The habitable zone

I know that I've discussed this before, but I think that new ideas have appeared that cast some light on what will become of humanity if it survives billions of years into the future. Think of this as a guide to how humans might survive the inevitable doom that's coming in the future.

First of all, we now know that the location of the so-called habitable zone around a star is defined as the minimum and maximum distances the planet must orbit within the region to have liquid water on its surface. This assumes that the planet has an atmosphere.

As you know, the planet Earth lies in the habitable zone relative to our star, the Sun. The Earth is 93 million miles away from the Sun and this is defined as 1.0 AU, or one astronomical unit. When we speak of the habitable zones we're concerned with rocky planets like the Earth. For comparison, the other three rocky planets in our solar system are not in the habitable zone. Mercury is at 0.38 AU and is tidally locked to the Sun. One side is very hot and the other is very cold. Venus is at 0.72 AU, and is the hottest planet in our solar system because of run-away global warming. Mars is at 1.52 AU and is mostly cold. Water there is frozen in its polar regions.

The thing to keep in mind here is that Venus was once in the habitable zone about 2 to 3 billion years ago and was probably like Earth is now. Venus could have supported life, but that was destroyed when Venus experienced run-away global warming. It's now a steamy planet with an atmosphere that's over 50 times the pressure of ours and experiences temperatures that would melt lead.

The way to know where the midpoint of the habitable zone of a particular star is to take the square root of the ratio of the star's luminosity compared to our Sun. That provides the AU value for the habitable zone. For example, if a star has twice the luminosity of our Sun, the AU would work out to the square root of 2 or 1.41 AU. From this it's obvious that if a star is brighter it means that the habitable zone is further away from it.

So, what does this mean? Well, it means that humans are going to have to leave the Earth and move further out into the solar system when the Sun becomes hotter. The Sun is thought to be very stable over the four billion and a half years it has been around, but that's not exactly true. The luminosity of the Sun increases about 10% every billion years. But, this is not the biggest threat because as the Sun gets brighter and heats the Earth hotter, many species, even humans, will need to adapt to the warmer climate.

Astrophysicists believe that the Earth will be so hot in 500, 000 years the summers will be unbearable and the winters will be when people will go out of their shelters, and I say shelters because that's what it will take to survive a 20 degree increase in global temperature. By a billion years from now the oceans will begin to boil away and fill the atmosphere with the most effective greenhouse gas, water vapor. This action will begin to turn our planet into a Venus where the temperature will increase rapidly to the point that all life will be destroyed except for thermal bacterial that like it hot. By this time it will be a good idea to move out to Mars, which will be in the habitable zone.

So, here is the timetable that humans must move away as the habitable zone of the Sun advanced further out into the solar system. First stop is Mars. Mars in two billion years will be habitable. All mankind has to do is create a breathable atmosphere there.
Next stop is the moons of Jupiter in three billion years. The ice moons like Io, Ganymede and Europa will melt and be like ocean moons. Hopefully, there will be a easy way to create oxygen from water using electrolysis.

About four to five billion years, the Sun will run out of fuel in its core and the core will begin to collapse because all that helium it created from fusing hydrogen is denser and will squeeze the core until it gets hot enough to fuse helium. This will be a dramatic change in the Sun. Our nice orange dwarf star will begin to expand into a red giant that will ultimately consume the inner planets, including Earth. The new red giant Sun will be 3000 times as bright as it is now. Jupiter will be heated up to the point that ammonia will vaporize in its atmosphere and will change it to a blue color. By this time Earth will be changed to molten lava and then be absorbed into the Sun. Mercury and Venus will be long gone before that.

Bu this time mankind needs to be a far out as Saturn. Saturn has some ice moons like Encellides that could be made habitable. However the pace of the warming effect will accelerate so it's out to Uranus and eventually Neptune's moons. But, even that won't be far enough away towards the end. Maybe humans will end up on Pluto. Think about that.

Ultimately, the red giant will blow up in 7 to 8 billion years to form a planetary nebula and the core will be the only thing left as a white dwarf, which will not be very effective as a star. The only thing that humans could do then is move to another star system because the solar system will be a very dark cold place.

Thanks for reading.