Building a Sci-Fi Fantasy Solar System & Planet

[scifiwriter888]

I'm hoping that some of you brilliant scientists can help me understand the realities of a world that I'm building for a story in a distant solar system...

I’m building a sci-fi world for a story that takes place on a distant planet that is multiple times the size of earth, but hosts similar, larger, and more varied forms of life that survive on the same basic elemental building blocks as on earth.

Without getting too bogged down in the potential scientific impossibilities of something like this, I do want to make sure that my fictional world doesn’t blatantly disregard the foundations of physics as we understand it. I would be grateful for any advice, thoughts, or ideas about how something like this might exist and how it would affect seasons, length of day/night, weather patterns, etc. I intend to take a lot of creative liberty with it, but want to be aware of the basics. Thank you in advance!

Here are some of the questions I've been researching:

What if a distant planet hosted life like ours but was three times the size of earth? How would this be possible?

What if other planetsthe size of Earth were closer to Earth so that they were even larger and more visible in the sky than our earth’s moon? How would this be possible?

How would larger planets that are closer together affect gravity, tides, day/night, temperature, weather patterns?

 

[Buzz Bloom]

What if a distant planet hosted life like ours but was three times the size of earth? How would this be possible?

Hi scifiwriter888:

For me, this is the easiest of the three questions to suggest one aspect of an answer. I assume that the average density of the stuff the distant planet (DP) is made of is about the same as the Earth's density. (It would be difficult for me to consider the consequences of a different assumption.) With this assumption DP would have 27 times the mass of the Earth and 3 times the radius. This would make DP's acceleration of gravity at the surface three times that of the earth. The weight of creatures of the same size would be three times as great, so the radius of a supporting limb would have to to be about 3^1/2 ≈ 1.732 that of similar sized animal or the earth. For example, and elephant sized creature on DP would need legs 1.732 times bigger in radius than an elephant's leg on earth. Another away to think about this is that an animal on DP with the general shape of Earth elephant would be about 58% as tall as an Earth's elephant.

What if other planets the size of Earth were closer to Earth so that they were even larger and more visible in the sky than our earth’s moon? How would this be possible?

I do not understand his question. I have the feeling that you must have some typos in it. The context "closer to earth" seems to be a change of topic.

How would larger planets that are closer together affect gravity, tides, day/night, temperature, weather patterns?

Again I don't understand this question. "Larger planets" than what? "Closer together" than what?

Hope this helps some.

Regards,
Buzz

 

[scifiwriter888]

Hi scifiwriter888:

For me, this is the easiest of the three questions to suggest one aspect of an answer. I assume that the average density of the stuff the distant planet (DP) is made of is about the same as the Earth's density. (It would be difficult for me to consider the consequences of a different assumption.) With this assumption DP would have 27 times the mass of the Earth and 3 times the radius. This would make DP's acceleration of gravity at the surface three times that of the earth. The weight of creatures of the same size would be three times as great, so the radius of a supporting limb would have to to be about 3^1/2 ≈ 1.732 that of similar sized animal or the earth. For example, and elephant sized creature on DP would need legs 1.732 times bigger in radius than an elephant's leg on earth. Another away to think about this is that an animal on DP with the general shape of Earth elephant would be about 58% as tall as an Earth's elephant.


I do not understand his question. I have the feeling that you must have some typos in it. The context "closer to earth" seems to be a change of topic.


Again I don't understand this question. "Larger planets" than what? "Closer together" than what?

Hope this helps some.

Regards,
Buzz

Thanks Buzz! Your answer to the first question brings to mind some really interesting ideas. It also makes me wonder -- and I know you said that this might be hard to imagine -- but what if this planet was made of less dense material? Is there any physical way for less dense material to make up a planet with a surface that is firm and easy to walk on like earth? Or would less dense material essentially mean the life forms are walking through quicksand or gas or something like that? Please excuse my ignorance!

Sorry that my second and third questions were unclear. Hopefully this helps for both: if a solid planet like Mars were much closer to Earth (so that it appeared much larger in our night sky), how would that affect physical forces or weather patterns on earth? Essentially, imagine if you were standing on this alien planet and some of the other planets in the solar system were so close that they filled a large portion of the sky. For example, what if Earth's moon appeared five times larger in our night sky than it does now (whether that's because it's bigger or much closer)? Does that make sense? I love the idea of this visual, but have a feeling the scientific implications would disrupt the idea of "earth-like" life on this alien planet.

