Billions of years in the future, when our Sun bloats up into a red giant, it will expand to consume the Earth's orbit. But wait, you say, the Earth travels the Earth's orbit... what's going to happen to our beloved planet? Will it be gobbled up like poor Mercury and Venus? Astronomers have been puzzling this question for decades. When the sun becomes a red giant, the simple calculation would put its equator out past Mars. All of the inner planets would be consumed. However, as the Sun reaches this late stage in its stellar evolution, it loses a tremendous amount of mass through powerful stellar winds. As it grows, it loses mass, causing the planets to spiral outwards. So the question is, will the expanding Sun overtake the planets spiraling outwards, or will Earth (and maybe even Venus) escape its grasp. K.-P Schroder and Robert Cannon Smith are two researchers trying to get to the bottom of this question. They've run the calculations with the most current models of stellar evolution, and published a research paper entitled, Distant Future of the Sun and Earth Revisted. It has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. According to Schroder and Smith, when the Sun becomes a red giant star 7.59 billion years, it will start to lose mass quickly. By the time it reaches its largest radius, 256 times its current size, it will be down to only 67% of its current mass. When the Sun does begin to bloat up, it will go quickly, sweeping through the inner Solar System in just 5 million years. It will then enter its relatively brief (130 million year) helium-burning phase. It will expand past the orbit of Mercury, and then Venus. By the time it approaches the Earth, it will be losing 4.9 x 1020 tonnes of mass every year (8% the mass of the Earth). But the habitable zone will be gone much sooner. Astronomers estimate that will expand past the Earth's orbit in just a billion years. The heating Sun will evaporate the Earth's oceans away, and then solar radiation will blast away the hydrogen from the water. The Earth will never have oceans again. It will eventually become molten again. One interesting side benefit for the Solar System. Even though the Earth, at a mere 1.5 astronomical units, will no longer be within the Sun's habitable zone, much of the Solar System will be. The new habitable zone will stretch from 49.4 AU to 71.4 AU, well into the Kuiper Belt. The formerly icy worlds will melt, and liquid water will be present beyond the orbit of Pluto. Perhaps Eris will be the new homeworld. Back to the question... will the Earth survive? According to Schroder and Smith, the answer is no. Even though the Earth could expand to an orbit 50% larger than today's orbit, it won't get the chance. The expanding Sun will engulf the Earth just before it reaches the tip of the red giant phase. And the Sun would still have another 0.25 AU and 500,000 years to grow. Once inside the Sun's atmosphere, the Earth will collide with particles of gas. Its orbit will decay, and it will spiral inward. If the Earth were just a little further from the Sun, at 1.15 AU, it would be able to survive the expansion phase. Although it's science fiction, the authors suggest that future technologies could be used to speed up the Earth's spiraling outward from the Sun. I'm not sure why, but thinking about this far future of the Earth gives an insight into human psychology. People are genuinely worried about a future billions of years away. Even though the Earth will be scorched much sooner, its oceans boiled away, and turned into a molten ball of rock, it's this early destruction by the Sun that feels so sad. Subscribe to stay updated and educated.

• published: 08 Jun 2014
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