Reality Check: Rogue Planet Flyby Scenarios and Survival Feasibility

[cassius]

Hello everyone,

I’m looking for some feedback on the scientific realism of a specific catastrophic scenario for a story I’m working on. Here are the parameters:

• The Event: A Mercury-sized rogue planet passes Earth at a speed of 200 km/s.
• Closest Approach (Perigee): 70,000 km from Earth's surface.

The proposed consequences in the story:

1. Tidal Forces: Severe enough to rupture the Earth's crust, causing massive magma outflows.
2. Atmospheric Effects: The atmosphere heats up significantly due to the planet's passage. A portion of the atmosphere is "siphoned off" toward the rogue planet, leading to a dramatic drop in air pressure (making survival impossible without a pressure suit).
3. Surface Devastation: The superheated atmosphere causes all vegetation to ignite.
4. Orbital Shift: Earth’s orbit is deflected closer to the Sun, leading to the eventual "baking" of any remaining life.
5. The Survival Attempt: The protagonist wants to witness the flyby face-to-face. He prepares a bunker anchored into solid bedrock (to avoid being swept away) in a vegetation-free zone. During the perigee, he steps outside wearing a fire-resistant suit over an old pressurized spacesuit with a filtered air supply. He survives just long enough to see the surface of the rogue planet before dying.

Is this at least approximately realistic?
I’d appreciate any insights into the physics or orbital mechanics of this scenario!

 

[Bandersnatch]

So, back of an envelope, that's approx. 250 times the tidal forces of the Moon, at its closest approach, which would rapidly decrease on either side of that point.
The effects of the tidal forces are much harder to ascertain than their strength, but - fwiw - my intuition suggests that'd result in some nasty tidal waves and winds, maybe a bunch of earthquakes, but nothing of the apocalyptic magnitude you desire.
Other than that, I don't see a plausible mechanism for superheating or siphoning off the atmosphere. The latter maybe at a much, much closer approach.

You can get the feel for the magnitudes involved by remembering that
1- tidal forces scale linearly with mass, but with the third power of the inverse of the distance.
2- Lunar tides affect the crust on the order of centimetres, the seas - metres, the atmosphere - kilometres.

So you can sort of see that from the combination of mass and distance from the planet as compared to the Moon, you get the aforementioned 250 times stronger tides, and that these tides should cause maybe a couple metres of deformation of the crust - on the scale of the entire globe, mind you - maybe a hundred metres of global sea surface deformation, and a few hundred kilometres-high bulge in the atmosphere.

As for the orbital deflection - this a very small planet. Some 5-6 percent of the mass of the Earth. Rapid, one-off passage. Even in ideal flyby conditions you wouldn't get much of a change in the orbit. I'd be much more likely to mess up the Moon, though.
What I'd do, is I'd put the scenario into an n-body simulator, and see what happens. It'd let you adjust the mass and distance parameters until you get what you want out of the flyby.

 

[Hornbein]

I'm not going to do any calculations but feel that's not nearly enough to have such a large effect.

 

[cassius]

Thanks for the answers! So, the distance needs to be reduced (probably below 5,000 km) for the effects to remain the same. Except for the deflection of the planet towards the Sun... for that, a more massive planet would be needed. How massive exactly? At least half the size of Earth? Or would it have to be much more massive than Earth?

 

[Filip Larsen]

You may want to consider that the premise of a massive (natural) body of nearly same mass as the Earth passing so close, will be a very, very unlikely event (1) thus it will as such probably not make your story that more believable to those "in-the-know" anyway. So perhaps you are better off with a setup that are consistent for your story you want to tell and just leave the details hazy enough that noone will be the wiser.

(1) I would say it is extremely unlikely because
a) The presence of interstellar bodies that massive are (as far as I know) believed to be extremely rare (e.g. borderline non-existing) in the first place since the planetary body must have accreted around a star (which require a somewhat stable orbit over millions of years) and then sudden gotten ejected by something more massive (which also have to have been accreted around same star for even longer).
b) The likelihood of collisions between two massive objects in the galaxy, each on their own "random" trajectory, is astronomically unlikely. To give a scale of the likelihood of (near) collision consider that (if I recall correctly) in order for a 50% chance of at least two stars to have physically collided during the lifetime of our universe you would need to include around 15 billion galaxies in that measure.
c) The event takes place right in the timeline of your story, i.e. it singles out a very narrow point in time for the above events to happen in.
So, if you multiply the chances in a), b) and c) you get immeasurable close to zero for that event.

Even if you somehow involve aliens in the setup (we are in the sci-fi part of PF after all) I would still think it would require high-caliber magic to pull this off.

