Tuesday, August 26, 2008


Does finding another Earth get more or less likely by the week?

'Finding an Earth-like planet gets more likely by the week', asserts New Scientist (21 June 2008, p. 7). Why think that? Well: 'Three more alien worlds - ranging from 4 to 9 times Earth's mass - have been found in tight orbits around their host stars by Michael Mayor of the Geneva Observatory in Switzerland and colleagues, as part of the HARPS survey.' The odd thing about this little paragraph (in the '60 Seconds' column) is that the latter statement, far from supporting the former, actually undermines it!

The fact that we have discovered yet more extra-solar planets, and have found that they (like all the others discovered thus far) are unlike Earth, actually supplies an even larger data set of non-Earth-like planets, a data set that therefore supports an even stronger inference to the (falsifiable) conclusion that Earth is a unique - or at least very rare - type of planet!

If an average measure of the parameters of the set of planets we had discovered were gradually converging upon the parameters exhibited by the Earth, then one would have some grounds for saying with New Scientist that
'Finding an Earth-like planet gets more likely by the week'. However, this doesn't appear to be the case. These latest discoveries, for example, are not just a little different from Earth (they are not even as 'alike' to the Earth as is Mars!), they are very un-Earth-like. The fact that we have discovered three more planets that are very unlike the Earth (they are both 4 to 9 times more massive that Earth, and orbit more closely to their stars than does the Earth) actually goes to support the conclusion that Earth is a very unusual type of planet, and thereby the statement that finding an Earth-like planet gets less likely by the week!

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