The ‘habitable zone’

Credit: The Habitable Exoplanets Catalog, PHL@UPR Arecibo (

[1]In around 5 billion years the Sun will have exhausted the hydrogen fuel in its core and will begin to expand. At its largest, it will approximately reach the orbit of the Earth, before losing its atmosphere completely as a planetary nebula and leaving the core to become a white dwarf. The evolution of the Sun into and through the red-giant phase has been extensively modelled, but it is still unclear whether the Earth will be engulfed by the Sun or will barely survive. At its brightest, the red-giant Sun will be several thousand times more luminous than today but be at about half the temperature.

[2]If we remain on Earth we will surely become extinct, and probably long before an expanding Sun roasts our planet.

[3]Findings published today in the journal Astrobiology reveal the habitable lifetime of planet Earth – based on our distance from the sun and temperatures at which it is possible for the planet to have liquid water. The research team looked to the stars for inspiration. Using recently discovered planets outside our solar system (exoplanets) as examples, they investigated the potential for these planets to host life.

The research was led by Andrew Rushby, from UEA’s school of Environmental Sciences. He said: “We used the ‘habitable zone’ concept to make these estimates – this is the distance from a planet’s star at which temperatures are conducive to having liquid water on the surface.”

“We used stellar evolution models to estimate the end of a planet’s habitable lifetime by determining when it will no longer be in the habitable zone. We estimate that Earth will cease to be habitable somewhere between 1.75 and 3.25 billion years from now. After this point, Earth will be in the ‘hot zone’ of the sun, with temperatures so high that the seas would evaporate. We would see a catastrophic and terminal extinction event for all life.

“To date, no true Earth analogue planet has been detected. But it is possible that there will be a habitable, Earth-like planet within 10 light-years, which is very close in astronomical terms. However reaching it would take hundreds of thousands of years with our current technology.

“If we ever needed to move to another planet, Mars is probably our best bet. It’s very close and will remain in the habitable zone until the end of the Sun’s lifetime – six billion years from now.”


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