The future looks bright for the Earth – but not in the way we’d hoped. The slim chance our planet will survive when the Sun begins its death throes has been ruled out.

In a few billion years, the Sun will fuse the last of its hydrogen into helium, turn into a red giant and expand to 250 times its current size. At first, the Sun’s loss of mass will loosen its gravitational pull on Earth, which will allow the planet to migrate to a wider orbit about 7.6 billion years from now.

This process has led some to speculate that theEarth might escape destruction – but survival now seems impossible, says Peter Schröder of the University of Guanajuato in Mexico and Robert Smith of the University of Sussex in the UK.

They created the most detailed model to date of the Sun’s transition to a red giant, based on observations of six nearby red giant stars. Sure enough, they found that Earth’s orbit will widen at first. But Earth will also induce a “tidal bulge” on the Sun’s surface, with its own gravitational pull. The bulge will lag just behind the Earth in its orbit, slowing it down enough to drag it to a fiery demise.

There is one last hope for anybody still living on Earth, the researchers say. In the past, some have suggested that Earth’s orbit could be tweaked by arranging thefly-by of a nearby asteroid to tug at it. This method could potentially maintain Earth’s speed enough to keep it in a widening orbit, they say.

Journal reference:Monthly Notices of the Royal Astronomical Society (DOI: 10.1111/j.1365-2966.2008.13022.x)