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Forming the planets
- The Molecules that Fell to Earth

There does, then, seem to be convincing evidence that biologically- interesting molecules exist throughout the cosmos. The question being raised now is whether these molecules could have seeded the Earth and provided an impetus in the development of life. Specifically, while there may very well be complex organic molecules floating about all over the ISM, how could they have safely travelled to, and arrived on, Earth?

Fig 2. The solar system (not to scale)

As indicated earlier, dense molecular clouds don't remain as clouds for very long and when a star eventually forms within the cloud, so may an accompanying solar system, although astronomers still don't know how common it is for the latter to occur. Nevertheless, theory states that once a star has been created within the cloud, the rest of the cloud begins to orbit the star, eventually forming a flat, planar mass, known as an accretion disc. Because the constituent particles in the disc are much closer together than in the dense cloud, they begin to agglomerate, forming larger and larger particles. This is the process that eventually creates the planets.

It is also one of the ways that biologically-active species could have found their way to Earth; by helping to form it. Not surprisingly, most astronomers consider that the extremes of temperature and pressure involved in the formation of a planet such as Earth would quickly destroy any complex structure that a molecule may possess. The molecules would simply add to the elemental composition of the planet.

This is not always the case, however. There is a temperature gradient within the accretion disc, depending on the distance from the central star. During the formation of our solar system, for particles in the accretion disc that were closer to the sun than 5AU (about where Jupiter is now), the heat would have caused the icy mantles to sublime away from the grain particles. According to theory, the remaining solid grains then combined to form the rocky planets of Mercury, Venus, the Earth and Mars, as well as the 1020 asteroids that today form the belt between Mars and Jupiter.

This process would have destroyed most complex organics, but some of the tougher organic molecules, such as microdiamonds, aliphatic hydrocarbons and PAHs, have been identified in meteorites that have landed on Earth. Even more intriguing is the fact that the PAHs detected are deuterium-enriched. Astronomers theorise that PAHs will become so enriched because of unimolecular photodissociation in the ISM, by which a UV photon breaks a carbon-hydrogen bond, the hydrogen atom then being replaced by either hydrogen or deuterium. The resulting carbon-deuterium bond is not as easily broken by a UV photon and, because hydrogen and deuterium are both available, interstellar PAHs should gradually become deuterium- enriched. The detection of deuterium-enriched PAHs in meteorites thus suggests an interstellar heritage.

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