What will happen to the Earth when the Sun dies?

It might be a good idea to read this post about what happens when a star dies for some background info before reading this post!

Our star, the Sun, should last another 5 - 8 billion years or so, but when it does eventually die, what will happen to our home planet, the Earth?

When the Sun has used up most of its fuel, as described in this post, it will start to expand to become a red giant star. The sun will expand so much during this phase that it will swallow up Mercury and Venus, and continue expanding until its diameter is a bit larger than Earth's current orbit.

The diagram shows a comparison between the sizes of the Earth, the Sun as it is now, and the size of a typical red giant (diagram nicked from elsewhere- click to visit).

So the Earth will be swallowed by the Sun?

Not necessarily. Yes, I just said that the sun will expand to fill Earth's current orbit, but the Earth may not still occupy that orbit by the time the Sun dies.

As the Sun turns into a red giant, its expansion is not smooth. It pulsates as the processes occurring inside it produce energy at varying rates. This pulsation is accompanied by the generation of a stronger stellar wind, which is the mechanism by which the Sun loses mass. As the Sun's mass reduces, the handhold it has on the planets (gravity) weakens, and the planets will start to spiral slowly outwards.

The question now is which will happen first: the Sun expanding to just beyond Earth orbit, or the Earth spiralling outwards from the Sun's reach? The calculations are difficult, and there is contention on this issue: we just don't know yet! So there are two possible fates for Earth as the Sun dies:

1. The Sun expands too fast for the Earth to get out of the way

The Earth will have moved slightly, but eventually the Sun's atmosphere will engulf the Earth. Earth will gradually be slowed in its orbit by particles from the Sun's atmosphere, and it will begin to spiral inwards and be consumed by the Sun.

2. The Earth moves outwards faster than the Sun expands

The Earth will escape the grasp of the Sun, but it will no longer be within the Sun's habitable zone. These means that although it won't be destroyed, the oceans will evaporate, and then the hydrogen will be stripped from the water by the solar wind. Over time the Earth's surface will heat up and gradually become molten.

What about life?

We will have moved on, in one way or another^[1], by the time the Sun dies. The habitable zone will have moved outwards from the Sun, much further out than it is now, possibly even towards the Kuiper belt, where there are many icy bodies that can provide plenty of water, the main necessity for life.

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1. We may have moved outwards to other bodies within the solar system, or started to colonise the galaxy. We could even have become extinct. [back]