Thanks again for your help.

 

[Buzz Bloom]

Is there any physical way for less dense material to make up a planet with a surface that is firm and easy to walk on like earth? Or would less dense material essentially mean the life forms are walking through quicksand or gas or something like that? Please excuse my ignorance!

Hi scifiwriter:

That's the hard part of analyzing that assumption. You will need some experts in physical chemistry, and that ain't me.

if a solid planet like Mars were much closer to Earth (so that it appeared much larger in our night sky), how would that affect physical forces or weather patterns on earth?

What I think is needed is to pick a particular example of "how much bigger" in the sky. You also need to decide if twice the size means twice the angular diameter as seen from the Earth, or or twice the solid angle area. In particular the moon has an angular diameter as it appears in the Earth's sky of about 1/2 degree, or about 1/720 of a complete great circle in the sky. It also fills a certain fraction of the entire spherical sky that I don't have the time now to calculate.

In general the tidal force is inversely proportional to the cube of the distance, or the cube of the angular size in the sky. So if the angular size doubles, the tidal doubles, and the tides increases in size about eight times. I don't think there is an effect on weather, except perhaps indirectly dues to larger tides. There might be more frequent and stronger Earth quakes, but that is just a guess. There is a limit on how close a moon made of rock material can be to a planet without being pulled apart by the tidal forces of the planet. See

https://en.wikipedia.org/wiki/Roche_limit .​

Hope this helps.

Regards,
Buzz

 

[snorkack]

Earth is almost 4 times as big in Moon's sky than Moon is in Earth's sky - in linear sense.

 

[scifiwriter888]

Hi scifiwriter:

That's the hard part of analyzing that assumption. You will need some experts in physical chemistry, and that ain't me.



What I think is needed is to pick a particular example of "how much bigger" in the sky. You also need to decide if twice the size means twice the angular diameter as seen from the Earth, or or twice the solid angle area. In particular the moon has an angular diameter as it appears in the Earth's sky of about 1/2 degree, or about 1/720 of a complete great circle in the sky. It also fills a certain fraction of the entire spherical sky that I don't have the time now to calculate.

In general the tidal force is inversely proportional to the cube of the distance, or the cube of the angular size in the sky. So if the angular size doubles, the tidal doubles, and the tides increases in size about eight times. I don't think there is an effect on weather, except perhaps indirectly dues to larger tides. There might be more frequent and stronger Earth quakes, but that is just a guess. There is a limit on how close a moon made of rock material can be to a planet without being pulled apart by the tidal forces of the planet. See

https://en.wikipedia.org/wiki/Roche_limit .​

Hope this helps.

Regards,
Buzz

This is all very helpful! Thank you for taking the time to help!

 

[scifiwriter888]

Earth is almost 4 times as big in Moon's sky than Moon is in Earth's sky - in linear sense.

Ah, good point, thanks!

 

[snorkack]

Regarding day and night: Earth is almost 4 times bigger than Moon in linear size, and about 14 times bigger in area. But in terms of brightness, look at this:
https://www.nasa.gov/feature/goddar...nasa-camera-shows-moon-crossing-face-of-earth
Far side of Moon is brighter than near side, because near side has more seas.
The estimate is that full Earth is about 43 times brighter than full near side Moon.

 

[newjerseyrunner]

If you have two objects that are close enough together that they look larger than the moon in our sky, your planet will have wicked tides. The increased mass of the planet would mean that it would probably attract a lot more gas during its formation and have a lot more of it seep out of rocks as it cools, so your atmosphere would be much thicker than Earth's, this is good because in order to have large animals, you need high concentrations of energy. Your creatures would probably have powerful legs to overcome the gravity, but birds and insects would probably look a lot like Earthy counterparts. Gravity would weigh them down more, but the thicker atmosphere would allow them to basically swim through it.

 

[scifiwriter888]

Regarding day and night: Earth is almost 4 times bigger than Moon in linear size, and about 14 times bigger in area. But in terms of brightness, look at this:
https://www.nasa.gov/feature/goddar...nasa-camera-shows-moon-crossing-face-of-earth
Far side of Moon is brighter than near side, because near side has more seas.
The estimate is that full Earth is about 43 times brighter than full near side Moon.

