5 oceans seems like an extravagance. On the dimensions of a planet, it’s surprisingly modest. NOAA estimates that Earth’s ocean contains about 1.335 billion cubic kilometres of water. Multiply that by 5 and unfold it throughout an Earth-sized sphere, and the result’s a layer roughly 13 kilometres deep.
That may be a intentionally easy calculation. It divides about 6.7 billion cubic kilometres by Earth’s roughly 510 million sq. kilometres of floor. It doesn’t reproduce the best way water would truly fill low basins first, nor the best way crust and mantle would reply to the additional load. What it does present is scale: the ensuing world common is comfortably higher than Everest’s 8.8-kilometre peak above current sea degree.
If most of that water remained on the floor, an Earth-sized rocky planet may plausibly lose its final islands and turn out to be a steady ocean. But 5 will not be a magic quantity. Ocean basins, buoyant continental crust, gravity, mountain peak, tectonic regime and the quantity of water saved contained in the mantle all have an effect on when the ultimate patch of land disappears.
A waterworld boundary, not a common flood line
Planetary scientists usually specific water inventories in “Earth oceans,” however even Earth’s stock is split amongst reservoirs. There may be floor water within the seas and ice, chemically certain water in minerals and a a lot much less specific amount within the mantle. Shifting water between these reservoirs can decide whether or not a planet is blue with scattered continents or blue from pole to pole.
A model developed by Nicolas Cowan and Dorian Abbot confirmed that pressure-dependent trade between oceans and the mantle may assist tectonically energetic planets retain uncovered land over a wider vary of sizes and water inventories than easier fashions implied. Their work additionally emphasised that stronger gravity can mute topography on a bigger rocky world, lowering the capability of continents and ocean basins to rise above a typical sea degree.
So the assertion that 5 oceans “may” drown each continent carries actual scientific weight in that single phrase. Put 5 current terrestrial ocean volumes onto right now’s solid-Earth aid and the arithmetic readily covers the excessive floor. Construct a special planet, let its mantle take in extra water or give it uncommon crust, and the shoreline can transfer.
Why slightly land does a lot work
Continents should not merely dry locations the place terrestrial life occurs. They’re energetic components of Earth’s local weather equipment. Carbon dioxide dissolves in rainwater, reacts with silicate rock and is carried towards the ocean in dissolved merchandise. Carbon can ultimately turn out to be carbonate minerals and sediment, whereas volcanoes return carbon dioxide to the environment.
The vital characteristic is suggestions. Hotter situations typically speed up chemical reactions and rainfall, rising weathering and drawing down extra carbon dioxide. Cooling slows the method, permitting volcanic carbon dioxide to build up. This carbon-silicate cycle doesn’t maintain Earth at one precise temperature, and it has not prevented ice ages or hothouse intervals. Over geological time, nonetheless, it may well resist a sustained push in both path.
Drown the continents and most weathering in rain, soil and rivers disappears. Weathering doesn’t cease, as a result of seawater nonetheless reacts with the ocean ground, however the acquainted surface-temperature suggestions turns into weaker and fewer direct. Deep-water temperature might not monitor adjustments in air temperature as promptly as rainfall and reactions on heat uncovered rock do.
A 2023 study of land-fraction diversity described a number of potential long-term outcomes even amongst Earth-sized, plate-tectonic worlds: land planets, ocean planets and extra balanced planets like Earth. Their local weather histories retain a reminiscence of early inside evolution. There is no such thing as a rule saying that each roughly terrestrial planet should converge on Earth’s 71 p.c ocean protection.
The nutrient drawback beneath the local weather drawback
The lack of land additionally closes a serious provide route for vitamins. On fashionable Earth, chemical weathering of continental rocks releases phosphorus that rivers carry to the ocean. Phosphorus is constructed into DNA, cell membranes and the ATP molecules used to switch vitality inside cells. On lengthy timescales, its provide can restrict organic productiveness.
This results in an intuitively persuasive image of a waterworld as a blue desert: plentiful solvent, however too little fertilizer. A planet may possess liquid water from pole to pole and nonetheless help a sparse biosphere. It would by no means produce sufficient oxygen or one other organic fuel to be acknowledged from light-years away.
There are a number of cautions inside that image. It describes a contemporary Earth-like, phosphorus-limited biosphere, not each potential metabolism. It additionally treats continents as the primary nutrient supply as a result of that’s how right now’s oxygenated Earth works. A younger ocean-covered planet may have a special environment, completely different seafloor chemistry and a special stability between nutrient sources and sinks.
A seafloor will not be chemically silent
The strongest motive to not declare waterworlds useless is that water nonetheless touches rock. Oceanic crust reacts with seawater, whereas hydrothermal circulation attracts fluid via fractures, heats it and returns it carrying altered chemistry. The path of the phosphorus trade relies upon partly on oxygen.
On right now’s oxygen-rich Earth, seafloor weathering is commonly a phosphorus sink. In laboratory experiments, nonetheless, researchers weathered basalt under oxygen-free conditions and located that it may launch bioavailable phosphorus at ratios comparable with fashionable river inputs. With much less oxygen obtainable to show dissolved iron into compounds that scavenge phosphorus, extra phosphorus can stay within the water.
