Would our Sun really eat Earth?

February 5, 2008 7:29 AM

Would an expanding Sun really swallow the inner planets?

Not that it keeps me up at nights, but would the Sun's projected eventual expansion into a red giant actually swallow up the inner planets? Is there actual astronomical evidence of this occurring in other star systems? I have an unfounded theory that the orbits of the planets would somehow be rebalanced further away from the sun by some combination of less gravity from the Sun (red giants are less dense) and other factors I'm not taking into account. Any research to support this?

posted by Burhanistan to science & nature (17 comments total)

Since you're offering an unfounded theory, please accept one in return. Although the Sun would expand and become less dense, its total mass would not change. And that mass would still be centered at the present center of the Sun. So the gravitational forces created by the interacting masses of the Sun and of the planets would not change.
posted by JimN2TAW at 7:35 AM on February 5

The sun is big. It eats whatever it wants. If it wants to get bigger (and it will, all young suns are like that), then it'll pull the inner planets into it without breaking sweat. I don't think there's anything to balance 'cause the size and power difference are already so imbalanced. We are the sun's bitch.
posted by Brandon Blatcher at 7:36 AM on February 5 [2 favorites]

like a parent orbiting a teenager, it's apparently possible to survive phases. but also: no, we're tiny sun candy.
posted by soma lkzx at 7:44 AM on February 5

Is there actual astronomical evidence of this occurring in other star systems?

I think this is where the answer to your question lies. There is very little known data concerning planets from other solar systems (we haven't progressed far enough to see them quite yet, and base our observations off the polarization of light emitted as they orbit their respective stars...and the bulk of those observed are fairly large...roughly Jupiter sized).

I think that once we're able to visually see other planets (which may or may not happen in our lifetimes) we will unlock the answers to many questions like this one. Here is an article related to the scope of your question...although its a little dated it should give you a good idea of what information is available on other stars.
posted by samsara at 7:45 AM on February 5

*Answer to your question lies* WTB edit function :P
posted by samsara at 7:46 AM on February 5

red giants are less dense

It matters not. A change in density doesn't affect gravity.
posted by cmiller at 7:46 AM on February 5

JimN2TAW is quite correct. A spherical object - which the sun is close enough to for our purposes - behaves exactly the same, gravitationally speaking, no matter what its diameter is. Less dense, sure - but still basically the same mass.
posted by Tomorrowful at 7:46 AM on February 5

If all stars did was get bigger when going red giant, then of course the gravity would remain the same. However, they do experience loss of mass, throwing off their outer layers while they expand.

See this, specifically:

Mass loss proceeds rapidly during the second Giant Branch phase.

Eventually, almost 46% of the Sun’s original mass will be lost.

The remaining planets move further outward in response to the reduced central mass of the Sun:
Venus at 1.22 AU
Earth at 1.69 AU

This is just enough to keep them from being engulfed by the swelling Sun.

posted by Freaky at 7:52 AM on February 5

IANAA (I am not an astronomer) but also consider that the sun will likely go through 2 red giant stages - the first will be as the hydrogen fuel source is almost used up, after which there will be a helium flash at which point the sun will contract again, shedding some mass and now burning helium at its core. According to a lecture found here
the sun will only expand to engulf Mercury at this stage. Additionally, the loss of mass and force of expansion could push the remaining inner planets further out.

Many years later, when the helium is exhausted it will again grow in its second red giant phase, although whether or not it will grow enough to engulf the rest of the inner planets will depend on where they are and how long the whole process takes. Of course the planets would likely be nothing more than the shells of their former greatness at this point.

Note: Please do not try to hasten the sun's demise to test these theories.
posted by langeNU at 7:52 AM on February 5

Note 2: I guess I shouldn't have previewed (as Freaky wasn't there when I was) and then I would have beaten him to the punch.
posted by langeNU at 7:55 AM on February 5

There is some evidence that planets can survive their sun's red giant phase.

According to this site, "when the sun becomes a red-giant star in four billion years, its photosphere will remain inside the orbit of Mercury. From Earth, the Sun in this phase would have a diameter of 3° on the sky, versus the current 0.5°."

Furthermore, the sun's mass would change. Through fusion the sun converts about 4 million tonnes of mass into energy. The decrease in mass has an attendant decrease in the Sun's gravitational pull and the planets' orbits expand, though the effect is negligible over the short run. The orbit expands by about 1 meter over 100 years.

