How will the expansion of the Sun influence Earth as a celestial body after 5 billion years?

Question

Science news have flooded our news feed again about how our Sun could swallow Earth. This time due to a real documentation of a similar event in an other system. theguardian washingtonpost

From what I've read over the years in various cosmology articles about the future of our Sun, there are three scenarios:

1. The Sun expands swallowing Earth
2. The Sun expands and pushes Earth to a new orbit
3. The Sun becomes a dwarf, Earth turns to an ice planet

What I don't understand is how Sun's expansion can influence a planet's orbit since the expansion is a product of internal nuclear reactions and the mass is essentially the same. What changes is the volume. An orbit does not change due to volume changes right?

By "swallowing" do they mean the Sun will literally expand by 150.000.000 km to actually reach Earth or is the "swallow" a result of an Earth orbit destabilization dropping it in the Sun ?

Answer 1

Yes, the Sun will expand and will probably reach a radius of about 1 au (the current distance between the Earth and the Sun). This wil occur in just over 7.5 billion years time (not 5 billion, e.g. Schroder & Smith 2008).

However, it will also lose mass while it is doing so. This loss of mass, in the form of a vastly enhanced solar wind, means the Earth's orbit will get wider.

It is as yet unclear whether the Earth will escape engulfment. It depends on the exact timing and rate of mass loss. Most likely it will, but then will be dragged into the giant Sun's outer layers by tidal forces, since the giant Sun's rotation period will be slower than the Earth's orbital period.

More detail and references in the answer to this question.