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Astronomers from the University of Amsterdam have come up with a new explanation for why our planet contains ten thousand times less carbon than expected. According to Lucia Klarmann, Chros Ommel and Carsten Dominik of the Anton Pannekoek Institute for Astronomy, much of this missing carbon burned up when the atmosphere around the still-forming Earth was scorched at temperatures topping 800 degrees Celsius. At the same time, incoming carbon was most likely blocked by Jupiter.

For years, scientists have been searching for an answer to the question of why Earth contains much less carbon than expected. For every ten thousand atoms of silicon, the Earth has only one atom of carbon. This ratio is completely different for comets, the Sun and the space between the stars and planets, where the amount of carbon is equal to or even up to ten times greater than that of silicon.

Dusk disk. Image: ESO

Explanation unlikely

Lucia Klarmann, Chris Ormel and Carsten Dominik used calculations and models to show that this prevailing explanation is unlikely. Firstly, the majority of carbon is not found in light dust particles, but instead in heavy pebbles that cannot simply end up in the top layer of the dust disk. Secondly, it takes too long for a small dust particle to be transported up, and new carbon would have arrived from the edges of the disk in the meantime.

The astronomers have come up with an alternative scenario based on two conditions. The first is that the inner regions of the dust disk, where the Earth was formed, could have been scorched by temperatures of over 800 degrees Celsius, thus causing the carbon to burn up and thus disappear. This temporary flare-up, called an FU-Orionis Outburst, has previously been observed in other stars. The second condition is that the influx of carbon-rich pebbles must have been blocked by Jupiter.

An exceptional case

According to principal investigator Klarmann, who hopes to obtain her doctorate on 4 October, the new models and calculations show that a low-carbon Earth may be exceptional. ‘This also means that other Earth-like planets might indeed contain a great deal of carbon. This has been suggested before, but now we can demonstrate it with our models.’

The researchers aim to continue investigating their scenario in the future. Among other things, they would like to explore whether just a single flare-up could have caused Earth’s lack of carbon. They also want to calculate how much earlier Jupiter would have had to form than Earth in order to actually block the supply of new carbon.

Publication details

Radial and vertical dust transport inhibit refractory carbon depletion in protoplanetary disks. L. Klarmann, C. W. Ormel, C. Dominik. This article will soon be published in the scientific journal Astronomy & Astrophysics, and is already available on ArXiv: