Why isn't the Earth larger?

[eromlige]

When discussing the formation of the planets in our solar system I've always had one question that no one has seemed to touch on yet. Why is the earth so small? Let me explain where i'm coming from. Isn't the earth, and other dense rock-like objects out there, more likely to have a greater effect (gravitationally) than say a sphere of gas of about the same size? How is it that we have and find, these gigantic gas planets and no correspondingly gigantic rocky planets? And why not a combination of the two? A gigantic rocky-gas monstrocity? I don't know the physics, so if someone that does would offer an explanation, I'd love to hear it.<br /><br />-e <div class="Discussion_UserSignature"> <p><font color="#999999">------------------------------------------------------------------</font> </p><p>When you get to my age, and I'm 66 now, you realize that the world is a madhouse and that most people are operating in fantasy anyway. So once you realise that, it doesn't bother you much.</p><p>-- John Cleese</p> </div>

 

[qso1]

The short answer is that we don't have enough actual observational data to know if rocky bodies larger than earth exist. There may be a physical limit as to what the density of a planet can be as far as rocky planets. As for rocky gas monstrosities, the presence of rocky cores for Jupiter like worlds has not been ruled out AFAIK. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[vandivx]

first the answer you are not interested in<br /><br />the Earth is not bigger because there was only so much material in the orbit left for it to scoop up<br /><br />now, if there was material aplenty it would have continued to grow and as it did, the gravitational pressure would be squeezing it more and more untill its interior got very hot and nuclear reactions would start and that would increase the heat in the core further still<br /><br />if still more mass got piled on to make it equal to Jupiter's mass, the Earth would become very much like Jupiter, it would have huge gaseous envelope and no firm surface to walk on etc, the high pressure would at some point liquify the gas as you proceeded into interior (not sure on that count, it might stay gaseous or some sort of exotic form of matter), all due to the heat from the core that wouldn't allow any crust to form and cool off and which would keep elements evaporated in gaseous state<br /><br />I think that's the rough idea as I am no planetary or astronomy expert from this point of view<br /><br />simple answer would be that what makes Jupiter a Jupiter is its mass it has and the same can be said for Earth<br /><br />if you would over do it, you could say that you can never have the rocky planet of the mass of the Sun because it it had its mass it would be the Sun if you get what I am trying to say, mass makes cosmic bodies what they are<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[qso1]

A much better answer than mine and one that I might be able to add to a little bit by looking at exoplanet research. You have basically answered the question as to the transition zone between planets and stars. That is mass. Brown dwarfs are several times the mass of Jupiter and are partially fusing stars as best I can describe it. When the mass threshold reaches beyond brown dwarfs, you get a full fledged fusion process star.<br /><br />However, this does not preclude the possibility of finding a rocky earth maybe two or three times or even ten times earths mass. But I doubt we'd find a Jupiter or even Neptune mass rock. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[MeteorWayne]

As I understand it, by the time earth grew by accretion of planetestimals to a size large enough to attract a lot of gas, the solar wind had begun to clear out the neighborhood. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[docm]

If you believe the giant impact hypothesis Earth I was a smaller body than the current Earth II. <br /><br />The extra material came from Theia/Orpheus, a Mars sized body whose location at an L point became unstable, causing the moon-forming impact. <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

What evidence do you have that the impactor came from an L point?<br />I had not heard that before.<br />Considering the state of the solar system at the time, it seems unlikely that anything would last long in a LaGrange point considering the number of rubble bits floating around. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[docm]

