So, I've been writing short genre fiction for various classes and workshops going on about 4 years now, and I think I've finally amassed enough writing experience to try my hand at the light sci-fi / heavy fantasy novel I've had stewing in my brainpot for months.

I've got a pretty interesting magic system worked out, but I'm having a wee bit of trouble with the physics bits. I understand that the 'how' doesn't really matter all that often in fantasy, that characters and plot are the driving forces, and that I can always employ the "because magic" standby, but I really would prefer to craft the world with a semblance of plausible physics (however wildly improbable those theories might be).

So, without further ado, I have just a few questions for those of you out there with a mind for theoretical astrophysics and potential effects on alien life.

(Q1)

I've been reading a fair bit about mega-earths, planet size/density, and about how massive a planet can get before deuterium fusion occurs (classifying the celestial mass as a star).

Do you think its possible, somewhere in the universe, for there to exist an oceanic planet roughly half the size of Jupiter with 1, maybe 2 small silicate-based volcanic continents? If there was a lack of gaseous particulates in the solar system, or perhaps a much larger giant in the same system that would siphon more of the free gaseous material, is it theoretically possible for a planet of this size to not become a gas giant?

(Q2)

I want daylight on one side of my planet to last hundreds of earth years. Is there any reason why a planet couldn't theoretically be closely, but not completely tidally locked with a star?

(Q3)

Given a prograde satellite rotation that is only a small percentage faster than a perfect tidal lock (moon earth example), to a continent on the daylight side of the satellite, the star would still set in the west, yes? Just at a very slow rate?


(Q4)

On a planet of such scale and mass, assuming it has a moon, could tides occur? Or would gravity keep ocean flow relatively stationary?

(Q5)

I've already come up with a few suitable lifeforms that might exist on the planet, with some tweaks to account for the extreme gravity. The thing I'm having trouble with is food sources and the food sources of the food sources. I'm thinking some monstrous mammalian-esque sea creatures, some really large, flattened spid- ...ahem... creatures that use octopedal locomotion, and an eclectic selection of vegetation a bit like rhyzomes, with widespread tubular roots. Do those options sound plausible...ish for such conditions?


Thanks in advance for any answers and replies you might have. When and if I have a novel outline / draft of a chapter I'll come post it for the ol' DLP rip apart.

 

Can't help you with too much of the sciency stuff, but where do the majority of people live on this planet? It would have to be close to wherever day is becoming night, would it not?

So the civilisations live in perpetual twilight, as the sun would be roasting half the planet for hundreds of years, while the other half lives in frozen darkness? As it slowly spins over the years the civilisation moves with the twilight?

 

Yes. Exactly. The star moves about a fingerspan across the horizon every few generations. And the story essentially opens in the final months before the last sliver of starlight disappears.

Several groups of characters will live in various city states on the daylight continent, which is mostly arid savanna, some rainforest, and desert. Another group have been traveling on a floating city, keeping to the shadow and cold.

I'm probably going to call it Starfall. Or Duskrise. I haven't quite decided if I want to do multiple novels yet either.

 

While I'm not brilliant at physics I think that it would be possible for this planet to have a moon and tides but it would all depend on the mass of the planet and the moon. Basically, if the moons mass is large enough (the size of the moon doesn't matter) it would be able to cause tides.

I don't believe so.

Also, on a side note, you should go to this site: http://arxiv.org/ftp/arxiv/papers/1405/1405.1025.pdf
and read that pdf. The first few pages aren't too important but it does talk about what life would be like on a tidally locked planet, and the atmosphere and a few other things. This may help you answer Q5.

