Earth is the only inhabited planet we know of, which makes it our only template for how a living world works. The geological record provides a guide to how a young habitable planet may look and how it may evolve with time. Earth today shows us how a middle-aged world with a rich biosphere operates. But what about the later stages of a living planet's lifetime? By looking at how stars like the Sun age, we can predict what conditions might be like on Earth millions, hundreds of millions and even billions of years from now. This gives us clues about what types of life could and could not live there and ultimately tells us how long a habitable planet can actually remain habitable.
The future of life on Earth is ultimately dependent upon what happens to the Sun as it ages. Observations of stars that resemble older versions of the Sun show us that the energy released by sun-like stars per unit time (their luminosity) will increase as they age. What does this mean for our Sun?
Over the next billion years, the energy Earth receives from the Sun will increase such that, a billion years from now, the Sun’s luminosity will be 10% higher than it is today. This does not seem like a huge change, but this extra energy will be amplified by Earth’s climate system. Predicting these changes is a much fuzzier and less precise art than the highly detailed simulations we have for changes to Earth’s climate over the next few decades. Nevertheless, we can still make some general predictions about what could happen and what these changes would mean for life on Earth. This is the approach I took for a series of papers that cover the fate of life on Earth and how this fate influences the biosignatures of a living planet over time.
The Swansong Biospheres papers chart the gradual heating of the Earth over the next few billion years and how this causes a decline in habitable conditions, eventually resulting in the extinction of plants and animals within the next billion years. However, microbes can cope with a much wider range of environmental conditions. So, after the loss of plants and animals, Earth becomes a microbial world (similar, at least initally, to the early Earth before multicellular life evolved). Yet even this world has an expiry date; an end to the planet's habitable lifetime after a further 1-2 billion years. I recently summarised this work for a general audience in a Cornell Cosmic article.
The 'end of the world', even if it is not due to happen for billions of years, is an evokative topic. As a result this work received a lot of media attention, giving me opportunities to talk about it on televsion and radio (including BBC World News, the BBC's Human Universe series, BBC Radio 4, NHK's documentary 'Cosmic Front: Journey to the Earth's Future') and resulting in articles in publications such as New Scientist, National Geographic and Science et Vie (a full list of media coverage can be found here.)
'In Search of Future Earths' builds upon the timelines outlined in the Swansong Biospheres papers to explore the question of how many habitable exoplanets in the galaxy are likely to resemble older versions of Earth. The results suggest that these 'Old Earth Analogues' may be rare in our galaxy. Nevertheless, finding one nearby would present a great opportunity to test some of the predictions about our own planet's far future.
In Search of Future Earths: Assessing the possibility of finding Earth analogues in the later stages of their habitable lifetimes: O'Malley-James J.T., Greaves J.S., Cockell C.S., Raven J.A. (2015), Astrobiology 15: 400-411.