2005 paper by Edward Belbruno & J. Richard Gott III of Princeton, which is now quoted almost everywhere;<br /><br />http://adsabs.harvard.edu/cgi-bin/bib_query?astro-ph/0405372<br /><br />http://arxiv.org/PS_cache/astro-ph/pdf/0405/0405372.pdf<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p><b>Abstract:</b><br /><br />The current standard theory of the origin of the Moon is that the Earth was hit by a giant impactor the size of Mars causing ejection of iron poor impactor mantle debris that coalesced to form the Moon. But where did this Mars-sized impactor come from? Isotopic evidence suggests that it came from 1AU radius in the solar nebula and computer simulations are consistent with it approaching Earth on a zero-energy parabolic trajectory. But how could such a large object form in the disk of planetesimals at 1AU without colliding with the Earth early-on before having a chance to grow large or before its or the Earth’s iron core had formed? We propose that the giant impactor could have formed in a stable orbit among debris at the Earth’s Lagrange point L4 (or L5). We show such a configuration is stable, even for a Mars-sized impactor. It could grow gradually by accretion at L4 (or L5), but eventually gravitational interactions with other growing planetesimals could kick it out into a chaotic creeping orbit which we show would likely cause it to hit the Earth on a zero-energy parabolic trajectory. This paper argues that this scenario is possible and should be further studied.<p><hr /></p></p></blockquote> <div class="Discussion_UserSignature"> </div>

 

[weeman]

Based on our knowledge of our Solar System, rocky and gaseous planets exist in relation to the Sun. Rocky planets, or terrestrial planets, are very solid, and have less gaseous atmospheres compared to the gas giants (jovian planets). Earth, Mars, and Mercury might have had thick atmospheres of helium and hydrogen in the early days of the solar system. However, they were burned off by the intense heat of the Sun when it ignited into a full fledged star from nuclear fusion. <br /><br />The jovian planets have not experienced their atmospheres being boiled away because they are at such greater distances from the sun. So it seems to me that it would be difficult to have a very gaseous-rocky planet. <br /><br />As for the size of Earth, I would also believe that it was just a limitation due to the availability of gas and dust in the early stages of the solar systems formation. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

 

[bonzelite]

<font color="yellow"><br />The jovian planets have not experienced their atmospheres being boiled away because they are at such greater distances from the sun.</font><br /><br />some of the largest and gaseous planets yet observed exist nearly touching their sun. <br /><br />core accretion theory is severely lacking in truth. <br />

 

[MeteorWayne]

docm, thanx! I'll read up on it. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[doubletruncation]

<font color="yellow">When discussing the formation of the planets in our solar system I've always had one question that no one has seemed to touch on yet. Why is the earth so small?...</font><br /><br />I think this is a really interesting question! As I understand it, the general core accretion theory is (similar to how VanDivX mentioned):<br />1. formation of dust bunnies and boulders from electrostatic forces<br />2. boulders - /> planetesimals -> protoplanets via gravity (with distinction being whether an object is big enough to attract other boulders etc. to it, or if it just has enough gravity to hold on to something that happens to hit it).<br /><br />The only things that accrete at this stage would be solid stuff. So how big you can get depends on how much solid stuff is around. There are two competing factors here - 1. as you get farther away in the disk the density of material decreases, 2. if you're close in less of the material in the disk would be in solid form. Beyond the snow-line things like water/ammonia/methane freeze into ices, so at some point (beyond mars) there is a big jump in the amount of solid material present - so you build up bigger solid proto-planets (or cores). <br />3. If the core can grow bigger than ~10 Earth masses it can start accreting the gas directly from the disk at this point the planet undergoes runaway growth (as it accretes gas it gets bigger and becomes more efficient at accreting gas) until the gas is cleared away from the disk. <br /><br />So the cartoon explanation would be that Saturn/Jupiter grew to be rocks that were ~10 times the size of Earth and were able to do that because they are farther away from the sun where more stuff is solid, and then they rapidly accreted gas and became giant planets. Neptune/Uranus formed farther out still where the density decrease in the disk prevented them from getting big enough to undergo runaway accretion, but they still grew to be quite a bit larger than the Earth. (N <div class="Discussion_UserSignature"> </div>

 

[eromlige]

I'll take it!<br />On to the next question. <div class="Discussion_UserSignature"> <p><font color="#999999">------------------------------------------------------------------</font> </p><p>When you get to my age, and I'm 66 now, you realize that the world is a madhouse and that most people are operating in fantasy anyway. So once you realise that, it doesn't bother you much.</p><p>-- John Cleese</p> </div>

 

[MeteorWayne]

One of my top ten post of all my time here at SDC.<br />Thanx!!!<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[robnissen]

Agreed. Tremendous and informative post!!!