The Universe is 13.8 billion years old, while Earth formed 4.5 billion years ago. Some cosmologists think this time gap means that life on other planets could be billions of years older than ours. However, a theoretical study by scientists from the University of Oxford and Harvard University suggests that life is actually premature from a cosmic perspective.
This is an artist’s impression of an exoplanet. Image credit: Ron Miller.
“If you ask, ‘When is life most likely to emerge?’ you might naively say, ‘Now.’ But we find that the chance of life grows much higher in the distant future,” said Harvard University scientist Dr. Abraham Loeb, lead author on the study.
In the cosmological context, life as we know it could not have started earlier than 10 million years after the Big Bang since the entire Universe was bathed in a thermal radiation background above the boiling temperature of liquid water.
Later on, however, the Universe cooled to a habitable epoch at a comfortable temperature of 32-212 degrees Fahrenheit (0-100 degrees Celsius) between 10 and 17 million years after the Big Bang.
The first stars in the Universe formed 30 million years after the Big Bang. Within a few million years, the first supernovae seeded the cosmos with the elements necessary for life.
Our Sun formed 4.6 billion years ago and has a lifetime comparable to the current age of the Universe. But low-mass stars could live a thousand times longer, up to 10 trillion years.
Dr. Loeb and his colleagues, Dr. Rafael Batista and Dr. David Sloan, both from the University of Oxford, considered the relative likelihood of life between those boundaries.
“The dominant factor proved to be the lifetimes of stars. The higher a star’s mass, the shorter its lifetime,” they said.
Stars larger than about three times the Sun’s mass will expire before life has a chance to evolve.
Conversely, the smallest stars weigh less than 10% as much as the Sun. They will glow for 10 trillion years, giving life ample time to emerge on any planets they host.
As a result, the probability of life grows over time. In fact, chances of life are 1,000 times higher in the distant future than now.
Dr. Loeb said: “so then you may ask, Why aren’t we living in the future next to a low-mass star?”
“One possibility is we’re premature. Another possibility is that the environment around a low-mass star is hazardous to life.”
Although low-mass, red dwarf stars live for a long time, they also pose unique threats. In their youth they emit strong flares and UV radiation that could strip the atmosphere from any terrestrial world in the habitable zone.
To determine which possibility is correct – our premature existence or the hazard of low-mass stars – the scientists recommend studying nearby red dwarfs and their planets for signs of habitability.
Future space missions like NASA’s Transiting Exoplanet Survey Satellite and James Webb Space Telescope should help to answer these questions.
The paper describing this work will be published in the Journal of Cosmology and Astroparticle Physics (arXiv.org preprint).
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Abraham Loeb et al. 2016. Relative Likelihood for Life as a Function of Cosmic Time. Journal of Cosmology and Astroparticle Physics, accepted for publication; arXiv: 1606.08448
