I want a planet with roughly the same mass and volume of earth to have tides on a much larger scale than earth, enough that the coast would be extended for at least several hundred feet at low tide in many places. I don't mind if the frequency of the tides is altered along with it. It would also be very convenient if I could do this without significant change to the day/night cycle. I'm afraid that I have practically no knowledge on the matter, so I apologize if I'm wasting your time. Initially, I was thinking of just making the moon larger and closer than earth's, but I'm not sure if that would be enough to change the tides that much without complications like tidal locking. To summarize:


  • Much larger tides


  • Planet must remain habitable
  • No significant change to the day/night cycle

Is this possible? If so, how? (If this could be answered in layman's terms, that would be appreciated)

  • 1
    $\begingroup$ Tough one to accomplish with a human habitable planet, there's a nice dramatic explanation of the Earth's history here that might inform how that couldn't have happened on our planet. Heavy-moon capture is tough too, unless it's arranged by superior aliens - would outside intervention be an acceptable answer? $\endgroup$ May 15, 2021 at 1:12
  • $\begingroup$ I would be ok with outside intervention, yes. Thanks for the link, it was helpful. $\endgroup$
    – Mrpenguin
    May 15, 2021 at 19:02
  • 1
    $\begingroup$ Well, the Earth's tides were (originally) some 400 times as strong,when the Moon had just formed and was ridiculously close to Earth. We seem to have done alright. ("we" in this context being life, in general) $\endgroup$
    – PcMan
    May 15, 2021 at 20:02
  • $\begingroup$ @PcMan I also thought about it, but it brings another problem. If he wants habitable planet with awesome tides, the tricky part is how impressive the tides have been a few billion years earlier. $\endgroup$
    – Shadow1024
    May 17, 2021 at 9:05

4 Answers 4


Short Answer:

The problem with a human habitable planet having very large tides is that astronomical parameters - including those responsible for tides - change very slowly but steadily with time, and a lot of time is necessary for a planet to become habitable for humans, thus a planet which starts out with large tides might have very small tides by the time that it becomes habitable for humans.

And as L.Dutch - Reinstate Monica♦ said, if the moon is too large or too close, the planet may become tidally locked to the moon and thus no longer have moving tides but permanent tidal bulges on opposite sides of the planet.

Long answer in Eight Parts:

Part One: How High Can Tides on a Habitable Planet be?

You should look at Habitable Planets for Man Stephen H. Dole, 1964, which discusses planetary habitability for humans.


Pages 61 to 63 discuss the necessary age for a planet to be habitable, outlining the various successive processes and stages which eventually resulted in Earth acquiring an atmosphere with a high oxygen content. And ends with:

In general, it is probably safe to say that a planet must have existed for 2 or 3 billion years, under fairly steady conditions of solar radiation, before it has matured enough to be habitable.

The relationship between a planet and its satellite are discussed on pages 72 tp 75.

Dole discusses both the case of a habitable planet with a large satellite and a habitable "planet" which is actually the satellite of a giant planet many times as massive.

He notes that if a habitable planet has a companion moon or planet whch is massive and/or close enough, the tidal effects will slow down the rotation of the planet until the planet is tidally locked to the companion world.

Dole says that a planet tidally locked to a companion world can still be habitable, if the days are short enough that the variations in heat and cold between day and night are not too intense. Dole suggests that a day of 96 hours or four Earth days would be the more or less arbitrary maximum length for the day of a habitable planet tidally locked to its companion world.

The circumsteller habitable zone is the zone around a star there a planet can have temperatures suitable for liquid water, and thus possibly have life if other conditions are good. Dole uses the word "ecosphere" to describe the circumstellar habitable szone.

In the section about the suitable types of stars for habitable planets, Dole wrote that the lower mass stars would have "ecospheres" very close to them, and thus planets in those "ecospheres" would experience very strong tides, and so would soon become tidally locked to their stars. Thus one side of the planet would have eternal day, and the other side would have eternal night. Dole believed that would make a planet uninhabitable.

In the section on satellite relationships, Dole says that if a planet or moon becomes tidally locked to their companion planet or moon, they won't become tidally locked to their star, and thus they will have an alternation of day and night and could remain habitable.

As we have seen, for stars at the low temperature end of the main sequence, there is an incompatibility between the tidal braking effect and the temperature requirements of a habitable planet. the lower limit of mass of stars that could have freely rotating planets withing an ecosphere was determined to be 0.72 solar mass. In this section we see a mechanism whereby the lower limit on stellar mass may be reduced still further. If a planet had a large, close satellite that maintained the planet's rotation rate so that the solar day was shorter than 96 hours, it could orbit within the ecosphere of a star less massive than 0.72 solar mass. However, a new limit would be reached when the tides on the lanet due to the primary reached a destructive level. If we assume that the destructive tide limit is 20 feet, the new limit on stellar mass would be 0.35 solar mass.

