Do planets have lives?

I was talking to my 13-year-old sun recently about how all things are born, live, and die. We talked about stars and agreed that a star has these three stages. But what about planets?

Aren't planets born when gasses come together by way of gravity to form a mass? But how do planets die? My son thought through erosion, but there isn't any friction in space.

Do planets eventually break up? Isn't this what asteroids come from?

Do planets eventually return to dust and to the matter that originally made them?

Ours will probably expand enormously at the end of its life, engulfing Earth and turning it to vapor.

But what of the remaining planets? Some of the other posts to this thread have interesting concepts which might explain how Neptune (for example) might "die."

They are big rocks. If something hits them, or they fall into something (a black hole, or a star) they will get cooked.

Planets untethered from a star that might go nova could conceivably exist forever, although I suppose radiation pressure can have some erosive effect, but this is negligible.

We are a product of the planets fecundity, not the reverse.

but that is a very loose interpretation of the meaning of 'alive'.

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It's a new discipline, which looks at life not as something that happens on planets, but rather to planets. Tectonic activity may be a requirement for a carbon cycle that sustains life over hundreds of millions of years. When Earth's plates stop recycling the carbon in another billion or three years, life is expected to die with it. Some of the moons of the outer planets are likely to be warmed by the expanding sun, and might conceivably begin their own tectonic/carbon cycles.

That's probably not what you're asking about, however. No, planets aren't going to defy gravity and return to dust, but tidal forces might tear the occasional world apart, while others are swallowed by their aging stars (the latter is a likely fate for Earth). Worlds can collide, too; the Earth-Moon system seems to have resulted from the collision of two planets.

This would help explain how the "life" of planets is shorter than the "life" of the universe...

...which we are right in the middle of. We are also in the middle of a (perhaps) one-billion-year "window" during which complex (multicellular) life is possible. On either side of the window lie two or three billion years of single-celled organisms. We've passed what may have been the peak of biodiversity for life on Earth.

What I got from Stephen Baxter's novel "Evolution" (which is scientifically pretty accurate):

Once the planetary core cools down and turns solid, the planetary magnetic field breaks down. Stellar wind blows the atmosphere away.

Also, hard radiation from the sun can split complex molecules, turning H²O into H² and O². Those are gas and prone to be blown away.

And: Natural erosion by wind and water destroys any and all landmarks (even mountains) given enough time.

I'll pass this information on to my son.

....though probably a bit much for a 13 year-old to wrap his brain around (much less an adult) some thoughts kind of, sort of along these lines, (I feel):

http://www.rainbowbody.net/Hathayoga/Swarayog.htm

http://www.shout.net/~jmh/clinic/science_of_breath/ch13.htm

http://www.yogasuppliesonline.com/yoga_styles/kundaliniyogayog.html

...for what it's worth ~ ~ ~ ~ :-)

It fails all the usual definitions for life. It's got a "life" like my DVD player has a life. It came into existence; it works for a while (increasingly better, as I learn how to use it); it starts having problems requiring treatment; then it dies and is decomposed. Not life. Just a momentary cessation of entropy.

Solar system's life cycle (I think of it as a general time course, to get away any suggestion of "life"):

1. The remains of planetary disk produced by the explosion of a star are hanging in space. The dust and gases are too diffuse to coalesce.

2. Another star goes supernova somewhere nearby. The shockwaves create compression waves. Think of them as concentrations of matter, sort of like what happens to air when sound moves through it. Except that the density is enough to let gravity keep the matter together, allowing the formation of a protoplanetary disk that becomes the Solar System.

3. The sun forms in the middle. The planets slowly form around them.

4. At some point the sun has enough heat and mass to start nuclear fusion. When the light/heat get to the surface, it's enough to clean the gas out of the solar system.

5. There are remains of the original state. Far outside the orbit of Nepture is the Oort Cloud, where most comets originate, and probably asteroids and meteoroids as well. It's vaguely spherical (we think), with a spherical hollowed out portion in which the solar system's planets and sun are found. Closer in, past Neptune, is the Kuiper Belt. It starts off fairly flat and merges with the inside perimeter of the Oort Cloud. Lots of asteroids and meteoroids out there.

The asteroid belt is usually held to be the remains of the protoplanetary disk between Jupiter and Mars that would have formed a planet but which gravitational forces prevented for coalescing.

6. Shortly after the Earth was formed it seems that a roughly Mars-sized protoplanet slammed into it. This produced two things: The Moon and an oversized iron core, and gave the Earth/Moon system a lot of angular momentum (i.e., made it spin faster). This produces, very neatly, a strong magnetic field. This magnetic field protects the Earth's atmosphere from being eroded by the particles produced by the Sun, the solar wind.

7. The Earth probably had an atmosphere probably prior to the production of the Moon. It was lost to the solar wind.

8. The Sun is following it's own life cycle. It started off brighter than it is now. It's dimmed for a while, and now is getting gradually bright. No, not fast enough to account for global warming. In a billion years the Earth won't support life as we know it, if the Sun follows the usual sequence of stages. The Earth will be far too hot, greenhouse gases won't matter much at that point. By then the Earth's core may have cooled enough that it won't have a strong magnetic sphere, so it's possible it won't have much of an atmosphere, anyway.

9. Fastforward 3-4 billion years after that and the Sun will have exhausted much of its hydrogen. It will expand and become a red giant, it's circumference somewhere near Earth's orbit (possibly closer to Mars). The Earth will have cooked pretty well by then. It may continue to orbit inside the red Sun, simply because the outer layers of such stars tend to be pretty thin stuff. Needless to say, Mercury and Venus are further in; if Mars is still outside the Sun, it'll be baked. Jupiter will heat up a bit. Some helium fusion and perhaps even carbon fusion will take place. But the Sun's not massive enough for that to go on long.

10. Regardless of the fate of the other planets, the Sun isn't big enough to go supernova nor produce a black hole. It will become a white dwarf. The inner portions of the Sun will collapse and the resulting shockwave will cause the outer layers--most of the Sun--to be ejected at great speeds.

You have to know that the Sun is kept from collapsing by a variety of forces. In part, the shere amount of radiation produced in the core helps prop up the outer layers. At some point, though, the particles that make up atoms, electrons and protons and neutrons, simply push against each other. It takes more force than the Sun's gravity produces to crush them any closer together. As the fusion in the center cools those forces start to win and the Sun swells. When the fusion in the core essentially stops, though, gravity wins. The outer layers rush inward, compress, and rebound. (Think of a ball bouncing.)

The Solar system is essentially gone at that point, only the Sun's core remaining, a superdense chunk of white-hot matter that is producing no new energy. It will eventually cool from white dwarf through red dwarf to black dwarf.

The Universe has billions of years of life in it after that. Around the time the Sun goes nova and becomes a white dwarf is when we're scheduled to interact with the Andromeda Galaxy. The matter--the atoms making up your body and that of your kid--will be blown off into space where there's a decent chance they'll coalesce with lots of other atoms and molecules to form another protoplanetary disk that may give rise to planets.

as they're not alive. Planets can "die" when their cores cool or freeze.