The development of life on any planet, including Earth, is a function of the age of the relationship between the planet itself and its host star, a prominent astrobiologist suggests.
In a paper soon to be published in the journal Astrobiology and currently available on the pre-print site arxiv, Jacob Haqq-Misra of the Blue Marble Space Institute of Science in Seattle, Washington, US, builds on previous research to explore the idea that the advent of life depends on the availability of “free energy”.
In other words, he writes, “the mean time required for the evolution of complex life is a function of stellar mass”.
The idea, which he and colleagues have previously explored, builds upon a current predictive method for the development of exoplanetary life, which is known as the equal evolutionary time (EET) hypothesis.
EET attempts to overcome the primary problem faced by astrobiologists – the fact that so far there is only evidence of one biological system in the entire universe – by assuming that Earth life is unremarkable and can be considered typical. In other words, the timing of the evolution of life on this planet can be regarded as an average case for evolution in general.
Thus, if Earth is in the vicinity of 4.5 billion years old, and unicellular life popped up around 3.7 billion years back but multicellular life didn’t emerge until a couple of billion years after that, a broadly similar pattern can be expected on other suitable planets. The idea has significant support among other astrobiologists.
Haqq-Misra, however, suggests that the EET hypothesis has problematic limitations, not least because it assumes all planet-star relationships within a habitable zone are equal.
Together with colleague Ravi Kumar Kopparapu he suggests a more complex hypothesis, dubbed “proportional evolutionary time” (PET).
This holds that life only has a chance to develop on any given planet when it is able to take advantage of a “biological available window” in which the energy radiated by the host star is sufficient to provide “free” energy. In other words, Haqq-Misra writes, the mean or expected value for the evolution of multicellular life is “proportional to the main sequence lifetime of the planet’s host star”.
“For example, if the accumulation of abundant atmospheric oxygen is a requirement for the evolution of complex life, then the timing of oxygenation will likely depend upon the free energy on the planet available for photochemistry and greenhouse warming,” he writes.
The PET hypothesis has implications for any estimate of the likely frequency of complex life in the universe.
It does not significantly alter estimates for the possibility of life on exoplanets orbiting main sequence stars that are broadly similar to the sun. However, when it comes to red dwarf stars (also known as M-dwarf stars), which are smaller and dimmer, and a slightly larger form, known as K-type, the numbers head rapidly south.
And that is important, because red dwarfs are by far the most common type of star in the universe.
“The proportional evolutionary time hypothesis predicts that late K- and M-dwarf stars are too young to host any complex life at the present age of the universe,” Haqq-Misra concludes.
Andrew Masterson is a former editor of Cosmos.
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