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I recently read this article (http://www.space.com/5985-hottest-planet-discovered.html) about a hot Jupiter that orbits it's star about once a (earth) day. This and remembering how high of an albedo Venus has (inefficient if you want to maximize temperature) made me think about how hot you could make a terrestrial planet, since I'm not sure of any observed terrestrial exoplanets that have all the right factors.

So for this question I am only accepting planets that one are terrestrial planets, two can survive and maintain their temperatures for billions of years. The planet's temperature will be considered based on surface temperature, I also want the planet to actually have a well defined surface, say something you could float a unobtainium boat on or whatnot.

Ideally I would prefer models that would make a reasonable person think that we should expect to eventually observe exoplanets with many of your stated properties. Though the specific perfect storm of properties might be extremely rare or whatnot requiring a perfect storm of variables, but they ought to be able to plausibly occur in our universe.

Some obvious (and less so) properties that would contribute to heat would be: Distance from star, extreme greenhouse effects, very high albedo, massive tidal effects from a dual planet system or massive moon or maybe the star itself at extremely close orbits, radioactive decay, and potentially multiple stars.

Problems that arise are that you would have to worry about much of the planet vaporizing at extremely high temperatures, and having a orbital period of one earth day is predicted to be about as close as you can get without being vaporized, though it might be different for a terrestrial planet.
Another problem is that thicker atmospheres grant greater greenhouse effects, however they are also likely to get blown away if close to the parent star, especially over the required geological timescales.
There's also the fact that the hottest stars wouldn't even last the required billions of years.
All in all it's more complicated than just maxing out every quality that we can know would increase temperature on it's own.

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  • $\begingroup$ This is seriously pushing the term "terrestrial planet". The question would be easier to understand if you outline the parameters directly. They seem to be 1) a well defined solid or liquid surface, 2) will last billions of years at that temperature. $\endgroup$
    – Schwern
    Jun 1, 2016 at 6:35
  • $\begingroup$ I mean terrestrial as a contrast to gas giants, in this case I'm assuming most of the mass of the planet should be rock, and metal. @Schwern $\endgroup$
    – Vakus Drake
    Jun 1, 2016 at 7:13
  • $\begingroup$ A side note that might or might not be relevant to what you want. There is a time in the birth of a terrestrial planet where its surface has not yet solidified, ie. is molten. For Earth this would be pre-Hadean or Chaotian era. Obviously surface temperatures would be in thousands of degrees. While this era is geologically fairly short (not even billion years!) it would be long enough to be observed. $\endgroup$
    – Ville Niemi
    Jul 6, 2016 at 14:36
  • $\begingroup$ @VilleNiemi I was aware of this, however the temperatures early on a earthlike world not even approach some exoplanets we've observed. $\endgroup$
    – Vakus Drake
    Jul 11, 2016 at 1:59
  • $\begingroup$ Getting a terrestrial planet significantly hotter than this presumes that it formed at a cooler location and even then the surface temperature of the star acts as a limiting factor. So "would not even approach" is probably an overstatement? The reason my comment is off-topic is because it is just a phase and you seemed to want a more permanent state? $\endgroup$
    – Ville Niemi
    Jul 11, 2016 at 4:29

2 Answers 2

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You can have a planet as hot as you want. The key parameter you can play with is the orbital distance.

There are planets known that are almost touching their stars. Wasp-12b is a gas giant planet that may be in the process of being tidally shredded by its host star (https://en.wikipedia.org/wiki/WASP-12b). It orbits its star once every 1.1 days.

But there are far more extreme terrestrial examples. Kepler-78b is Earth-like in its size and bulk density but has an 8.5 hour day (see here: https://en.wikipedia.org/wiki/Kepler-78b). The shortest day of any known planet (that I'm aware of) is about 4 hours.

That close to the star, a planet's spin is likely to be tidally locked to its star, and the surface temperature may be hotter than the melting point of rock (~1500 Kelvin). That said, the temperature on the night side (which always points away from the star) may not be too too high, as long as the atmosphere is thin.

Here are a couple of articles discussing this type of planet that is tidally locked to its star. Hope this helps https://planetplanet.net/2014/10/07/real-life-sci-fi-world-2-the-hot-eyeball-planet/ http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets

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  • $\begingroup$ Ok but the thing is that you can't go as hot as you want necessarily. There's a tradeoff between having a thick atmosphere and being closer to one's star as a stated in my question. And there's a lot of other interacting factors, the goal is to combine as many factors as possible to imagine what the theoretical limit to this is. $\endgroup$
    – Vakus Drake
    May 31, 2016 at 10:22
  • $\begingroup$ There are indeed different ways to heat a planet. External heating from the star is the simplest one, but not the only one. A planet can be heated from the inside-out by the decay of long-lived radionuclides like Uranium-238. Strong tidal dissipation within the planet can create strong internal heat too (just look at Jupiter's moon Io, which is covered with tidally-driven volcanos). A thick atmosphere can heat the surface too. The only theoretical limit on this process in my mind is when there is no surface left: it is entirely molten or evaporated. $\endgroup$
    – Sean Raymond
    May 31, 2016 at 11:34
  • $\begingroup$ Yes but that's the point as I said in the question there are a good deal of trade offs between different heating methods. And the point where the planet is vaporized by the heat will vary based on composition. It may be a scenario where the maximum heat depends on how much a certain composition can take before the planet starts to be lost to space. The point is I really doubt there's an easy way to actually answer this question without doing a bit of research, and given my knowledge of the topic is based on a few youtube videos and wikipedia articles its rather beyond me. $\endgroup$
    – Vakus Drake
    Jun 1, 2016 at 3:56
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Since you mention unobtainium, can the planet be made of something with a fictitiously high melting point?

There are theoretical alloys of hafnium which have a melting point of 4,126°C. So if the planet is made out of Carbon, Hafnium and Nitrogen, it could maintain solid parts at under 4,126°C.

If the planet is close enough to the sun, perhaps it could melt partially during the day, and still maintain it's shape, thus being even hotter than that.

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  • $\begingroup$ Where did I mention unobtainium? The point of this question is about something theoretically possible in our universe. Oh I see where I said unobtainium, but that's not about the properties of the planet. The planet itself should be natural. $\endgroup$
    – Vakus Drake
    Jul 11, 2016 at 1:53
  • $\begingroup$ If a alloy is only theorized You need a fair bit of explanation to conceive of how it could possibly arise in a natural system. $\endgroup$
    – Vakus Drake
    Jul 11, 2016 at 1:57
  • $\begingroup$ From what I've read, the reason for the theoretical melting point is that it's difficult to test a melting point that high in an experimental setting. I'm not quite sure how the alloy could be created, but there are definitely circumstances where alloys can be created naturally in space that cannot occur naturally on Earth. $\endgroup$
    – Brian McClure
    Jul 11, 2016 at 14:54
  • $\begingroup$ Hmm, I think I know what you mean, 0-G is needed for some alloys, but i'm not sure if there any alloys which require this that would matter in this scenario $\endgroup$
    – Vakus Drake
    Jul 21, 2016 at 4:48

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