So Dole estimated - accurately or not - that tides of 20 feet would be incompatable with habitability for humans.

Part two: Comparison with Tides on Earth.

But what did Dole mean by tides?

Earth tide (also known as solid Earth tide, crustal tide, body tide, bodily tide or land tide) is the displacement of the solid earth's surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours and longer. The largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational force causing earth tides and ocean tides is the same, the responses are quite different.


According to the table, the vertical component of semi-diurnal Earth tides is 384.83 milimeters or 1.2625 feet. So the highest vertical compnent of Earth tides when the different tides work together should probably be about one meter or 3.2808 feet.

Ocean tides in the open ocean have a similar range.

The typical tidal range in the open ocean is about 0.6 metres (2 feet) (blue and green on the map on the right).

The range of tides on coastlines, where people notice them, can vary greatly.

The tidal range has been classified[9] as:

Micro-tidal, when the tidal range is lower than 2 metres.

Meso-tidal, when the tidal range is between 2 metres and 4 metres.

Macro-tidal, when the tidal range is higher than 4 metres.

So the micro-tidal range is less than about 6 feet, the meso-tidal range is about 6 to 13 feet, and the macro-tidal range is above about 13 feet.

Closer to the coast, this range is much greater. Coastal tidal ranges vary globally and can differ anywhere from near zero to over 16 m (52 ft).3 The exact range depends on the volume of water adjacent to the coast, and the geography of the basin the water sits in. Larger bodies of water have higher ranges, and the geography can act as a funnel amplifying or dispersing the tide.4 The world's largest tidal range of 16.3 metres (53.5 feet) occurs in Bay of Fundy, Canada,3 a similar range is experienced at Ungava Bay also in Canada6 and the United Kingdom regularly experiences tidal ranges up to 15 metres (49 feet) between England and Wales in the Severn Estuary.7


So did Dole mean that a habitable planet could not have a tidal range of more than 20 feet anywhere on the planet?

Obviously not, since he should have known, like many children, let alone many scientists, that there are a number of coasts on the habitable planet Earth which have tidal ranges greater than 20 feet or 6.096 meters. Tides of 20 feet or more do not make their coasts lifeless deserts, nor do they make the entire planet Earth lifeless and uninhabitable.

Perhaps Dole meant that the average mid ocean tide range on a habitable planet could not be more than 20 feet. Since that is about 10 times the height of typical mid ocean tide range, the maximum tidal ranges on various shores of a planet with 20 foot mid-ocean tides would vary from much less than 20 feet on shores which have very low tides, to ten times the meso-tidal range of 6 to 13 feet - 60 to 130 feet, to ten times the macro-tidal range of 13 to 53 feet - 130 to 530 feet.

So if Dole's limit of 20 foot tides means 20 foot mid ocean water tides, and if it is correct, and if the planet has various shores which amplify tides as much as some shores on the planet Earth do, then the higest tidal ranges in the shores with the highest tidal ranges could be up to about 530 feet.

Of course, it is possible that some planets could have configurations of land and sea which amplify the tides much more than any place on Earth does, and so some places could have much greater tidal ranges than the approximately 530 feet calculated.

Tidal flats or mud flats are flat areas which are covered and uncovered by the tides.

Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A recent global analysis suggested they are as extensive globally as mangroves. 1 They are found in sheltered areas such as bays, bayous, lagoons, and estuaries; they are also seen in freshwater lakes and salty lakes (or inland seas) alike, wherein many rivers and creeks end.2 Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and aquatic animal detritus. Most of the sediment within a mudflat is within the intertidal zone, and thus the flat is submerged and exposed approximately twice daily.


If a tidal flat has a rise of one percent, it will rise one foot for every 100 horizotal feet. Thus a tidal flat with a rise of one percent will extend 10,000 feet or 1.89 miles horizontally for every hundred feet of tidal range. And I supect that many tidal flats are much more level than that.

Added 06-06-2021

I quote:

In this earlier ice age, sea levels along the Gulf Coast were about 400 feet lower than they are today, and the Gulf shoreline was between 30 and 60 miles farther offshore than our modern beaches.


So if about 400 vertical feet equals 30 to 60 horizontal miles, or 158,400 to 316,800 horizontal feet, there is an average rise of between about 396 to one and 792 to one. If a location where the shore is that flat has a tidal vertical range of 530 feet it could have a horizontal range between about 209,880 feet (39.75 miles) and about 419,760 feet (79.5 miles).

And possibly there are even flatter landscapes where a tide 530 feet high would travel much farther horizontally.

