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The Question

(Submitted February 18, 1998)

I was wondering what would happen to our solar system when the Sun goes into the next phase in its life cycle.

The Answer

You ask a very intriguing question that scientists still discuss.

First, have you read the information in our site about the Sun and the evolution of stars.

If you look at those topics on the above page, you will find a lot of information about what may happen in the VERY distant future. The Sun should burn normally (though gradually getting hotter) for the next 5 billion years or so, which means most elected officials won't even call for studies of the problem for about the next 4.99 billion years or so.


contains an archive of past questions, some of which also ask about the life cycles of stars. You may find some interesting information there.

I may also point out that human beings (Homo Sapiens) as a species are only about 1 Million years old. According to present theory the Sun should use up its Hydrogen fuel, and start to evolve into a red giant star in about 5 Billion years. In such a long period of time it seems strange to even consider humans as being around. Some other species, possibly directly descended from human beings, would have evolved to best adapt to the environment where they are living.

There was an Astrophysical Journal (ApJ) paper a few years back which dealt with the future evolution of the Sun (1993, ApJ, 418, 457). You may be able to find this at a large library, or a college library. It is also available via the Internet at: (

They write:

..... The Sun eventually reaches a luminosity of 2300 L(sun) and a radius of 170 R(sun) on the RGB, shedding 0.275 M(sun) and engulfing the planet Mercury. ... the Sun climbs the AGB, encountering four thermal pulses .... the Sun reaches its largest extent at 0.99 AU ... However, at this point the Sun's mass has been reduced to 0.591 M(sun) and the orbits of Venus and Earth have moved out to 1.22 and 1.69 AU, respectively - they both escape being engulfed. ...

As the Sun sheds mass, the gravitational attraction that the planets feel will decrease and their orbits will get larger.

Mars will definitely become more comfortable (Of course that is a relative term, for me comfortable is about 20 degrees F, with snow falling at a rate of 12 inches per hour) but it will be warmer. To actually guess as actual conditions is pretty tough since predicting the exact weather 1 week in advance is still pretty hard here on earth where we have a lot of information.

Hope this helps,
Mike Arida and John Cannizzo
for the Ask an Astrophysicist team

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