
Previous studies have suggested the gas disks that planets first begin to form from usually last for only one to 10 million years, whereas rocky planets like Earth took a minimum of 30 million years, and possibly a maximum of 100 million years, to form from the slow accretion of larger material available later in the solar system's evolution.
How exactly could Jupiter and Saturn form so rapidly that they could accumulate so much of the fast-disappearing gas in the early solar system?
"The timescale problem has been sticking in our throats for some time," says SRI researcher Hal Levison, the study's lead author.
"It wasn't clear how objects like Jupiter and Saturn could exist at all," he says.
Writing in the journal Nature, the researchers suggest collisions and accumulation of the so-called pebbles would have allowed the cores to form much more rapidly that the accepted theory of the accretion of larger bodies.
The researchers created computer models that suggest Jupiter and Saturn may have formed well within the 10-million-year time frame if they grew by slowly accumulating a population of smaller planetary pebbles, icy objects averaging around a foot in diameter.
Accretion of the pebbles couldn't be too fast, they note, or the growing planets would not have enough time to begin gravitationally interacting with one another.
"If the pebbles form too quickly, pebble accretion would lead to the formation of hundreds of icy Earths," says SRI scientist Katherine Kretke. "The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed."
The researchers say the new model accurately predicts what we see in the outer reaches of our solar system: two gas giants — Saturn and Jupiter — and two ice giants — Neptune and Uranus — and a Kuiper belt of leftover solar system material.
Queen's University researcher Martin Duncan states that it's a relief to find a model that is highly successful after the years of computer simulations that were performed of the standard one.