The end result didn’t show that each waterworld is fertile. It demonstrated that uncovered continents should not the one conceivable nutrient supply. The whole provide would nonetheless depend upon how shortly contemporary rock is produced, how a lot water circulates via it, the ocean’s redox state and the way quickly phosphorus is buried once more.
A 2025 geochemical modelling study added one other complication. On a hypothetical Earth-sized planet with a roughly 10-kilometre world ocean, dissolved phosphorus depended strongly on the composition of the seafloor, particularly its calcium content material. Extra calcium favoured formation of fluorapatite, a mineral that locks phosphorus away and lowers the quantity left in resolution.
That turns “does it have continents?” into the start of the query, not the tip. Two planets with equally deep oceans may have very completely different nutrient budgets as a result of their crusts shaped from completely different beginning materials. The chemistry of the host star might supply clues to these rocky substances, however translating a star’s abundance sample into a person planet’s seafloor stays unsure.
Habitability doesn’t require an Earth copy
The acquainted carbon-silicate thermostat is one path to a sturdy gentle local weather, not the one conceivable route. Models by Edwin Kite and Eric Ford adopted rocky waterworlds with between 10 and 1,000 instances Earth’s water stock. Many retained liquid floor water for greater than a billion years with out counting on an energetic Earth-like geochemical cycle.
In these fashions, excessive seafloor strain restricts trade of carbon between the convecting mantle and the ocean-atmosphere system. Early reactions between water and crust can then go away the planet with an inherited mixture of dissolved ions and carbon dioxide that occurs to stay temperate as its Solar-like star slowly brightens.
This can be a matter of preliminary situations, not a brand new common thermostat. Some beginning chemistries fail. Orbit, atmospheric mass, stellar spectrum and the timing of water supply all matter. The helpful conclusion is narrower and extra strong: the absence of uncovered land doesn’t logically require a runaway greenhouse, a frozen ocean or a dull world.
“Liveable” additionally units a decrease bar than “Earth-like.” A worldwide ocean could be poor in vitamins, weakly buffered in opposition to local weather change and inhabited solely sparsely, whereas nonetheless providing temperatures, liquid water and chemical vitality appropriate with microbes. Whether or not such a biosphere would turn out to be productive, oxygenate its environment or help complicated life is a separate and far more durable query.
Depth ultimately adjustments the physics
At 5 terrestrial ocean volumes, the crude common depth is about 13 kilometres. Stress on the backside can be round a tenth of a gigapascal, relying on gravity and water density. That’s formidable, however it’s typically under the strain required to put a thick layer of unique high-pressure ice throughout a temperate ocean ground. Liquid water may nonetheless contact rock.
Add tens or tons of of ocean volumes and the inside turns into a special system. Higher strain can suppress volcanic outgassing and, at nonetheless higher depth, create ice phases denser than bizarre ice. These solids can sit beneath liquid water and separate it from silicate rock, weakening the chemical trade that makes shallow ocean flooring fascinating.
Coupled climate and geochemical models published in 2019 discovered that some very water-rich planets may endure runaway cooling. Melting close to scorching ridges can expose rock, promote seafloor weathering and take away carbon dioxide, whereas high-pressure ice elsewhere adjustments the circulation. Counterintuitively, including water might help produce a snowball reasonably than a steam tub.
This distinction echoes SpaceDaily’s earlier look at hidden oceans in the outer Solar System. Water quantity will not be a habitability rating. Contact with rock, obtainable vitality and the circulation of important components can matter as a lot because the mere presence of liquid.
What 5 oceans actually tells us
5 Earth oceans whole about 6.7 billion cubic kilometres and, utilizing the mass of terrestrial seawater, solely slightly over 0.1 p.c of Earth’s mass. That small planetary fraction may nonetheless reorganize the complete seen floor. A compositional distinction that’s delicate in a mass-and-radius measurement may be decisive for shorelines, local weather feedbacks and biology.
It might most likely erase the continental rivers and soils that make Earth’s carbon and nutrient cycles work of their acquainted type. It might not erase chemistry. Basalt may climate, hydrothermal techniques may flow into, impacts may ship materials and biology, if it started, may recycle scarce vitamins many instances.
Astronomers can not presently rely oceans on an Earth-sized exoplanet with this precision. Mass and radius enable many inside mixtures to mimic each other. Water vapour in an environment doesn’t set up a world floor ocean, whereas a dense environment or ice layer can complicate the inferred radius. Even the apparently easy distinction between land and water might stay past direct remark for a lot of targets.
The five-ocean thought experiment is due to this fact most useful as a warning in opposition to shortcuts. A planet within the liveable zone will not be essentially liveable, and a planet with out land will not be essentially sterile. Between these claims lies the tough work: figuring out how a lot water is current, whether or not it touches rock, what the crust is fabricated from, how carbon strikes and whether or not the local weather has endured reasonably than merely existed for a second.
