when the sun becomes a red giant, what planet will be on the habitable zone then?

None of the major planets, nor moons of the major planets will be habitable then.

MAYBE during its gradual expansion stages, Mars or eventually some of the outer planets might be habitable zone...but not once it fully expands to a red giant.

If you do the calculation predicting the sun to have a radius of 250 solar radii and a photosphere temperature of 3000 Kelvin (these are typical values for red giants, and according to http://en.wikipedia.org/wiki/sun, they are predicted values)...

Use the formula

T = Ts*sqrt(Rs/d), and solve for d (careful with units, of course)

This formula will provide a first order temperature estimate for a planet, based solely upon parent star radius, parent star temperature, and separation distance from star to planet.

d = Rs*(Ts/T)^2

Plug in T = 280 Kelvin, the present first order temperature estimate for Earth. You will get d=124 Astronomical units. That is 124 times as distant from the sun as Earth is now.

For purpose of comparison, Neptune's orbital radius is roughly 30 AU. Pluto's aphelion distance is roughly 50 AU.

So, perhaps it may be some tail end Kuiper belt object about twice as far from the sun as Pluto.

Mars will be at the ideal point in the habitable zone, Earth will not be at all and the some of the Jovians will be in the mix as well, so as a result the moons of those planets will also be in the habitable zone

One day Jupiter's gas will be stripped away and will become habitable possibly however the sun will lose some of its gravitational pull as it gets much bigger and it will eventually get 250 times bigger as it gets as it enters the giant stage and then its lifespan will be very short during this period and eventually none of the planets will be habitable the other Giants will always be frozen it's likely to never engulf Earth although that still debatable because like I said it will lose some of its gravitational pull but it will certainly eat Mercury and Venus

I saw a special on the history channel. Scientists are predicting that the sun will expand out to completely engulf Mercury and Venus. Earth will still orbit the sun, but will become so hot that the atmosphere and oceans will evaporate into space and the earth's crust will become nothing but hot black metal and flowing rivers of magma.

This would not be habitable at all for life. Perhaps humans will advance enough to escape the solar system before this happens, who knows.

The liveable zone will flow out to Jupiter, Saturn, Neptune, who knows. It relies upon on precisely how great the solar turns into. If Jupiter is already occupying the "liveable zone" then it truly is merely no longer attainable to position the different planet everywhere in that zone for any prolonged length of time. This journey received't happen for yet another 4 or 5 billion years. what percentage comet impacts do you assume in that factor body? what percentage rogue planets do you foresee stepping into our photo voltaic gadget. it truly is unclear if our photo voltaic gadget is even strong over this variety of time body. interior the time body of geological historic previous, the prevailing time will be seen as a time of mass extinction. Humanity is busy replacing this planet right into a planet of one species. when we are the in user-friendly words species left, the planet will be merely one step from starting to be sterile. on account that we are also busy hard all mineral elements from this planet, no matter if another species replaces us in a pair million years, they could don't have any mineable elements to exploit and could be always stuck on the stone age. If we want to keep the biosphere we could do it now, no longer in 4 billion years time. if you're writing a e book perchance you could evaluate those elusive information. Cheers!

Mars and then some moons on Jupiter, because Jupiter and the planets beyond are gas planets.

Although Mars will be in the zone , habitability for humans or other mammals is limited as the hydrogen / oxygen { water } content is limited .

In order for a planet to be habitable for humans it has to have an atmosphere, certain temperature, gravity, pressure, nitrogen, oxygen, etc. Earth is the only planet in our solar system that has the appropriate habitability factors for humans.

Jupiter

Hopefully by then we will have populated planets outside of our solar system.