Thus it seems possible for a habitable planet to have a few large tidal flats where the tide goes out and comes back in much faster than a human can travel, even if the human doesn't get stuck in the mud. I note that on Earth, even in places with normal tidal ranges, people have often been caught by incoming tides and drowned. See the famous stories of the lost treasure of King John for example.


Part Three: The Evolution of the Orbits of Moons over Time.

But can a planet keep extremely high tides long enough to become habitable?

There are many natural satellites or moons in our solar system. Some of them are believed to be regular satellites, formed with the planets they orbit. Others are believed to have been captured by the planets millions of years after they formed.

All regular satellites have prograde orbits, orbiting their planets in the same directions as the planets rotate. Captured satellites can be captured in either prograde or retrograde orbits, which are orbits in the opposite direction to the rotation of the planet. The captured satellites in our solar system include ones with prograde orbits and ones with retrograde orbits.

If a prograde satellite orbits its planet at less than the synchronous orbital distance, tidal interactions will cause the satellite to slowly spiral closer and closer to the planet, and eventually that will result in the destruction of the satellite and probably the extermination of all life on the planet. A few moons in our solar system orbit below the synchronous orbits of their planets and are slowly approaching their planets.

If a prograde satellite orbits its planet at more than the goesynchronous orbital distance, tidal interactions will cause the satellite to slowly move away from the planet, so that after billions of years when the planet finally becomes habitable for humans the satellite will be orbiting many times farther away than at first, and the tides will be many times smaller than originally.

Retrograde satellites will spiral inward towards their planets.

All retrograde satellites experience tidal deceleration to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible. https://en.wikipedia.org/wiki/Retrograde_and_prograde_motion#Natural_satellites_and_rings[4]

Tidal interactions also cause Triton's orbit, which is already closer to Neptune than the Moon's is to Earth, to gradually decay further; predictions are that 3.6 billion years from now, Triton will pass within Neptune's Roche limit.[25] This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn.[25]

Part Four: A Planet With a Retrograde Moon.

So if Triton has already been orbiting Neptune for billions of years, and still has another 3.6 billion years to approach closer and closer to Neptune before being destroyed, it shows that in some cases a captured retrograde satellite can get closer and closer to the planet for billions of years before the final disaster. Billions of years during which the planet could become habitable for humans.

So hypothetically an Earth like planet could capture a large moon in a retrograde orbit, and the the planet might endure for billions of years and become habitable for humans, before that retrograde moon crashes into the planet and turns the entire crust of the planet into molten red hot lava, destroying all life.

So maybe the planet in your story captured a large object into a retrograde orbit and that moon is now very close to the planet and about to cause a terrible disaster in a very short time by astronomical and geological standards,though perhaps in a very long time by human standards. In that case the tides produced by the moon would be almost as high as they will ever get.

Part Five: A Old Planet with a Recently Acquired Prograde Moon.

Or maybe the planet formed and developed for billions of years and became habitable for hamans, and then relatively "recently" on an astronomical time scale chanced to capture a large object into a prograde orbit.

If the newly acquired moon was in a prograde orbit just above the synchronous orbital distance, it would start to slow down the day of the planet and move outward from the planet, lessening the height of its tides on the planet. But if the capture was relatively recent on a cosmic time scale, the moon would not have moved far away and the tides would still be almost as strong as at first.

If the newly acquired moon was in a prograde orbit just below the synchronous orbital distance, it would slowly spiral down toard the planet and a terrible disaster which would wipe out all life on the planet. But if the capture was relatively recent on a cosmic time scale, the moon would not have moved much closer and the tides would still be only a little bit stronger than at the synchronous orbital distance.

Part Six: A Young PLanet Which Has been Terraformed While its Moon is Still Close.

Another possiiblity is that the planet cold be young and have a newly formed large moon whch is still very close to the planet, and an advanced ciivization has terraformed the planet to make it habitable.

Part Seven: A Habitable Exomoon Orbiting a Giant Exoplanet.

Or possibly your planet isn't a planet, but a giant, planet-sized, habitable, exomoon orbiting a giant exoplanet. Thus the planet would be tidally locked to the giant exoplanet. That would normally freeze the tidal bulges into two oppose sides of hte exomoon, one directly facting the planet and one directly facing away from the planet. But if the giant planet has other large moons orbiting it, their gravity will cause the moon to have tides which move over its surface as the relative positions of those moons change.

If you want the day of the tidally locked exommon to equal about one Earth day, the exomoon will have to orbit the exoplanet with a period of about one Earth day. And since the exomoon and exoplanet will also be orbiting the star of the system, the day-night cycle of the exommon will actually be somewhate longer than its orbital period around the exoplanet. See synodic day and sidereal day for an explanaton.

