Hal Clement's planet Mesklin
This first appeared in his novel Mission of Gravity (1953). Mesklin is a gold standard hard science planetary setting.
Mesklin is a fictional supergiant planet created by Hal Clement and used in a number of his hard science fiction stories.
It is distinctive for the interaction of its strong gravity with the
centrifugal force due to its fast rotation, originating, according to
Clement's original calculations, a gee force gradient, starting at 3 g
on the equator, and ending at 665 g on the planet's poles.
Further details can be explicated below.
Clement described the basic characteristics of Mesklin in the article
"Whirligig World" in Astounding Science Fiction (June 1953). He
based the world on an object then thought to exist in the 61 Cygni
system, which had been detected by analysis of the motion of the two
already known stars in the system. Further analysis with more
extensive data led to the conclusion that the find had been erroneous.
Clement decided, since its mass was 16 times that of Jupiter, Mesklin
would have an extremely large angular frequency to partly counter its
gravity in order to allow humans to visit part of it. He wanted the
equatorial gravity to be 3 g, so he determined the period necessary to
make this occur: each Mesklin day is 17.75 minutes long given that the
planet rotates approximately 20 degrees a minute.
As a result of this extremely large rate of spin, Mesklin is not even
slightly spherical; it has a large equatorial bulge. Mesklin's
equatorial diameter is 48,000 miles (77,250 km), while from
pole-to-pole along its axis of rotation it is 19,740 miles (31,770
km). Then Clement attempted to calculate the polar gravity, finding it
surprisingly difficult. He admits, "To be perfectly frank, I don't
know the exact value of the polar gravity; the planet is so oblate
that the usual rule of spheres... would not even be a good
approximation..." "Whirligig World" reports his initial calculations
of the pole gravity to be 655 g; the dust jacket of Heavy Planet
reports it as 700 g. A later program created by Clement computed it as
275 g, as did a similar program written by the MIT Science Fiction
Society. The MIT group also concluded that the planet would have had a
sharp edge at the equator.1 Clement also gave Mesklin a set of rings
and massive moons. The inner moon is 90,000 miles (140,000 km) from
the planet's center, with a period of 2 hours 8 minutes.
Clement assumed Mesklin's orbit around its star (which he decided
would be 61 Cygni A) took 1,800 Earth-days, and was highly elliptical:
at its closest point the average temperature would be −50 °C, while at
the furthest its average temperature would be −180 °C. Since the orbit
is eccentric it moves rapidly past its sun at the closest point, so
its temperature would be around −170 °C most of the time.
Clement decided this imaginary world would have native life-forms,
that they would be based on methane (CH4), and there would be oceans
of methane. However, methane has a low boiling point, suggesting that
Mesklin's sun might boil its oceans and cause the methane to escape
the planet entirely. Thus, the writer arranged the planet so its
northern hemisphere's midsummer occurs when it is nearest its sun.
Thus, the northern hemisphere would develop a large frozen methane cap
during most of its year; the southern hemisphere (where most creatures
live) is protected from the sun's closest approach by the rest of the
planet. He also asserted the planet would have a fairly rapid
Considering Clement published his novel about Mesklin in 1953, yet it sounds like one of the exoplanets discovered in the last couple of decades.
Importantly, Clement made the following magnanimous offer to open this planetary setting to other authors.
In "Whirligig World", Clement stated he gave "official permission to anyone who so desires to lay scenes there [in Mesklin]. I ask only that he maintain reasonable scientific standards, and that's certainly an elastic requirement in the field of science fiction."
Effectively this offer for other authors to use Mesklin has been available since 1953. To date no-one has taken it up. As a courtesy, any author writing stories set on Mesklin should contact the Clement estate before doing so. Hal Clement (real name Harry Stubbs) died in 2001. Please note the only restriction is maintaining reasonable scientific standards, and even there Clement was indicating that some measure of flexibility is allowed.
Clement has done all the hard work of building the planet. While various studies have expanded and extended on the characteristics of Mesklin. More recently the Australian physicist Ditmar Jenssen analyzed Mesklin in detail. Tjis was published in the fanzine Interstellar Ramjet Scoop circa 2004.
This isn't a shared world, they came much later, circa the 1980s or 1990s, and it isn't open source fiction where creative commons licenses are expected. It is, surprise, surprise, a science-fiction setting that is open to any science-fiction author who wants to write a story there.
A point of clarification about the intellectual property rights inherent in Clemen's Mesklin. Although Clement publicly granted access to his created imaginary planet, the ownership of the IP rights presumably now reside with his estate, therefore, it will be necessary to contact his heirs and successors.
They may have the power the rescind this open access, and it is for this reason any writer intending to use Mesklin as a setting they should first contact his estate both as a courtesy and ensure it is permissible to do so.
However, a good case could be put to his surviving family that having other authors write about Mesklin was well within Hal Clement's own explicitly stated wishes, even as long ago as 1953, and to do so would be a fitting tribute to his memory and a celebration of his work.
As the question said: "The world building has been done for you - the logic holes filled, the possibilities mapped and explored, the implications accounted for." Mesklin has been open for business by other writers since 1953. Hal Clement was scrupulous worldbuilder. He had given a clear indication that other writers were allowed to play there and in a manner that should permit an open legal license to do so. The pity is no-one had taken him up on the offer.