What kind of planet would have a “megamareal” tidal range?

Question

A large group of people has been banished from Earth and punished to scrape out a perilous existence on a strange planet. The planet has breathable atmosphere, and consists of an immense ocean teeming with wondrous marine species (some of which are catchable and edible), and small islands far apart from each other that are mostly barren and with very little flora and fauna. Partly because they need a sufficient source of nourishment, and partly because of their large numbers and need for living space, people are compelled to live near the shore. They have a problem, however:

The planet has incredibly high tides. Accordingly, the shoreline moves many kilometres back and forth with the tide while the sea level rises and falls hundreds of metres. At low tide, there is a considerably larger area to live in but food is far away. As their dwellings need to be on dry land and they have no resources or tools with which to build floating vessels, at high tide, the living and breathing space becomes so cramped that people start suffering from phobias and can barely stand each other’s presence.

In order to survive and not lose their sanity, people will have to adapt. There is a plethora of unexplored questions concerning the society that will emerge as well as the abilities these humans will develop. The first question, however, has to be:

What kind of planet could have such extremely high tides? What unique features do its geosphere and hydrosphere and/or planetary system need to have?

Thank you for your help!

Note: If you find the living area constraint unrealistic, please consider for instance an island that consists of a high plateau rendered unreachable by fabulously tall vertical cliffs. (I am not sure how geologically possible this is, but I guess this can be dealt with in another question :). The humans live at the foot of these cliffs and during high tide there remains only a narrow strip of dry land.

In any case, while researching, I found a nice simulator of what it would look like to have a higher sea level on Earth.

Answer 1

Your planet is very strange.

Assuming that your tidal range is not locally produced, like the Bay of Fundy, there is really only one possible source of the tides, which is a moon which is both large and close.

Let's assume that the planet resembles the earth in size, and the moon is like ours in size. With no local tide amplification mechanisms, the tide is caused by the tidal bulge. The height of the bulge is proportional to the inverse of the cube of the distance separating the two bodies. The earth's tidal bulge is about 1 meter, so a 200 meter bulge requires an orbital distance R $$R = \frac{1}{200}^{\frac{1}{3}} = \frac{1}{5.8} = \frac{385,000}{5.8} = 66,000 \text{ km} $$ In other words, the moon would have to be about 1/6 the distance it is now. This has the unfortunate effect of requiring that the moon orbit much faster, and from Kepler's 3rd Law $$T = \frac{1}{5.8}^{\frac{2}{3}} = 0.3 $$ In other words, the month will be about 10 days long.

Furthermore, the planet will be rather more geologically active than earth, since the tidal squeezing of the crust will tend to heat it.

However, all of this has happened on earth long ago, and this is where the problem starts. The energy losses will fairly quickly slow down the moon and cause it to recede, so the fact that the moon is still close means that the planet is very young. So how did all those marine species evolve?

Actually, you've planted the answer in your description of the oceanic life as being (at least in part) edible. This clearly suggests that the planet was seeded with earth marine life, with all the entertaining results which that implies.

Answer 2

An extreme example written about in 1982 is Rocheworld.

Rocheworld is a double planet in which the two elements are close enough that they share an atmosphere. Each element is also deformed into an egg shape by the gravity of the other.

Generic Rocheworld:

Please note that it takes quite a bit of extreme tuning of each body to get a configuration like this that keeps all of the mass of each body within its own Roche Lobes. If any part of either body (e.g. atmosphere) falls outside of its Roche Lobe, then that planet will begin losing mass - which usually only accelerates the problem.

The Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately tear-drop-shaped region bounded by a critical gravitational equipotential, with the apex of the tear drop pointing towards the other star (the apex is at the L1 Lagrangian point of the system).

An example in which a body over flows its Roche Lobe:

At this point you might ask yourself, "what does this have to do with the question?"

A pair of bodies like this will share the following traits:

1. To share atmosphere, they must be relatively close together (the actual distance depends upon the details of the configuration). For two Earthlike bodies, I'd assume a distance of 100 km or so.
2. For both bodies to almost fill but neither overflow its Roche Lobe, they must possess similar (but not necessarily identical) mass.
3. They also must also possess similar (but not necessarily identical) density.
4. Both bodies will be egg shaped (as shown in above image).
5. Both bodies must rotate very fast around the other to keep from falling into one another (how fast depends upon the details of the configuration). When I plug the numbers in for two bodies identical to Earth, I get 1 orbital revolution every 84 minutes.
6. Both bodies will be tidally locked to each other - this means there are no tides. They might not have tidal locking if they have the unusual pole - pole configuration but they still won't have tides then :(.
7. From a configuration like this, it is extremely easy to reach space. Anyone who can get to the L1 point (the equipotential point between the planets) can get to space with an arbitrarily low $\Delta V$.
8. Over geologic timescales, I can't imagine this configuration being stable. However, over human timescales, there's plenty of time for adventures on the planets.

A note on tides: When I said this configuration has no tides, I only mean this in the sense of terrestrial water tides. The reality is the egg shape of the planets is one giant tide. However, since the planet rotates at the same speed as this tide, then the inhabitants won't really notice that change.

OTOH, if the planets haven't completed tidal locking and one or both are librating around this tidally locked configuration, the waves would be devastating. There would be tremendous heating of the crust, 10.0+ magnitude earthquakes, volcanoes, etc. In fact, the surface might not be livable. Water would be flowing in giant waves ala Miller's Planet from Interstellar.

Giant wave from rotational libration around tidal locked configuration:

Here's what Kipp Thorne (noted physicist) has to say about that:

As I describe in the book, I imagine this planet is tidally locked, keeping the same face toward the black hole so that tidal forces don’t rip it apart. But it hasn’t been tidally locked for all that long, it was deposited in its orbit relatively recently, so it’s actually wobbling back and forth slightly relative to the tidal-locking position, and as a result huge tides are created in the ocean at the planet’s surface. And these tidal forces are so great that they create the huge waves you see in the film.

Answer 3

There is one world in fiction that is similar to the one you are picturing: Dxun and Onderon from Star Wars.

Dxun is the moon of planet Onderon, and its orbit comes close enough (once a planetary year) to the planet that the atmosphere of both celestial bodies merge, and sometimes creatures are sucked and moved from one to another.

In this world of Onderon, the tides are huge, during the approach of Dxun.

The flora and fauna adapted "well" to this setting.

Answer 4

You might try multiple moons. Perhaps they orbit in such a way that periodically they all line up and cause a larger tide. Between these constructive events, the tides might be chaotic. It wouldn't give you the low tide, high tide scenario you were looking for, but it might be workable.

There are wave calculators online that can help with this.

Hope this helps.

Answer 5

It would make more sense, and a gentler planet, to make all of the continental slopes gradual, so that a small tide rise or fall (a few feet) would cover or uncover many miles of bottom. This might indicate a geologically old or tectonically dead planet that still has enough weather to erode the surface.

Unless your real goal was to have formations like "Devil's Tower", a volcano remnant, sticking right out of the water, with the challenges that might entail.