 

I think you might be underestimating the power of gravity here:

Jupiter's radius: ~70,000,000 metres

You say you want a planet half the size of Jupiter, so:

Planet X radius: 35,000,000 metres

It's not a gas giant like Jupiter though. Presumably we want roughly Earth-like structure. So let's take Earth's density:

Earth's density: 5520kg/m^3

Mass = density * volume

Mass = density * (4/3 * pi * r^3)

Mass of planet X = 5520 * 4/3 * pi * 35,000,000^3

Mass of planet X = 9.9 * 10^26 kg

Acceleration due to gravity is determined thus:

g = MG/R^2

Where G is the gravitational constant: 6.67 * 10^-11 m^3 kg^-1 s^-2

So:

g = (9.9*10^26)*(6.67 * 10^-11)/(3.5 * 10^7)^2

Gravity on planet X = 53.9 m/s^2

Earth gravity is 9.81 m/s^2

So gravity on your planet is about five times greater than on Earth. I recommend reading this:

http://en.wikipedia.org/wiki/Gravitational_biology

Edit:

You might also be interested in this:

http://en.wikipedia.org/wiki/G-force#Human_tolerance_of_g-force

Any human on your planet would be experiencing a constant vertical force > 5g. That force is pulling their blood down to their feet, away from their brain. 5g is the limit of what a typical human can endure without losing consciousness. I don't know if you're planning on having humans on your planet, but it's doubtful that they'd be able to survive.

 

Unless there's a much more exotic planet formation mechanism going on than what we understand today, when rocky planets get bigger than around 10 Earth masses they tend to grab all the surrounding gaseous matter and turn into gas or ice planets. (Since planets are held together by gravity, the heavy stuff (rocks) tends to sink to the core and the lighter stuff (hydrogen, ammonia, CO2, etc.) tends to rise to the top.) This sets an effective upper bound on rocky planets' size provided we're not so far out on the Kardashev scale as to be capable of hurling planets into one another to design massive super-planets.

This is technically possible, but highly unlikely. I suppose one way might be to start with a tidally locked planet and then perturb it, say with a colliding object of just the right size and trajectory. Over time, however, tidal locking would eventually be recovered since the same flows that applied a torque to the planet to achieve tidal locking in the first place would remain.

Yes. If I understand you correctly, it'd be much like the Earth, just slowed down a lot.

Provided you have sufficient coverage of fluid on the surface that can flow about, tides could still occur (in principle). It would depend on the mass and orbital distance of the moon. Like Earth's tides, what matters is the difference of the gravitational force of the attracting body--star, satellites, other planet--on either side of the planet. (The Sun's net gravitational force on Earth is about 175 times that of the Moon's, yet the Moon's attraction dominates the tides because of our rather large Moon's proximity to Earth.)

As Taure has already aluded to, whatever high-gravity ecosystem you try to devise for your fictitious planet, I'd suggest you take into account allometry concerns as you go about it. Plants and animals (and people) on Earth evolved to the scales they did because of some fundamental mechanical and fluid-mechanical considerations.

 

Thanks that was quite helpful.

I have new questions now. Adjusted to a more neptune sized planet, suppose the core is small and composed of something considerably lighter than iron. Like a lightweight alloy or pumice rock with a silica carbide mantle, truly massive oceans thousands of miles deep, and a thin pumicey crust with only 1 or 2 Australia sized contents. Is that within the realm of possibility?

Aye. I'd already read both of those pages and had 4-5g in mind for the xenobiology of the main chars. They aren't human at all. They're essentially short marsupialesque humanoids with two circulatory systems, incredibly dense bones and muscles, and 3 very powerful hearts, and while they do have cranial brains, they have distributed secondary hind brains all along their spine. They can only remain bipedal using their thick tails for counterbalance and only for short periods. This is a very alien world.

I'll probably have the first 2 or 3 chapters that really establish how the world works in a few months. Hooray writing.

 

What concept of time do the creatures on this planet have? The obviously wont be able to think in days, like we do, and how will they measure time as things that we used a long time ago to measure time, such as the sun dial, wont have been invented.

 

I was going to give the planet about 3 moons for coolness factor and to help keep simple time.

One revolution of the High Moon is roughly 350 revolutions of the Low Moon.
One revolution of the Low Moon is roughly 25 revolutions of the Bright Moon.
One revolution of the Bright Moon is akin to an hour.

So akin to earth time but they can really only measure in semi-accurate hour long blocks.

I was actually going to make a funny little supporting character who, when asked, would mysteriously be able to point out the exact position of each moon even when it was darkside.

 

Note that planets harboring life most likely need a magnetic field to protect against solar wind.

Mars lost its atmosphere over billions of years due to lack of magnetic field. Lack of magnetic field would also result in massive increase of harmful radiation what would probably make life on land far less likely.