Wow that's an amazing image. Something like that would only be visible with a high powered telescope, right? Is there any feasible way that someone on this distant planet might see a sight like that (a moon crossing in front of another planet) with the naked eye while standing on the surface of this distant planet? Thanks for your help.

 

[scifiwriter888]

If you have two objects that are close enough together that they look larger than the moon in our sky, your planet will have wicked tides. The increased mass of the planet would mean that it would probably attract a lot more gas during its formation and have a lot more of it seep out of rocks as it cools, so your atmosphere would be much thicker than Earth's, this is good because in order to have large animals, you need high concentrations of energy. Your creatures would probably have powerful legs to overcome the gravity, but birds and insects would probably look a lot like Earthy counterparts. Gravity would weigh them down more, but the thicker atmosphere would allow them to basically swim through it.

Hm that sparks some cool ideas. By a thicker atmosphere, do you mean something like fog?

 

[snorkack]

Wow that's an amazing image. Something like that would only be visible with a high powered telescope, right?

Yes, because Moon is far from Earth.

Is there any feasible way that someone on this distant planet might see a sight like that (a moon crossing in front of another planet) with the naked eye while standing on the surface of this distant planet? Thanks for your help.

Well, the problem is with definition of "planet".
Io crossing in front of Jupiter would certainly look bigger than Moon from Earth for a naked eye observer on Europa.

 

[Loren]

I'm hoping that some of you brilliant scientists can help me understand the realities of a world that I'm building for a story in a distant solar system...

I’m building a sci-fi world for a story that takes place on a distant planet that is multiple times the size of earth, but hosts similar, larger, and more varied forms of life that survive on the same basic elemental building blocks as on earth.

Without getting too bogged down in the potential scientific impossibilities of something like this, I do want to make sure that my fictional world doesn’t blatantly disregard the foundations of physics as we understand it. I would be grateful for any advice, thoughts, or ideas about how something like this might exist and how it would affect seasons, length of day/night, weather patterns, etc. I intend to take a lot of creative liberty with it, but want to be aware of the basics. Thank you in advance!

Here are some of the questions I've been researching:

What if a distant planet hosted life like ours but was three times the size of earth? How would this be possible?

What if other planetsthe size of Earth were closer to Earth so that they were even larger and more visible in the sky than our earth’s moon? How would this be possible?

How would larger planets that are closer together affect gravity, tides, day/night, temperature, weather patterns?

Well, I hate to bust your bubble, but the answers are not going to be what you like.

1. Significant gravity increases will drastically impact life. Vertebrates are going to have a tough time with the extra mass. You can probably scratch the idea of birds. Creatures are more likely to be smaller, not larger.

2. Closer planets will most likely provide a very shaky solar system. It's doubtful that the system would last before worlds are cast into unforgivable orbits. The chances of collisions would be higher and the overall result much less likely to support life.

If you want the physics to work out I suspect that you will have a hard time with your proposals. You might want to start researching exobiology. There are many books out there on the art of science fiction world building. You can start there and read and read and read. Once you do that you will be in a better position to derive realistic scenarios.

Or, you could just write stories and steer clear of exotic worlds to play it safe. Or you could just write fantasy genre. Whatever you enjoy.

I have been writing my novel for one year now. Fortunately, most of my time is with the writing, but I still have a significant amount of research devoted to designing systems that are within 10% of Earth's parameters just to avoid excessive effort sorting out the physics.

In the end you don't want to get too wrapped up in the setting. The real story is about the characters and the conflicts they must overcome. Just saying...

 

[scifiwriter888]

Thank you! This is all very helpful. A good reality check!

 

[Khashishi]

Insect-size creatures should have no problem existing with increased gravity.

 

[Jenab2]

This might help.

Part 1. Constants.

G = 6.67384e-11 m³ kg⁻¹ sec⁻²
M๏ = 1.98844e30 kg
R๏ = 6.96e8 meters
L๏ = 3.826e26 watts
T๏ = 5770 K
π = 3.1415926535897932384626433832795...

Part 2. The physical parameters of planets, in terms of each other.