However, over the longer run (i.e. once the Sun enters it's red giant phase) the decrease in mass would have been substantial, and the orbits of all of the planets would move further out likely preventing the Earth from being engulfed, however the complex process that create energy around the sun's core in the red giant phase will lead to considerably higher temperatures on earth. According to wikipedia, we have about 900 million years left.
posted by Pastabagel at 8:10 AM on February 5

Believe it or not, this actually does kind of keep me up at night, or at least it did for a while after I started learning about it. Astronomy kind of freaks me out. To chill out about it I've started taking this online course from Yale. I'm hoping more understanding will equal less laying in bed thinking about the nature of existence.
posted by loiseau at 8:23 AM on February 5

Whether or not the Earth is engulfed as the Sun becomes a giant star depends on how much mass the Sun loses and the rate at which it loses it (and a couple of other things like tidal forces and gas drag). Here's a recent article where the process is simulated. These authors find that in order to survive, a planet in our solar system would have to start with an orbital radius of 1.15 AU (that's 0.15 AU more than the Earth's orbital radius).

I don't think there is any direct evidence yet for this process happening in extrasolar planetary systems (but this isn't really my field and I could have missed it if there was). One signature people look for is enhancements in the metal content of a giant star's atmosphere. There are other competing effects that change the star's metallicity though and the signature of gaining an Earth mass of metals is pretty small, so it is not easy.
posted by kms at 12:01 PM on February 5

These guys are saying no in the Monthly Notices of the Royal Astronomical Society.
http://arxiv.org/abs/0801.4031

Found through this site, Universe Today which has more explanation.
posted by Idcoytco at 12:01 PM on February 5

Looks like the people at Universe Today are a week ahead of me! Thanks for the links and answers.
posted by Burhanistan at 12:39 PM on February 5

Some say no but I think it will depend on how fast the expansion contraction cycle is.

Depending on mass loss, etc, really the earth would continue its orbit (or possibly be pushed out if mass loss is great) but I imagine it also would depend that while the earth is in its orbit inside the atmosphere of the expanding sun, would drag bleed off enough of earths orbital speed to cause its orbit to decay? Much like a satellite in earth orbit experiencing atmospheric drag, would it happen fast enough to doom the earth physically?

At any rate, the Earth would be entirely uninhabitable by then - regardless of outcome - so best bet is to hope we have colonized off-planet by then and can watch from a safe distance :)
posted by clanger at 3:27 PM on February 5

Whoever's there to watch it, the term "we" probably won't apply. :)

I'm not a physicist or astronomer, but it seems one major factor that hasn't been addressed yet, affecting what exactly will happen, is the actual density (rather than increases up or down) of the Sun.

For comparison:
Air has a density of about .0012 x 10³ kg/m³.
Water has a density of about 1 x 10³ kg/m³.
Iron has a density of about 7.9 x 10³ kg/m³.

The average density of the Sun throughout its vast size is about 1.4 x 10³ kg/m³. Nearly half again as dense as water, about the same as honey. Since the core's density is about 150 x 10³ kg/m³, it seems reasonable to infer (I couldn't find any definite data) that there's a "surface" of sorts where the convection zone starts, that the convection zone is above the core and has some maximum density significantly less than that of the core, from which it drops off in density to 1.4 x 10^-4 kg/m³, ie .00000014 x 10³ kg/m³. Much like Earth's core, its ocean, and its atmosphere (although there may or may not be an ocean/atmosphere distinction in the convection zone). Actually, more like Earth's inner core and mantle.

The outer Sun isn't even close to the solidity of a cloud. You couldn't walk on it, even assuming you were made of indestructable unobtainium, and could tolerate the conditions. The gravity of 28g would pull you in until at some point the density of the material exceeds gravity's ability to pull you through it. You'd be "mired" in the Sun, or maybe you'd fall all the way down to the the core, the way a rock dropped in the ocean falls to the sea floor.

So what happens to a planet if its star's convection zone expands over it? There is vastly greater heat (the reason the star expands), so all of the mass of the planet will have its temperature greatly increased, but this heat comes from a near-vacuum plasma/gas. With the sudden heating up of the ocean and rocks and metals, all of which have different melting and evaporation points, there will be huge explosions, driving stuff off into space. The former Venus (it seems the most likely candidate to have this happen) would expand and leave a huge trail of gaseous matter, like a comet through the remnants of the Sun. This would probably be visible to sentients watching it.

Over time, my guess is that the planet would "dissolve" into a diffuse ring inside the expanded Sun. As to what happens to that ring of matter, I'm all out of guesses. :)
posted by aeschenkarnos at 4:59 PM on February 5