Astrobiology Workshop, Macquarie University July 12-13 2001

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Earth-Like Planets:Common or Rare in the Universe

Ross Taylor (Australian National University)

I begin by assessing the many factors involved in the formation and evolution of our planet. These include the initial size and angular momentum of the fragment that separated from a molecular cloud, crucial in determining whether a planetary system or a double star develops from the resulting nebula. Then an adequate concentration of heavy elements (metals) was needed to provide the two percent 'rock' and 'ice' components of the original nebula. An essential step in forming the rocky planets in the inner nebula was loss of gas and depletion of volatile elements, due to early solar activity, that is linked to the mass of the central star.

The lifetime of the gaseous nebula controlled the formation of gas giants. Fine timing was needed to form the gas giant, Jupiter before the gas in the nebula was depleted. Although Uranus and Neptune eventually formed cores large enough to capture gas, they missed out and ended as ice giants. Early formation of Jupiter was responsible for the existence of the asteroid belt (and our supply of meteorites) and the small size of Mars while the gas giant now acts as a gravitational shield for the terrestrial planets.

The Earth and the other inner planets accreted 10-100 m.y. after the giant planets in a gas-free inner nebula from volatile-depleted planetesimals that were probably already differentiated into metallic cores and silicate mantles. The accumulation of the Earth from such planetesimals was essentially a stochastic process, accounting for the differences among the four inner rocky inner planets, including the startling contrast between those two apparent twins, Earth and Venus. Impact history and accretion of a few more or less planetesimals were apparently crucial.

The origin of the Moon by a single massive impact with a body larger than Mars accounts for the spin and obliquity (and its stability) of the Earth in addition to explaining the angular momentum, orbital characteristics and unique composition of the Moon. The supply of water to the Earth from icy planetesimals was another stochastic process, but crucial to the subsequent evolution of the planet. Plate tectonics, a process unique among the terrestrial planets, led to the development of the continental crust on the Earth, an essential platform for the evolution of Homo sapiens. Random major impacts have punctuated the geological record, accentuating the directionless course of biological evolution.

This sequence of events that resulted in the formation and evolution of our planet were thus unique within our system and the individual nature of our eight planets is repeated among the 80-odd satellites: no two are identical while the four satellite systems of the giant planets differ in most respects (Taylor, 1999, 2000).

This survey of our solar system raises the question whether the random sequence of events that led to the formation of the Earth are likely to be repeated in detail elsewhere. Preliminary evidence from the extrasolar planets has revised earlier views that planetary systems would consist of a central star surrounded by an inner zone of rocky planets and an outer zone of giant planets beyond a few AU. Extrasolar planets are found mostly around metal-rich stars, supporting models for growth of gas giants around cores of rock and ice. Thus Jupiter-sized bodies in close orbits around other stars probably formed in a similar manner to our giant planets at several AU from their parent star and subsequently migrated inwards becoming stranded in close but stable orbits as 'hot Jupiters', when the nebula gas was depleted. Such events would prevent the formation of terrestrial-type planets in such systems. The prevalence of orbits with high eccentricities (except for those planets in close orbits) raises further problems in forming habitable planets like the Earth. Clearly orbits of low eccentricity are not a given in the construction of planetary systems, leading to a more pessimistic outlook for the presence of planets residing within an habitable zone that is essentially circular around a star.

Taylor, S. R. (1999) On the difficulties of making Earth-like planets (Leonard Medal address) Meteoritics and Planetary Science, 34, 317-329

Taylor, S. R. (2000) Destiny or Chance: Our Solar System and its Place in the Cosmos Cambridge University Press 229 pp.