Mass (M).
M(R,g) = gR²/G
M(R,v) = v²R/(2G)
M(R,ρ) = 4πρR³/3
M(g,v) = v⁴/(4gG)
M(g,ρ) = 9g³/(16π²ρ²G³)
M(v,ρ) = v³ √[3/(32πρG³)]

Radius (R).
R(M,g) = √(GM/g)
R(M,v) = 2GM/v²
R(M,ρ) = ∛[3M/(4πρ)]
R(g,v) = v²/(2g)
R(g,ρ) = 3g/(4πρG)
R(v,ρ) = v √[3/(8πρG)]

Surface gravity (g).
g(M,R) = GM/R²
g(M,v) = v⁴/(4GM)
g(M,ρ) = G ∛(16π²ρ²M/9)
g(R,v) = v²/(2R)
g(R,ρ) = 4πGρR/3
g(v,ρ) = v √[(2πρG)/3]

Escape speed from surface (v).
v(M,R) = √(2GM/R)
v(M,g) = ∜(4GMg)
v(M,ρ) = (32πρG³M²/3)^⅙
v(R,g) = √(2gR)
v(R,ρ) = R √(8πGρ/3)
v(g,ρ) = g √[3/(2πρG)]

Average density (ρ).
ρ(M,R) = 3M/(4πR³)
ρ(M,g) = [3/(4π)] √[g³/(G³M)]
ρ(M,v) = 3v⁶/(32πG³M²)
ρ(R,g) = 3g/(4πGR)
ρ(R,v) = 3v²/(8πGR²)
ρ(g,v) = 3g²/(2πGv²)

Part 3. Red dwarf stars, physical parameters.*

Radius.
R/R๏ = −6.1108746(M/M๏)⁴ + 13.1850512(M/M๏)³ − 8.9344001(M/M๏)² + 3.1283874(M/M๏) − 0.0969583

Effective temperature.
T = −128573.17631(M/M๏)⁴ + 196041.80636(M/M๏)³ − 103996.03306(M/M๏)² + 24245.95618(M/M๏) + 1012.30222

Luminosity.
L/L๏ = (R/R๏)² (T/T๏)⁴

Time on Main Sequence, τ, in billions of years.

If 0.5 ≤ M/M๏ < 0.6 then
τ = 10 (M/M๏) / (L/L๏)

If 0.25 ≤ M/M๏ < 0.5 then
τ = 5.10155782e-7 exp(34.779212 M/M๏) + 21395.6632 exp(−11.6197093 M/M๏)

If 0.08 ≤ M/M๏ < 0.25 then
τ = 89.05387 (M/M๏)^(−1.8586791)

Part 4. Primary physical properties of main sequence stars having mass between 0.5 and 2.0 solar masses*

M/M๏, R/R๏, L/L๏, effective temperature (K)

