Imperial College London, UK

A planetary scientist learns how comet dust gets from the inner to the outer Solar System.

I was lucky enough to be part of a team studying the grains of comet dust collected last year by NASA's Stardust mission. Comets are primitive, pristine objects, and the Stardust samples are changing the way we think about how our Solar System formed.

Among many surprising findings, perhaps the most significant is that a large fraction of the dust grains are minerals formed at high temperatures — temperatures expected only in the inner Solar System. How did this stuff get out to where the comet began its life, in the cold, outer regions of the Solar System?

At the recent Lunar and Planetary Science Conference in Houston, Texas, I learned about a numerical simulation that potentially offers a neat solution (F. J. Ciesla and J. N. Cuzzi, abstract here).

Observations of dusty disks around young stars show an inward flow towards the central star. Ciesla and Cuzzi's simulation suggests that this inward transport is confined to the top and bottom of the disk. It predicts that there is a narrow region near the disk's midplane where dust flows outwards — a flow sufficient to account for the Stardust results.

So now we know that comets contain a mixture of stuff from the inner Solar System, and we have a physical model that can explain how it got there. But we're still left with one question.

Virtually everything in the inner Solar System — Earth, Mars, the Moon, almost all meteorites — is depleted in volatile elements, which can't condense at high temperatures. But the cometary dust grains don't show this depletion signature. Why not? It'll be fun finding out.

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University of Oklahoma, Norman, Oklahoma, USA

An astronomer invites you to contemplate the history of some of the oldest stars in the Universe.

Much of my work is an attempt to determine what kinds of stars formed and what types of element synthesis occurred when our Galaxy was very young. This means that I am particularly interested in a class of stars referred to as 'carbon-enhanced metal-poor'.

The composition of a star reflects the properties of the interstellar medium at the time it formed, which evolves as generations of stars come and go. Metal-poor stars were born early in the history of our Galaxy, before dying stars enriched the interstellar medium with heavy elements. The fact that some of these metal-poor stars are carbon-enhanced provides insight into the types of stars that came before them.

Recently, one group reported that around 20% of metal-poor stars are carbon-enhanced (S. Lucatello et al. Astrophys. J. 652, L37–L40; 2006). A previous study had produced a lower figure (J. Cohen et al. Astrophys. J. 633, L109–L112; 2005), prompting a battle between the competing groups. But both papers agree that more metal-poor stars are carbon-enhanced than are younger, high-metallicity stars.

To me, this is one of the most interesting and compelling results to come from the study of such stars. Massive stars — with at least ten times the mass of the Sun — produce carbon efficiently, so this gives us a clear indication that massive stars, although very rare today, were much more common early in the history of the Universe.

The differences between the studies' numbers may lie in how the authors define a carbon-enhanced star, or could be a matter of statistics. I look forward to future papers that address these issues — and perhaps continuing the controversy.

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