The Why Files The Why Files -- whyfiles.org

Let's make a planet 17 MAR 2005

From dust unto ... planets?
Red gas in figure 8 shape surrounds bright star beginning.Picture a young sun, surrounded by gas 'n dust. Somehow, out of this swirling miasma, planets arose. Don't take our word for it. You can see the pictures of young stars cloaked in clouds of dust -- if you have a cool telescope.

MyCn18 is a young planetary nebula located about 8,000 light-years from us. Planetary nebulas were the birthplace of solar systems like ours, but how did planets start forming? This Hubble image shows the hourglass shape and intricate "etchings" in the walls. The picture was composed from three images, showing ionized nitrogen (red), hydrogen (green), and double-ionized oxygen (blue). Photo: Wide Field and Planetary Camera 2 Hubble Space Telescope

But there's a problem: Gravity alone is too weak to bind the dust together. Otherwise, those motes of dust you see in the beam of a movie projector would bind together and drop to the floor.

This matters. If dust in the nebula does not congeal, the solar wind -- the charged particles zooming outward from the sun -- would blow it all away before the planets could form.

Within a million years, the solar wind would strip away all the dust, and we would not have planets. No planets means no microwaves. No strip malls. Not even any Why Files. The horror. The horror! It's the ultimate existential question: Why didn't the solar wind just blow it all away?

Four large spheres form in black expanse of space.
The outer ("gas") planets may have gotten help from sticky ice: from top: Jupiter, Saturn, Uranus and Neptune. As no spacecraft has yet visited Pluto, it's not shown. Photo: NASA

Science -- flash frozen
tube of 'sticky ice' glueNow we hear from Martin Iedema and James Cowin of Pacific Northwest National Laboratory, who say the dust was slathered with a thin coat of flash-frozen ice. And their experiments show that ice forming at around 30° Kelvin is sticky enough to congeal into gunk-balls that eventually grow large enough for gravity to take over and cause further accumulation.

To understand what Cowin and Iedema are saying, slam your time machine into reverse, and imagine conditions in our planetary nebula, at the distance, say, of Jupiter, about 4.5 billion years back. The dust is so thick you can't even see the sun, which has recently begun "burning." Although it's dark, it's also windy. A solar wind of charged helium and hydrogen atoms is streaming away from the sun, eroding away the dust.

The infant Stingray nebula (Hen-1357) is the youngest known planetary nebula. The bright central star is in the middle of the green ring of gas. Photo: Hubble

Green, gaseous ring surrounds small, bright circle. You would expect the wind to strip the dust away in, say, a million years. Presto-chango -- no planets! Gravity is too puny to congeal the dust particles, says Iedema. "Gravity alone is insufficient, it doesn't have any effect when you are talking about objects smaller than a meter in size, it won't cause little dust grains to gather together. You need some other mechanism."

Don't count on the electrical attraction called van deer Waals forces. Again, too flimsy.

The benefits of being unbalanced
A different mechanism emerged from lab experiments at Pacific Northwest, where the researchers grew ice on a super-cold plate in a vacuum chamber. The ice turned out to have a peculiar electrical property: Like tiny electric versions of bar magnets, the ice molecules had a slight preference to align themselves.

At these frigid temperatures, Iedema explains, "as ice grows from a gas onto a solid, it's so cold that the ice doesn't have a chance to reorient, it gets frozen into position." At somewhat warmer temperatures, he says, ice particles "would be free to jostle, to find a better configuration, where the charges would cancel each other out. But it's so cold that they don't have that freedom to explore. They get stuck, and wind up having a slight alignment."

Even if an impact breaks the ice, the electrical imbalance remains, he says. "If they are hit and disintegrate, there is some attraction to fall back together, and recombine, rather than bounce away."

Graph of red dot follows path like a ball droppingBut there's more. The flash-frozen ice has a different crystal structure than regular, hard ice. It's less dense, making the surface less "elastic." And when the researchers tested this "squishy" ice in the lab, stuff didn't bounce off the way it does when a solid object whangs into hard ice.

Click for a (52 KB) movie showing a ceramic ball bouncing on the "squishy ice" that forms in supercold conditions. A ball dropped on normal ice would bounce to 80 percent of its original height. These "inelastic" collisions could help in the first stage of planet formation. Photo: Pacific Northwest National Laboratory

It's a nice explanation for something that needs explaining: how planets started forming. "People have had the hypothesis that this needs to happen in a certain time frame, or the material gets ejected from the forming solar system," says Iedema.

Eventually, when the electrically-attracted iceball grows to somewhere between a meter and a kilometer in diameter, gravity starts to play a role, he says, and the agglomeration accelerates.

But here's a caveat: The experiment was done at temps found at Jupiter's distance from the sun. Earth arose in warmer conditions, where ice would have been more "normal."

Yet Iedema suspects a similar electrical imbalance could have affected the heavier silicate dust -- the stuff that formed Earth in the early years of the Solar System and gave, for better or worse, an orbital platform for Jackass (the movie), Eminem (the rapper) and The Why Files (the nerd nuts).planet being glued together with sticky ice

At the distance of Earth, Iedema says, "some of the same electrical and mechanical properties we observed for water ice may be important."

-- David Tenenbaum

Bibliography
Sticky Ice Grains Aid Planet Formation: Unusual Properties Of Cryogenic Water Ice, H. Wang et al, The Astrophysical Journal, 620:1027-1032, 2005 February 20.


Related Why Files
Planet formation

Star formation

Comets: dirty iceballs

Moon's origin

Alpine iceman


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