the roots of planets
POSTED 24 NOV 2004


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In a protoplanetary disk, stars and planets develop within giant gas and dust clouds. At the right, the Hubble Space Telescope caught glowing gases moving toward the star being born. The gases are cut by a band of light-absorbing dust; most likely the planet-forming disk. Photo: NASA

Did planets form from crystals, or did crystals form in planets?

For years, scientists have scrutinized meteorites for clues to conditions in the ancient solar system. After all, lots has changed since our star and its nine planets started forming roughly 4.6 billion years ago. The sun, we believe, assembled through gravity from a dense ball of gas, mainly hydrogen.

Whispy clouds of gas and dust glow and surround newborn stars.But what formed the planets?

This week, European scientists reported in the journal Nature that they had detected mineral crystals in disks of gas and dust where planets are probably forming. The researchers looked at three "proto-planetary disks" located in our astronomical neighborhood -- just 400 to 800 light years distant. Each disk was rotating around a star somewhat larger than our Sun in size.

By analyzing the wavelengths of infrared light collected in two giant telescopes, and using a clever technique to precisely locate the origin of that light, the scientists concluded that the dust near the newly-formed stars contained microscopic crystals of olivine.

Olivine is a silicon-bearing mineral that is commonly found in rocks from Earth, Mars, and meteorites. In fact, olivine is the main component of Earth's mantle -- the huge, gooey layer beneath the crust.

Martian meteorites contain five types of igneous rocks, including this olivine, seen in polarized light. Photo: NASA

Yellow, green and red reflect from rock in polarized light.For many years, scientists have believed that planets form from the same disks of dust and gas that also create stars. But the details were sketchy -- for one thing, the first planet-forming disk was photographed just a few years ago.

Chicken or egg?
The study, the first detailed look at the composition of dust in planet-forming systems, answers a key question: Which came first: mineral crystals or planets? Did planets form from an agglomeration of crystals, or did crystals form inside planets?

Michiel Min, a graduate student studying with Prof. Rens Waters at the University of Amsterdam, contributed to the new Nature report. Min told us that crystals were already present in the three disks.

Indeed, close to one of those stars, all the dust was crystalline, Min told us via email and telephone. In all three cases, crystals were less common, but amorphous silicate more common, further from the star. One type of amorphous silica is familiar: plain 'ol window glass.

Standing on the shoulders of planets
If the new data withstand scrutiny, they will expand our longstanding understanding of the composition of planets. Every solar-system planet has a rocky core. The inner planets (Mercury, Venus, Earth and Mars) have thin atmospheres, while the outer planets (except for tiny Pluto) are wrapped in thick, cold, dense layers of gases like methane and ammonia.

Hunk of rock with olive green sheen on vivid blue stage.That difference results largely from temperature. Around the warmer, inner planets, most gas atoms move too quickly to be restrained by gravity.

Olivine is a green, glassy silicate mineral containing iron and magnesium that forms at high temperature. Photo: USGS

Now, it also appears that the planets' rocky cores may have formed from different materials, Min says. "The building blocks are different. While the inner planets are built from crystalline silicates, the outer, gaseous planets mainly had amorphous material at their disposal."

So here's the big picture: In a planet-forming disk, crystalline silicates -- the raw material of many rocks -- form close to the star, before any planets appear. And that makes sense, given the warmer temperatures found near a star. "To form crystalline silicates, we have to heat the material above about 530 ° C," Min told us. "Our new observations indicate that these crystals formed close to the star, where it is nice and hot, and then mixed out into the cooler regions."

-- David Tenenbaum

The Building Blocks of Planets within the "Terrestrial" Region of Protoplanetary Disks, R. van Boekel et al, Nature, 25 Nov. 2004, pp. 479-482.


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