Is Earth Nearly Unique?
Scientists argue that the earth is far from common.
In an article released by Space.com entitled "Earth is Rare, New Study Suggests", there is a suggestion that the study of extra-solar planets and comparions to Jupitar argue that the earth is much more rare than many people assume. According to the article:
Now, this discussion suggests that one of two things is true. Either we need to develop a new theory for planetary formation that will allow for the creation of Jupiter and the Earth, or we need to acknowledge that it is possible that the Earth was specially placed here. While I am certainly open to the first, isn't the latter possible?The leading model of planet formation -- conjured before anyone knew there were planets around other stars -- is called core accretion. A rocky core develops first, then an object either becomes a terrestrial planet (like Mercury, Venus, Earth or Mars) or it attracts huge amounts of gas and grows into something like Jupiter or Saturn.
An alternative method forms a gas giant planet via the gravitational collapse of a knot of material.
Scientists disagree which way the outer planets of our solar system were born. The core accretion model has shortcomings. For one thing, when run on a computer, Neptune and Uranus typically don't show up. Further, observations reveal that Saturn has a solid core but Jupiter does not.
The gravitational collapse model has been invoked to explain some these discrepancies. It is also appealing as a method for making the gargantuan gaseous planets found around other stars. Most are a few to several times the mass of Jupiter and orbit incredibly near to their host stars on wildly non-circular orbits.
In all but a handful of these setups, rocky inner planets are not possible because they'd be consumed by a giant or gravitationally booted out of the "habitable zone," a comfortable region that can support life, Beer explained. Earth and Mars both orbit stabily in just such a temperate area, thanks to the fact that the outer planets are far away and on nearly circular orbits.
If gravitational collapse formed the known extrasolar planets, then there's no need for rocky cores.
"Without these rocky cores, terrestrial planets (that is, Earth-like) do not form," Beer points out.