What makes iron core more or less necessary.

Is it really necessary to have such giant planet? Probably available space would be anyway unused in the story.

"planet roughly half the size of Jupiter" (I assume that it means half of radius) has surface 25 times greater than Earth. And even really filling just Earth-sized planet with something is unlikely to be done.

 

I was hoping to keep it fairly large yes, to maintain the notion that it would take a good long while to sail across.

I'll probably go with a deep ocean planet nearer to neptune size to maintain the high g force. We're talking a planet 2 to 3 times the diameter of Kepler 10c, so it doesn't stretch feasibility too much... just a little.

 

Well, Mars's density is about half that of iron, so there's some flexibility at least with net densities for rocky planets. That said, pressures in a Neptune-sized planet's core will preclude metallic alloys in the traditional sense or what we usually think of as atoms. A better characterization would be so-called "warm dense matter," i.e., a state of strongly coupled plasma.

The requirement of having a geodynamo is real; Mars once had a thick atmosphere, but it eroded away. Iron/nickel isn't mandatory (see Jupiter), but is one of the easiest ways for a large, rocky planet to sustain a magnetic field/magnetosphere.

 

I've been thinking about another way to go about this. Tidal locking occurs over hundreds of millions of years right? The earth used to spin faster than it does now, and given it's current rotation deceleration, it's postulated that we may eventually tidal lock.

To produce the desired effect, couldn't my planet simply be much closer to a tidal lock with its star than Earth is to the sun? Or am I misunderstanding the physics there?

 

AFAIK yes, though it may be unfeasible to achieve near tidal lock of large planet that is not really close to the star.

 

If you really want a solution to the gravity issue, you could say your 'planet' is in fact a shell around a gas giant. Put the gas giant in a near Earth orbit, position the shell at the correct orbit around the gas giant, have the gap between the shell and the gas giant be empty and you'll have normal gravity. Such a shell would clearly be the mega structure of some long gone alien species but that might be fun to run with in the right story.

 

This reminds of orbital resonance.

http://en.wikipedia.org/wiki/Orbital_resonance

This makes orbits extremely stable, and it sounds cool as well.

Also, I haven't done the math but I don't think any planet can have a moon orbiting every hour, even if the planet didn't have an atmosphere. Low Earth Orbits are about 90 minutes long, and gravity anomalies from landmasses and ocean tides are enough to destabilize the orbit, though atmospheric drag is by far the biggest problem.

Without some crazy external stimuli, I'm pretty sure the Earth will never be tidally locked with the Sun, mostly because of the Moon, and partially because the Sun only has another five billion years to try.

Stars and planets need rotation to form, and the reason they spin is because of that rotation. So no matter what, your large planet will have started out with a great deal of angular momentum. However, in the chaotic early days, it might receive a glancing blow at just the right angle with just the right force...

The upshot of this is, with the moons around, your planet can remain "almost tidally-locked" indefinitely. If the moons are there and large enough, it's not really tidally locked with the star (the star would have to be much larger and much nearer to do it within hundred of millions of years, and then it would be far too hot for liquid water), the rotation period just happens to almost coincide with the orbit.

 

By revolution do you mean orbiting the planet or do you mean turning on its axis?
Also, here are some other ways you might want to look into if you find that your method of timekeeping isn't feasible.

http://en.wikipedia.org/wiki/Candle_clock
http://en.wikipedia.org/wiki/Merkhet

 

Okay. Well at least the near lock phenomenon is physically possible. I'll do more research on the means.

And yes jp, I do mean orbiting the planet. And doesn't jupiter have a small moon that orbits every 4 hours? I could easily change to 4.

 

Io orbits Jupiter about every couple of days. That's the fastest orbiting large moon I know of (and one whose tidal forces nearly tear the moon apart).

 

Yeah I wasn't going to make the Bright Moon a really large moon. It was going to be tiny, but highly reflective, made of something like the pink spinel they found in moonrocks. Essentially a red dot that shoots across the sky.

I was thinking of Metis (one of Jupiter's inner satellite moons) by the way, and the orbital period is listed by NASA as about 5 hours.