0.50 | 0.412 | 0.02166 | 3450.8
0.51 | 0.421 | 0.02386 | 3494.5
0.52 | 0.431 | 0.02623 | 3537.9
0.53 | 0.441 | 0.02878 | 3581.0
0.54 | 0.450 | 0.03152 | 3623.8
0.55 | 0.460 | 0.03447 | 3666.3
0.56 | 0.470 | 0.03763 | 3708.5
0.57 | 0.480 | 0.04102 | 3750.5
0.58 | 0.490 | 0.04465 | 3792.2
0.59 | 0.499 | 0.04853 | 3833.6
0.60 | 0.509 | 0.05268 | 3874.8
0.61 | 0.519 | 0.05709 | 3915.7
0.62 | 0.529 | 0.06180 | 3956.4
0.63 | 0.539 | 0.06682 | 3996.8
0.64 | 0.549 | 0.07215 | 4037.0
0.65 | 0.559 | 0.07781 | 4077.0
0.66 | 0.569 | 0.08382 | 4116.8
0.67 | 0.579 | 0.09019 | 4156.3
0.68 | 0.589 | 0.09694 | 4195.6
0.69 | 0.599 | 0.10409 | 4234.7
0.70 | 0.609 | 0.11165 | 4273.7
0.71 | 0.620 | 0.11965 | 4312.4
0.72 | 0.630 | 0.12809 | 4350.9
0.73 | 0.642 | 0.13850 | 4394.6
0.74 | 0.654 | 0.15085 | 4446.7
0.75 | 0.667 | 0.16412 | 4498.7
0.76 | 0.679 | 0.17836 | 4550.6
0.77 | 0.692 | 0.19363 | 4602.5
0.78 | 0.705 | 0.20998 | 4654.2
0.79 | 0.717 | 0.22747 | 4705.8
0.80 | 0.730 | 0.24617 | 4757.4
0.81 | 0.743 | 0.26615 | 4808.9
0.82 | 0.756 | 0.28748 | 4860.2
0.83 | 0.769 | 0.31022 | 4911.5
0.84 | 0.782 | 0.33446 | 4962.7
0.85 | 0.795 | 0.36027 | 5013.9
0.86 | 0.809 | 0.38774 | 5064.9
0.87 | 0.822 | 0.41695 | 5115.9
0.88 | 0.835 | 0.44798 | 5166.8
0.89 | 0.849 | 0.48093 | 5217.6
0.90 | 0.862 | 0.51590 | 5268.4
0.91 | 0.876 | 0.55298 | 5319.0
0.92 | 0.889 | 0.59228 | 5369.6
0.93 | 0.903 | 0.63390 | 5420.1
0.94 | 0.917 | 0.67795 | 5470.6
0.95 | 0.930 | 0.72455 | 5521.0
0.96 | 0.944 | 0.77381 | 5571.3
0.97 | 0.958 | 0.82586 | 5621.5
0.98 | 0.972 | 0.88082 | 5671.7
0.99 | 0.986 | 0.93882 | 5721.8
1.00 | 1.000 | 1.00000 | 5771.8
1.01 | 1.013 | 1.04450 | 5796.0
1.02 | 1.027 | 1.09052 | 5820.2
1.03 | 1.041 | 1.13809 | 5844.1
1.04 | 1.054 | 1.18724 | 5868.0
1.05 | 1.068 | 1.23801 | 5891.7
1.06 | 1.082 | 1.29044 | 5915.2
1.07 | 1.095 | 1.34456 | 5938.7
1.08 | 1.109 | 1.40043 | 5962.0
1.09 | 1.123 | 1.45806 | 5985.2
1.10 | 1.137 | 1.51751 | 6008.3
1.11 | 1.151 | 1.57881 | 6031.2
1.12 | 1.165 | 1.64201 | 6054.0
1.13 | 1.179 | 1.70714 | 6076.7
1.14 | 1.193 | 1.77424 | 6099.3
1.15 | 1.207 | 1.84336 | 6121.8
1.16 | 1.221 | 1.91454 | 6144.2
1.17 | 1.235 | 1.98782 | 6166.5
1.18 | 1.249 | 2.06325 | 6188.6
1.19 | 1.264 | 2.14086 | 6210.6
1.20 | 1.278 | 2.22071 | 6232.6
1.21 | 1.292 | 2.30284 | 6254.4
1.22 | 1.307 | 2.38729 | 6276.1
1.23 | 1.321 | 2.47411 | 6297.7
1.24 | 1.336 | 2.56335 | 6319.3
1.25 | 1.350 | 2.65505 | 6340.7
1.26 | 1.365 | 2.74926 | 6362.0
1.27 | 1.379 | 2.84603 | 6383.2
1.28 | 1.394 | 2.94540 | 6404.3
1.29 | 1.409 | 3.04743 | 6425.4
1.30 | 1.423 | 3.15217 | 6446.3
1.31 | 1.438 | 3.25966 | 6467.2
1.32 | 1.453 | 3.36996 | 6487.9
1.33 | 1.468 | 3.48311 | 6508.6
1.34 | 1.483 | 3.59918 | 6529.1
1.35 | 1.497 | 3.71820 | 6549.6
1.36 | 1.512 | 3.84024 | 6570.0
1.37 | 1.527 | 3.96535 | 6590.3
1.38 | 1.542 | 4.09357 | 6610.5
1.39 | 1.557 | 4.22498 | 6630.7
1.40 | 1.573 | 4.35961 | 6650.7
1.41 | 1.588 | 4.49753 | 6670.7
1.42 | 1.603 | 4.63879 | 6690.6
1.43 | 1.618 | 4.78345 | 6710.4
1.44 | 1.633 | 4.93157 | 6730.1
1.45 | 1.649 | 5.08319 | 6749.8
1.46 | 1.664 | 5.23839 | 6769.3
1.47 | 1.672 | 5.35107 | 6788.8
1.48 | 1.678 | 5.45236 | 6808.3
1.49 | 1.684 | 5.55487 | 6827.6
1.50 | 1.690 | 5.65860 | 6846.9
1.51 | 1.696 | 5.76355 | 6866.1
1.52 | 1.703 | 5.86974 | 6885.2
1.53 | 1.709 | 5.97718 | 6904.2
1.54 | 1.715 | 6.08586 | 6923.2
1.55 | 1.721 | 6.19579 | 6942.1
1.56 | 1.727 | 6.30698 | 6960.9
1.57 | 1.733 | 6.41944 | 6979.7
1.58 | 1.739 | 6.53317 | 6998.4
1.59 | 1.744 | 6.64818 | 7017.0
1.60 | 1.750 | 6.76447 | 7035.6
1.61 | 1.756 | 6.88206 | 7054.1
1.62 | 1.762 | 7.00094 | 7072.5
1.63 | 1.768 | 7.12112 | 7090.8
1.64 | 1.774 | 7.24261 | 7109.1
1.65 | 1.780 | 7.36542 | 7127.4
1.66 | 1.786 | 7.48955 | 7145.5
1.67 | 1.791 | 7.61501 | 7163.6
1.68 | 1.797 | 7.74180 | 7181.7
1.69 | 1.803 | 7.86993 | 7199.6
1.70 | 1.809 | 7.99941 | 7217.6
1.71 | 1.814 | 8.13023 | 7235.4
1.72 | 1.820 | 8.26242 | 7253.2
1.73 | 1.826 | 8.39597 | 7270.9
1.74 | 1.832 | 8.53089 | 7288.6
1.75 | 1.837 | 8.66719 | 7306.2
1.76 | 1.843 | 8.80487 | 7323.8
1.77 | 1.849 | 8.94394 | 7341.3
1.78 | 1.854 | 9.08440 | 7358.7
1.79 | 1.860 | 9.22626 | 7376.1
1.80 | 1.865 | 9.50203 | 7419.5
1.81 | 1.871 | 9.73613 | 7453.6
1.82 | 1.877 | 9.97465 | 7487.7
1.83 | 1.882 | 10.21767 | 7521.7
1.84 | 1.888 | 10.46523 | 7555.7
1.85 | 1.893 | 10.71740 | 7589.7
1.86 | 1.899 | 10.97424 | 7623.7
1.87 | 1.904 | 11.23580 | 7657.6
1.88 | 1.910 | 11.50216 | 7691.5
1.89 | 1.915 | 11.77337 | 7725.3
1.90 | 1.921 | 12.04948 | 7759.2
1.91 | 1.926 | 12.33058 | 7793.0
1.92 | 1.932 | 12.61671 | 7826.7
1.93 | 1.937 | 12.90794 | 7860.5
1.94 | 1.943 | 13.20434 | 7894.2
1.95 | 1.948 | 13.50597 | 7927.9
1.96 | 1.953 | 13.81289 | 7961.5
1.97 | 1.959 | 14.12517 | 7995.1
1.98 | 1.964 | 14.44288 | 8028.7
1.99 | 1.969 | 14.76608 | 8062.3

