Far from traditional views that Earth and its neighbors emerged from nothingness quickly and passively, planets tend to be built over millions and millions of years, in stages punctuated by massive collisions between rocky bodies.

That is what just-released observations, based largely on NASA's Spitzer Space Telescope, have revealed. The primordial dust clouds that form around nascent solar systems are huge, turbulent and persistent. Components as small as grains of sand and as large as planetary bodies constantly and repeatedly smash into one another.

The young Earth seems to have experienced at least one such a catastrophe, when a Mars-sized object ripped the planet apart and adorned it with a ring of dust and rock that eventually coalesced into the moon.

It's a mess out there, said George Rieke of the University of Arizona in Tucson. We are seeing that planets have a long, rocky road to go down before they become full grown.

Spitzer, launched in August 2003, trails Earth around the sun in what is known as a heliocentric orbit. Protected from the star's heat and radiation by a shield, and supercooled to allow its sensitive detectors to function, it can capture the faint, infrared radiation from neighboring star systems. In that radiation, the one-ton telescope can observe the dusty aftermaths of protoplanetary collisions, as the events are called.

When embryonic planets, such as the rocky cores of Earth and Mars, crash together, they are thought either to merge into a bigger planet or splinter into pieces. Either way, the dust generated by these events is warmed by the host star, creating heat that Spitzer can detect.

Rieke and colleagues used new data from Spitzer and previous data from two other sources: the European Space Agency's Infrared Astronomical Satellite and the joint NASA, United Kingdom and the Netherlands' Infrared Space Observatory.

They looked for dusty discs around 266 nearby stars of similar size - no more than three times the mass of the sun - and various ages. Of those stars, 71 were found to harbor discs, probably containing planets at different stages of development, but instead of seeing the discs dissipate in older stars, the astronomers observed the opposite in some cases. The discs were aglow with infrared light, signifying that collisions were continuing - worlds and worldlets were bashing one another with regularity.

The findings mirror the turbulent past of Earth and its neighbors, abundant evidence of which is in full view of anyone with binoculars looking at the moon's crater-covered surface.

Our moon took a lot of violent hits when planets had already begun to take shape, said Rieke, the lead author of the research and a member of the Spitzer team.

As has been known for some time, rocky planets form somewhat like the way people build snowmen. Just as small wads of snow can be rolled into larger and larger balls, gravity pulls clouds of dust particles surrounding stars into rocky clumps. As the clumps interact, they continue to pull together. The problem is, the bigger the the clumps get, the more mass they accumulate and the more violent their collisions become.

The revelation runs counter to the assumption that planet building is a smooth and somewhat orderly progression, from minute dust the size of smoke particles, to grains of sand, to handfuls, and so on, as the gravity from the clumps attracts more and more of the particles from the cloud. Under this scenario, major collisions are rare events.

We thought young stars, about one million years old, would have larger, brighter discs, and older stars from 10 to 100 million years old would have fainter ones, Rieke said. But we found some young stars missing discs and some old stars with massive discs.

The brighter the disc detected by Spitzer, the hotter it is - the heat generated by violent collisions.

This implies planet-forming discs can remain dusty for hundreds of millions of years after the host star was formed, Rieke said. The only way to produce as much dust as we are seeing in these older stars is through huge collisions, he added.

Spitzer has provided the key to the findings. Before its exquisitely sensitive instruments were trained on the target stars, only a few dozen planet-forming discs had been observed around stars older than a few million years.

Spitzer has opened a new door to the study of discs and planetary evolution, said Michael Werner, the spacecraft's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Indeed. Spitzer, in its own way, has added to human capability to observe, directly, worlds outside the solar system. In this case, the research provides information about planetary history. Eventually, these instruments will help provide clues about space exploration plans for the future.

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