"The absorbing question is how one goes from a
disc of dust and gas to a solid object, in which
particles have condensed, then coalesced to
ultimately build a planet," said Dr Sarah Maddison.
Illustration: Paul Dickenson

The birth pangs of the Earth, how it drew its substance from the disc of gas that formed the early solar system, are still not well understood. In search of the solution to this celestial enigma, a team of Swinburne University of Technology scientists are seeking answers in what might, at first glance, seem an unusual 'birthing suite' - the roaring heart of a terrestrial blast furnace.

It turns out, says Swinburne astrophysicist Dr Sarah Maddison, that the processes by which iron forms in a blast furnace bear a strong semblance to those which took place when the infant Earth first coalesced out of gas and dust in the solar disc, some 4.6 billion years ago.

By applying knowledge gained from the well-understood processes by which metals form in modern smelters, the team hopes to discern how the elemental particles formed in the biggest natural 'smelters' of them all, the Earth and other planets.

Working with Swinburne colleague Professor Geoffrey Brooks, a leading authority on the physical chemistry of metal formation, CSIRO's Dr Kurt Liffman and French Masters student Vianney Taquet from University Paris-11, Dr Maddison is deciphering the physical conditions that led to planet formation - both in our own and other planetary/star systems - using principles drawn from metallurgy.

"By observing the pressure and temperature in blast furnaces and seeing what particles condense out and accrete under different conditions, we are applying these same principles to protoplanetary disks and hope to develop models that will explain how planetary formation occurs," she says.

The happy marriage of high-end astrophysics with utterly practical metals chemistry may help explain why the planets differ from one another in their composition, from the Moon and Mars with their apparently solid iron cores, to the Earth with its liquid iron core, to the gas giants of the outer solar system, whose inner cores remain, as yet, unknown.

The key to the mystery, Professor Brooks says, may lie in the behaviour of elements such as calcium, aluminium, magnesium, silicon, iron, nickel, manganese and phosphorus. In certain circumstances of heat, pressure and cooling, these elements transform into oxides and in others they prefer to assume their metallic form. This behaviour, well understood from study of the thermodynamics and chemistry of metal smelting, provides clues to the conditions that the proto-Earth underwent as it condensed from a cloud of gas.

"The Earth is really a giant smelter - I tell my students that," Professor Brooks says. "Like a smelter it has liquid metals in its hot core, liquid slags which make up the surrounding magma, and solidified oxides in its outer crust." This implies that the same computational methods by which metallurgists study smelting reactions can be used to investigate the complexity of planet formation.

"For example, under what conditions in planet formation do we expect iron to be present in a sulphide phase? If we find magnesium combined with aluminium and oxygen in particular crystal forms, what does it tell us about the conditions that formed them? These chemical models will ultimately be incorporated into various physical models for planetary formation."

The absorbing question, Dr Maddison says, is how one goes from a disc of dust and gas to a solid object, in which particles have condensed, then coalesced to ultimately build a planet. Clues to this process may be found in chondrules, small spherical formations found in the bodies of stony meteorites, which reveal the rapid heating and cooling processes to which they have been subjected.

If superheated gas chilled rapidly enough for metals to condense, something special must have happened: CSIRO scientist Dr Kurt Liffman proposed, in the early 1990s, that enormous jets of superheated gas formed at the inner edge of the disc surrounding the whirling infant sun and cooled in the relatively lower temperatures of nearby space at a rate that enabled metal and stone-like particles to form. Such particles would then rain out over the early solar system.

Just about all young stars produce such jets, and astronomers call them bipolar outflows. This theory led to the prediction that most of the disc around the early Sun would contain particles that were heated to very high temperatures. This prediction was confirmed in 2006 when NASA's Stardust mission obtained samples of the comet Wild 2 and found that just about all the dust particles had been heated to very high temperatures, even though the comet had formed in some of the coldest parts of the solar system. This is possibly what gave rise to meteorites and the material that formed the inner planets - and the team is testing the idea in their model for planet formation.

Whether the metals condense directly out of the gas on their own, or whether they nucleate - like raindrops - around microscopic particles of stellar dust, remains an open question which the team hopes to answer, Dr Maddison says. The key is cooling, both the rate and extent, from a scorching 1500˚C to 1800˚C in the jet, to the chilly, but not absolutely cold, ambience of circumstellar space. "We'd like to know if such jets are responsible for the formation of meteorites and can explain how different materials condense out in different parts of the disc," she says.

This ability to predict where different elements condense, accrete and solidify forms the heart of the team's work and will create the window through which they hope one day to peep in order, finally, to glimpse the very birth of the planets.