# What is the largest I can make my water planet, while remaining within Earth-like parameters?

What is the largest planet I can get, while still remaining within Earth-like parameters? My concern is with gravity, water pressure and atmospheric pressure. Escape velocity is less of a concern. Available elements in areas accessible for human resource collection should be somewhat Earth-like, but the composition of the planets inside is less important, as long as it is plausible.

I would like to create a planet that is mostly covered in oceans, with optionally some small islands sticking out above the ocean's surface. I would like to see as large a volume of the ocean being accessible to a human-like culture as possible. So, basically the question is about maximizing the accessible ocean volume, on a planet where humans can reasonably comfortable survive.

These humans have a technology level we could think plausible for the year 2,200 or so (sci-fi, not sci-fantasy). These humans live in underwater settlements, towns and cities and travel between them is submarines.

While this is not a hard-science question, I would minimize the liberties that have to be taken, to get the largest, human inhabitable, ocean planet possible.

• Here's your problem: everything is incredibly interconnected. Weather, volcanology, geology, oceans and currents, life... It's marvelously complex. Therefore, the only way to be "Earth-like" is to use the Earth as a model. This suggests no variation. If you want a larger planet, you must be willing to give up "Earth-like" stuff. You can't be bigger without increasing gravity or decreasing density, which would have serious implications for geology and weather. So, how "not Earth-like" can your planet be? What are you willing to give up to get a bigger planet? (*continued*) – JBH May 3 at 14:15
• But in the end, are you sure that you need this level of detail for your story? Are you worried that the average reader might read your story and think, "this planet is so unrealistic?" That won't be the case because ignoring the facts of reality is why people read stories in the first place (well... usually). Star Wars, Star Trek, and pretty much the entire Sci-Fi genre wouldn't exist if "reality" was a prerequisite. So, what about your story depends on scientifically justifying a large planet with "Earth-like" conditions? If the answer is "nothing," then declare it to be so and move on. – JBH May 3 at 14:17

## 1 Answer

Models of planetary structure indicate that ocean planets should reach peak size at a few hundred to a thousand Earth masses, reaching maximum radii of perhaps 4-5 Earth radii. Add in some iron and silicates and you decrease the size slightly. Indeed, this mass range features peak radii for planets of most compositions; go further and the planets shrink.

For these parameters, you see surface gravities about 12 times that of Earth - too much. If you want to retain a surface gravity like that on Earth, your mass limit is about 6 Earth masses and about 2.5 Earth radii. If you want a more massive planet, you in turn need a larger planet, and it turns out that for constant surface gravity, the required radius grows quickly. For a planet of a few hundred Earth masses, the required radius to maintain Earth-like surface gravity would mean that the world is composed largely of hydrogen and helium.

I made a plot of some mass-radius curves for a pure ocean planet and a pure silicate planet, as well as curves for planets with surface gravities of $$g$$, $$2g$$, and $$3g$$. Your planet should be in between the $$\text{H}_s\text{O}$$ and $$\text{MgSiO}_3$$ curves, given that solid land requires some solid materials, and arguably continental plates of some sort.

Earth-like mass, radius and surface gravity usually means an Earth-like atmosphere insofar as it should be composed of heavy gases like nitrogen and carbon dioxide. Small planets like water worlds typically do not hold onto hydrogen-helium envelopes for very long, losing them to thermal atmospheric escape.

• I love this chart. It's super useful. – Andon May 4 at 16:47