Could we exist on earth under a red giant star?

About five billion years from now, the sun’s usual source of nuclear energy will be depleted. It will begin to expand and cool significantly, becoming a ‘red giant’. Its outer layers will be thrown off into space. As the sun loses mass, its gravitational pull will weaken and the planets’ orbit will widen. We know that mercury and venus will not be able to outrun the expanding sun, and will be engulfed and incinerated.

Earth may just outrun the swelling red giant but its proximity, and the resulting rise in temperature, will probably destroy all life on Earth, and possibly the planet itself. However, there’s no reason that life could not survive on another planet (or moon) sufficiently far out from the sun, as long as it lies within the sun’s expanded ‘habitable zone’. Life could also survive on suitably hospitable planets around other Red Giants.

What shape does a planet orbiting

two suns make?

There are many possible orbital shapes that a planet can make in a binary star system. It depends on the masses of the two stars, their separation, the mass of the planet, its distance from each star and whether it is orbiting one star in the pair or both. At sufficient distances the planet will revolve around both stars in an elliptical orbit. Closer in, the orbit may be squashed into various shapes that slowly change orientation, much like the patterns you get from a Spirograph. There are even orbits that trace out long sausage shapes or the petals of a flower, or cross periodically between the two stars. Complex orbits such as these may be rare, since many of them are inherently unstable and would soon break down. Figure-of-eight orbits are not possible.