Part 5. The physical parameters of main sequence stars having mass between 2 and 20 solar masses.*

R/R๏ = (M/M๏)^0.57
L/L๏ = 1.505964 (M/M๏)^3.5 − 0.0252982 (M/M๏)^4.5
T = 5770K ∜(L/L๏) / √(R/R๏)

*The curvefits are mine, based on published astrophysical data.

 

[DHF]

If you made the planet with the same composition as Earth then in all probability you would not end up with a terrestrial planet. Much more then twice the mass of the Earth and your planet would begin to draw in more gas and dust and form into a gas giant.

Making the planet out of less dense materials would be the only way I could see it working. There are some other threads here that discuss larger planets with less dense materials. Keep in mind that removing all of the heavy elements is going to impact how life evolves on this planet as lack of all heavy metals and elements is going to give you a very different landscape.

 

[Algr]

I agree with the observation that a higher gravity planet would likely have smaller life forms, as these would resist gravity better. Yet another problem would be oceans: Higher gravity would flatten your mountains and raise your ocean floor. Too round, and a relatively small amount of water would submerge all your land masses, and you have an ocean planet. A planet could be larger without the excess gravity if it had a smaller iron core. But then you might not get the vital magnetic field to protect your atmosphere and life forms from radiation.

If you want something big in the sky, being the moon of a jovian is probably the best way - and you get other moons zipping by as well.

May I ask, what is the dramatic purpose of having such a big planet? Are there alternatives ways to accomplish the same thing?

Edit: Oops, I didn't notice how old this thread was. Sorry.