NASA has said there are ‘billions’ of planets in our own Milky Way galaxy – but a new study suggests that the idea that they are teeming with alien lifeforms may just be wishful thinking.
Two Princeton scientists used what’s known as ‘Bayesian analysis’ – a technque that ‘boils down’ ideas to the actual data, as opposed to scientists’ own ideas about what ‘should’ be true.
They suggest that it’s very possible Earth is a one-off aberration where life took hold unusually fast – and on the average extraterrestrial planet, the chances of life are very low indeed.
‘Fossil evidence suggests that life began very early in Earth’s history and that has led people to determine that life might be quite common in the universe because it happened so quickly here, but the knowledge about life on Earth simply doesn’t reveal much about the actual probability of life on other planets,’ said Princeton astrophysical sciences professor Edwin Turner and David Spiegel, a former Princeton postdoctoral researcher.
‘Information about that probability comes largely from the assumptions scientists have going in, and some of the most optimistic conclusions have been based almost entirely on those assumptions,’ he said.
‘If scientists start out assuming that the chances of life existing on another planet as it does on Earth are large, then their results will be presented in a way that supports that likelihood,’ Turner said.
‘Our work is not a judgment, but an analysis of existing data that suggests the debate about the existence of life on other planets is framed largely by the prior assumptions of the participants.’
Joshua Winn, an associate professor of physics at the Massachusetts Institute of Technology, said that Turner and Spiegel cast convincing doubt on the basis for expecting extraterrestrial life.
Winn, who focuses his research on the properties of exoplanets, is familiar with the research but had no role in it.
‘There is a commonly heard argument that life must be common or else it would not have arisen so quickly after the surface of the Earth cooled,’ Winn said.
‘This argument seems persuasive on its face, but Spiegel and Turner have shown it doesn’t stand up to a rigorous statistical examination — with a sample of only one life-bearing planet, one cannot even get a ballpark estimate of the abundance of life in the universe.
rgence of life on Earth gave reasons to be optimistic about the search for life elsewhere,’ Winn said. ‘Now I’m not so sure, though I think scientists should still search for life on other planets to the extent we can.’
Deep-space satellites and telescope projects have recently identified various planets that resemble Earth in their size and composition, and are within their star’s habitable zone, the optimal distance for having liquid water.
Of particular excitement have been the discoveries of NASA’s Kepler Space Telescope, a satellite built to find Earth-like planets around other stars.
While these observations tend to stoke the expectation of finding Earth-like life, they do not actually provide evidence that it does or does not exist, Spiegel explained. Instead, these planets have our knowledge of life on Earth projected onto them, he said.
Yet, when what is known about life on Earth is taken away, there is no accurate sense of how probable abiogenesis is on any given planet, Spiegel said. It was this ‘prior ignorance,’ or lack of expectations, that he and Turner wanted to account for in their analysis, he said.
‘When we use a mathematical prior that truly represents prior ignorance, the data of early life on Earth becomes ambiguous,’ Spiegel said.
Spiegel and Turner also propose that once this planet’s history is considered, the emergence of life on Earth might be so distinct that it is a poor barometer of how it occurred elsewhere, regardless of the likelihood that such life exists.
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