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In high school I messed around a lot with Celestia, a freeware "universe simulator" that allows you to view pretty much any known object in the solar system, the galaxy, or the universe. The key appeal of Celestia is the ease with which one can code add-ons, many of which can be found here. In college I once coded an add-on that contained every star, planet, and moon in the Firefly verse, which I never uploaded because I knew nothing about creating planetary textures.

Still don't.

One that I've always remembered is this interpretation of the HD 40307 solar system. At the time it was made there were only three "Super-Earth" planets known around that star; now there may be as many as seven. Regardless, it wasn't that big of a stretch for modder "AVBursch" to add a hypothetical fourth planet.

Clockwise from top-left: HD 40307 b, hypothetical e, c, and d, dubbed Lemnos, Naxos, Thebes, and Samothrace respectively by the modder. (Image source: Celestia Motherlode)

None of these planets could be habitable, and this modder doesn't try to make them so. Instead he comes up with four inhospitable worlds that seem incredibly real. This is helped by the flavor text he provides in the readme file that comes with the add-on.

The innermost planet, Lemnos, is covered in molten metal seas and rivers, tidally locked to its star, with day and night temperatures of 1880 degrees F and 1232 degrees F respectively. Zoom in on the surface and you'll find that the sky is bright red.

Thebes, the next planet, has a thick atmosphere but is described as being too hot for clouds to form (though the model has clouds - they can be toggled on and off).

Samothrace, however, has a thick cloud layer like Venus, though you can still view the surface through the clouds. Its atmospheric density is 4 times that of Venus.

Naxos, the hypothetical planet, does not quite match with any of the planets that have been discovered in real life since this mod was created. It's described as having a mass of 5.51 Earths and a semi-major orbital axis of 0.422 astronomical units. Compare HD 40307 f, with a mass of 5.2 Earths and an orbital axis of 0.247 AU, and HD 40307 g, with a mass of 7.1 Earths and an orbital axis of 0.600 AU (give or take 1 or 2 Earths and 0.1 or 0.2 AU). Described as having "ocean of water that is kept liquid in spite of the surface temperature of 605 kelvin (630 F) because of the immemse atmospheric pressure", the description for Naxos is where I first heard the term "Pelagic" used to describe an ocean planet.

These planets would almost never be used as settings for any mainstream sci-fi media, unless Hal Clement were still alive (look him up). This modder made these planets just to be creative, and it's a shame there aren't more well-thought-out mods like this, where scientific accuracy and realism are the top priority. Why can't they all be like this?