 

[cassius]

What is more likely? That we will travel through time, or that two massive bodies will collide? And yet, how many wonderful stories have been written about these two improbabilities?

Even greater magic would be required for two advanced civilizations to meet—considering the sheer size of the galaxy and the 'lifespan' of intelligent life, it’s a matter of extreme magic. Yet, you yourself mention the involvement of aliens :)

But perhaps it wasn't aliens who caused it, but God. At that point, we reach a probability of zero... and yet, the greatest bestseller in history was written about Him.

 

[Nik_2213]

Wiki, YMMV, has...
https://en.wikipedia.org/wiki/List_of_nearest_stars
This includes prior and pending passes...

https://en.wikipedia.org/wiki/Scholz's_Star
This mini-binary came close enough ~70ky ago to disturb Sol's Oort cloud.
But, IIRC, such objects need another million years or so to in-fall...

As we seem to be lacking mega-craters or astroblemes aged~70ky, seems Scholz' cometary objects missed Earth. Could Jupiter's Great Red Spot originate from such ??

Notes:
#1 the Toba mega-caldera *is* of that age, but totally volcanic.
#2 What if some in-fall came early ?? See my 'Hunters Night' which is #4 in...

Thread 'The Short Story Thread: Post Yours Here'

May 14, 2017

I see a lot of posts here in the writing and world-building forum that ask for help in creating a story, but I rarely see any actual stories! Perhaps you've never shown your writing to anyone else or maybe you've never gotten to the part of writing where you actually do some writing (it's quite common).

Whatever the case, that's what this thread is for. Type up a short story that can fit within a single post and post it! It doesn't have to be long. A few sentences is fine. Any amount of writing is a good amount!

However, please don't spam the thread with lots of short posts in a short...

• Drakkith
• Short Thread
• Replies: 22
• Forum: Sci-Fi Writing and World Building

• 

 

[Filip Larsen]

What is more likely? That we will travel through time, or that two massive bodies will collide? And yet, how many wonderful stories have been written about these two improbabilities?

Indeed. In my opinion such stories, when they work for me, work because they manage suspension of disbelief and have a catching narrative. Almost no matter how hard you try to make the setup physically plausible in all its presented details some parts will likely not hold up to detailed scrutiny anyway.

My point here is really just to say that while it is commendable that you want to make a physically realistic setup (and if you can do that "for free" in your story then by all means go for it) you may risk that desire for (too much) realism actually breaks suspension of disbelief. For instance, I like that Andy Weir in his foreword (I believe it was) for the Martian simply writes that while he strove to get as much realism in as possible he simply had to break it a bit in a few places to make for an interesting story. For me that made the story a more enjoyable read even when spotting situations with fishy physics.

 

[cassius]

Most sci-fi premises are based on improbable events. What’s worse is that some creators simply throw logic and physics down the toilet. So, setting aside the improbability of a rogue planet's close flyby, my main focus is on a realistic description of the consequences. These will be described in the final chapter, seen through the eyes of the protagonist, who dies witnessing the event. Otherwise, the book isn't about scientists or physics, but about an ordinary person witnessing the erosion of civilization before the rogue planet even arrives.

And part of that is determining a believable distance for the close flyby.

And I actually have one more question regarding that: when would the earthquakes start? Days before the perigee? Or just hours?

 

[Bandersnatch]

At 200km/s and 3600 seconds in an hour, the planet traverses the entire diameter of the lunar orbit in one hour. If it's a small body, having to pass close-by for the desired tidal effects, then you're looking at minutes of earthquakes at most.

 

[cassius]

It's a big body. It was originally supposed to be the size of Mercury. But it needs to be enlarged so that the first weak earthquakes can be felt at least a week before the flyby.

 

[Bandersnatch]

I don't think that's possible. It'd have to be another star passing by, and a sizeable one too. In which case earthquakes would be the least of your protagonist's problems.

 

[cassius]

So, if I tweak the parameters: A rogue planet with 80% of Earth's mass passes by our planet at a velocity of 200 km/s. Perigee is 10,000 km from the surface. That should be enough to trigger the first minor tremors and earthquakes several days before the flyby, followed by the total devastation of life on Earth.
Additionally, it could already be enough to nudge Earth towards the Sun (though that’s not a strict requirement for the scenario).
Could this work?

 

[cassius]

I don't think that's possible. It'd have to be another star passing by, and a sizeable one too. In which case earthquakes would be the least of your protagonist's problems.

