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I know my title isn't the most specific, but the planet I am looking for is. I have been looking through many exoplanets and I can't seem to find what I am looking for, the closest I have found is TRAPPIST-1h. The issue is I do not believe that an anti-greenhouse effect could lower the temperature enough to make it suitable.

What I am looking for is a terrestrial planet, under six earth masses, at an equilibrium temperature of around ninety degrees Kelvin, and is within the Aquarius constellation. The star of the planet doesn't have to be visible to an earthbound audience, and the equilibrium temperature can be above or below ninety, as long as greenhouse or anti-greenhouse effects could theoretically make the surface temperature average ninety. A preference for the star it orbits to be a population II star, but that is not a requirement. Based on these parameters are there any planetary candidates?

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    $\begingroup$ Probably none. Terrestrial planets that are that cold (and hence far from their star) would probably not be detectable by radial motion, and would be hard to detect by transit. But they are too small for direct imaging. So detecting such a planet would be very difficult. And for it to be in some random region of the sky makes it much less likely. What is your actual purpose for this? Why are you looking for a planet with this combination of properties? $\endgroup$
    – James K
    Aug 1, 2023 at 7:35
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    $\begingroup$ @JamesKI was recommended to posit this question in this S.E. from the Worldbuilding S.E. If this is inappropriate here I can close it out. $\endgroup$
    – Amoeba
    Aug 2, 2023 at 2:56
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    $\begingroup$ @eshayaI It is a term I have read in the context of Titan, if there is a better term I will happily use it. $\endgroup$
    – Amoeba
    Aug 2, 2023 at 2:57
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    $\begingroup$ @eshaya Anti-greenhouses are standard terminology in the literature since decades, see e.g. ui.adsabs.harvard.edu/abs/1999Icar..137...56M/abstract $\endgroup$ Aug 2, 2023 at 10:55

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There is no known planet with these specifications.

Terrestrial exoplanets are rather hard to detect with current technology. Most plant detections use one of 3 methods: Transit, radial velocity and direct imaging. The first two are most likely to detect a planet that orbits close to the star; the last is only able to detect very large bodies that are very distant from the star. So in the middle region (where temperatures would be about 90K, and hydrocarbon lakes might exist in polar regions) is a region in which it is very hard to see small planets. Nearly all known terrestrial planets have temperatures that are very hot.

The one exception might be Proxima c. This is a somewhat speculative detection of a planet doubted by some. If it exists, it would probably be a small ice giant, but it could be a very large terrestrial planet. However, it would be too cold. And it is not in Aquarius (whatever that means).

So, no such planet is known to exist. This is likely just to reflect current observation technology. For the sake of worldbuilding you are free to build a world with the properties that you desire. There are plenty of stars in Aquarius.

A moon orbiting the Neptune-like planet Gliese 876 e, in Aquarius, could be in about the right temperature range. Of course no such moon is known to exist, but it would seem likely that a Neptune-like planet would have moons.

Given luminosity 5e24 W, albedo of 0.9, and distance 5.2e10 m from the star, the temperature, ignoring the greenhouse effect, would be (5e24*0.1/(16*pi*(5.2e10)^2*5.6e-8))^0.25=90K

Population 2 stars formed at a time when the dust that forms into planets hadn't yet been created. So terrestrial planets around such stars are likely to be rare.

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