Bad news, fellow denizens of Earth: According to some new astrobiological research, our home planet will not be habitable for much longer. As our Sun gets older, it will get larger and warmer, eventually leading to the Earth becoming uninhabitable — first to humans and other complex life in fairly short order, and then, in around 1.75 to 3.25 billion years, to all cellular life as we know it. Due to anthropogenic climate change, and other variable factors, we don’t know exactly when human life will become untenable on Earth, but the conclusion of the study is pretty clear: Our time here on Earth is finite, and we better find our way off it sooner rather than later.
The research was carried out by Andrew Rushby and fellow researchers from England’s University of East Anglia. There are many factors that dictate a planet’s habitability, but in this case the astrobiologists focused on how long Earth-like planets remain in the habitable zone (HZ) of main sequence stars. Basically, most stars adhere to the main sequence — an order of events that begins with a star becoming dense enough to fuse hydrogen into helium. Over time, these stars get hotter and hotter, until they start to run out out of hydrogen. As the star gets hotter, nearby planets also get hotter, until eventually they become uninhabitable — usually due to a lack of surface water. (The photo above is of the remnants of a supernova, incidentally; our Sun probably won’t go supernova.)
In the case of the Earth, the Sun’s increasing warmth will push our planet out of the HZ in around 1.75 billion years, with an upper bound of around 3.25 billion years, depending on the model used by the researchers. Here they are talking about habitability in absolute terms; for humans, Earth will become very unpleasant much sooner. ”Of course conditions for humans and other complex life will become impossible much sooner – and this is being accelerated by anthropogenic climate change,” says Rushby. “Humans would be in trouble with even a small increase in temperature, and near the end only microbes in niche environments would be able to endure the heat.”
As part of the same study, the astrobiologists also investigated the length of the habitable phase for other exoplanets, such as Kepler 22b and Gliese 581d, which are both theorized to be within their parent star’s habitable zone. While 22b’s habitable phase is around the same length as Earth’s (4.3 to 6.1 billion years), Gliese 581d, due to the nature of its orbit around its small home star, is expected to be habitable for around 55 billion years.
How long planets remain in the habitable zone, versus host star size. (Smaller stars’ habitable zones last longer.)
Given the almost impossible-to-fathom scale of the universe, and early analysis of data from the Kepler space telescope, there could be up to 50 sextillion Earth-like planets out there in the cosmos. This is a mind-bogglingly large figure (5 followed by 22 zeroes), but in short, combined with this new data from UEA, it’s very, very likely that there are other planets in the universe that have managed to cultivate lifeforms during their multi-billion-year tenure in the habitable zone.
It’s also good news for humans: Assuming we can find a way off this planet in the not-too-distant future, we should have no problem finding a planet that can host us for a few billion years (assuming we don’t ruin it, like we did Earth, of course).
Research paper: DOI: 10.1089/ast.2012.0938 – “Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars” [Free PDF]