Assumming that the exoplanet is exactly like the planet Jupiter, an exomon with an orbital period exactly one Earth day long would orbit many thousands of kilometers beyond the orbit of Thebe, with a semi-major axis of 222,452 kilometers and aperod of 0.6778 days, and many thousands of kilometers inside the orbit of Io, with a semi-major axis of 421,70 kilometers and an orbital period of 1.7691 days.

It has been suggested that exomoons orbiting exoplanets at distances of 5 to 20 planetary radii can be habitable, if other factors are correct. The equitorial radius of Jupiter is 71,492 kilometers, so 5 to 20 planetary radii from Jupiter would be about 357,460 to 1,429,840 kilometers.

Part Eight: A Habitable Planet With Large Tides from Neighboring Planets.

The TRAPPIST-1 system has several planets orbiting in the circumstellar habitable zone of their star which are very close to their star and thus to each other.

The orbits of the TRAPPIST-1 planetary system are very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Except for b, they orbit farther than the Galilean satellites do around Jupiter,[43] but closer than most of the other moons of Jupiter. The distance between the orbits of b and c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth.[42] A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days.[40][37]

Four of the planets orbit within the circumstellar hitable zone, TRAPPIST-1d, TRAPPIST-1e,TRAPPIST-1f, & TRAPPIST-1g. Since all of the planets have many times the mass of the Moon, and their closest distances are only few times the distance from the Earth to the Moon, any tides they might raise on any water on their surfaces would usually be several times as strong as the tides on Earth.

  • $\begingroup$ Thank you so much for going to all this effort for me! It really helps. I think that a recently acquired prograde moon will work out great. $\endgroup$
    – Mrpenguin
    May 15, 2021 at 21:15
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    $\begingroup$ It occurs to me that there's another impending-disaster scenario: You have a moon in an elliptical orbit. Another body came along and passed deep into the Roche limit of the world, got shredded and then ran into the moon in a basically head-on collision when the moon was at periapsis. Since it's a whole bunch of bits rather than one object the impact isn't as cataclysmic as it would otherwise be--it was a very bad day for the planet but not an extinction event. $\endgroup$ May 17, 2021 at 2:06
  • $\begingroup$ If I knew what half of the terminology meant, I’m sure I would totally agree 😁 $\endgroup$
    – Mrpenguin
    May 17, 2021 at 4:00

Tides primarily come from the moon, not the sun. Making higher tides without touching the day/night cycle is easy--move the moon closer. Like it used to be long ago.

The problem is with the first constraint. The thing is tides don't magically only move water. It's just the land is a lot more solid, it moves a lot less. Make the tides huge and the land movement matters, also.

Observe some of the moons of Jupiter--nasty things happen because of the tides. I don't know how far you can go while keeping a habitable planet.

  • $\begingroup$ Maybe a planet with an older, thicker, more motion-resistant crust? Perhaps the world acquired the moon later. Still would have more vulcanism, etc. $\endgroup$
    – DWKraus
    May 15, 2021 at 14:07
  • $\begingroup$ @DWKraus Exactly. The limit comes when the seismic issues become too much of an issue. The early Earth was incompatible with life due to tides. The tides are not the issue, living through them is. $\endgroup$ May 15, 2021 at 14:46

I would be careful with doing the moon more massive: it would made higher tides during the transient, but those tides would on the other hand quickly tidally lock both bodies, leading to no tides, just a permanent bulge.

You might try with local geographical features to amplify the tide height

The tidal range of a particular location is dependent less on it position north/south of the equator than on other physical factors in the area; topography, water depth, shoreline configuration, size of the ocean basin, and others. For example, let's consider the southern coast of Alaska and British Columbia. The configuration of this coastline is very similar to a funnel, with the narrow end at Cook Inlet. The tides travel as a "wave" across the oceans, and in many other respects act as a "wave"; this type of configuration tends to accentuate the "wave" at the narrow end of the funnel. This is part of the reason for the large tidal ranges, 30+ feet, in the area of Cook Inlet. If you look at the tidal ranges for stations on the Bering Sea, outside this funnel but at the same latitude, you will find a tidal range of 5-7 feet.


Crowded sky.

First - how does there come to be occasional very high and very low tides on Earth?


sun and moon teams

The sun and moon either team up to pull on the ocean, or counteract each other.

On your world there are more bodies in the sky that affect tides. Maybe your world has 2 or more moons that line up, or a binary star. Maybe your world is the moon of a gas giant, and itself has a moon and sometimes the giant and the moon and the star all team up to make megatides.

As far as habitable, people who live in areas affected by the tide will not be caught by surprise. It will be pretty clear where high tide and low tide are and seafarers will make allowances.

Serious tides will also affect the crust and could produce quakes. Tidequakes will also be predictable. Tidequakes might augment the effects of plate tectonics on your world and those effects as regards earthquakes and possibly volcanism will be less predictable.


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