The thing is, I need a timeframe of 12, maybe 15 months max from the first detection. I don't want years of humanity preparing and coming up with rescue scenarios. I want a maximum of 15 months of brutal waiting for the inevitable end. That’s why it’s a rogue planet and why the velocity is so high. The core of the story isn't a heroic struggle for survival, but a chronicle of the rapid collapse of society.

 

[Bandersnatch]

Look, with some rounding of the numbers, at 200 km/s the body is covering 0.1 AU in a day. So to trigger an earthquake a day before perigee you need a body that is sufficiently massive at the aforementioned distance to have significantly higher tidal influence than the Moon, which is some 40 times closer.
I don't know what is sufficient for a tidal earthquake here, but let's assume for the sake of the argument, and ease of number juggling, that it's a hundred times stronger tide than lunar.
Which means that, due to the strong scaling with the inverse of the distance, the body needs to be at least 6 million times more massive than the Moon or a 100 000 more massive than Earth. It's a 100 Jupiters - i.e. a 0.1 solar masses red dwarf star.
If you now extend that time scale for the onset of earthquakes to ten days, you need something that produces a hundred times as strong tides as the Moon at the distance of 1 AU. That's a star of 100-200 solar masses - skirting the limits of what's permitted by the current understanding of stellar evolution.
Even if you cut the speed down to barely above escape velocity, and the strength of the required interaction by an order of magnitude, you still won't cut the mass down to a planet. Maybe a brown dwarf.

The distance scaling of the tidal forces is just not your friend here. With anything sensible mass-wise you get nothing for most of the time, then suddenly a lot, and then nothing again.

 

[Nik_2213]

FWIW, there is a wandering inter-stellar population of genuine 'Rogue Planets'.

IIRC, some were probably tossed 'wild & wide' when their host star 'evolved' off main-sequence. Some probably came via the 'Brown Dwarf' route, perhaps not accreting to full size. Almost certainly, some were ejected by gas-giant migrations. Don't some 'sufficiently robust' dynamical models suggest the Jupiter / Saturn 'Nice Shift' ejected an ice-giant from Sol-system ??

Against that, Space is big, very big, so getting a pass sufficiently within Oort Cloud to immediately affect inner system is, thankfully, improbable...

 

[Bandersnatch]

I feel like I should maybe be more constructive with my comments.
Do you need the apocalypse to be caused by the tides? I understand that the visual of a foreign astrophysical body looming over the sky as the world ends is the selling factor here, but maybe we can work out some alternatives.
I can currently think of two scenarios
1 - there is a rogue planet, of not insignificant mass, passing nearby. The more massive, the less 'near' the nearby has to be. The idea is that it disturbs the Earth-Moon system, so that after the passage the Moon ends up on a highly elliptical orbit with perigee underneath the Earth's surface.
Then you have the Moon as the civilisation-ending body, and the plausible few weeks of time for it to complete the orbit and impact the Earth as your period of dread before the end.
You can then have the rogue planet be detected long in advance, but not be the cause of immediate concern, as there's enough wiggle room in the uncertainty of many-body interactions to not at all be sure what the outcome of the passage would be, or even be sure how close the passage will be until maybe a few months prior.
And then the passage turns out to be very close, and the outcome turns out to be not a happy one, and the Moon comes crashing down, and you have your tides and a looming doom overhead.
2 - A somewhat more massive planet or brown dwarf passes by, and changes the Earth's orbit so that its perihelion is way too close for the survival of life on the planet. Then the grace period is the time for the Earth to approach the Sun, which could be counted in months of gradually growing temperatures.

 

[cassius]

Both scenarios are very interesting; I particularly like the one involving the Moon. However, everything in the story is already set up for a close flyby of a massive body—especially the main character's motivation. So, I now know I need to increase the planet's mass and bring the perigee closer. One thing remains to be settled: the movement of the atmosphere. With a perigee below 5,000 km and a rogue planet the mass of Venus—how much acceleration would the Earth's atmosphere experience? Would a portion of the atmosphere be siphoned off between the two bodies?

 

[DaveC426913]

Perigee is 10,000 km from the surface.

I'm sure someone else has mentioned it but Earth's Roche limit is about 18,000 km, as is your Venus-size planet's Roche Limit. The planets would suffer - not merely earthquakes - but disintegration.

The effect on the atmosphere will be lost in the midst of the liquefaction and disintegration of the body itself.

The effects follow so closely one upon the other, that's its' unlikely they will be perceived as discrete.
Try waving one neodymium magnet past another without them overpowering your reflexes and colliding. It's almost impossible.

 

[BWV]

curious if you really think these answers are better than what an LLM would provide