We're having an argument on another site about the Hart definition of the CHZ. In 1979, Hart provided a very conservative estimate of habitable zone: 0.958 - 1.004 AU. This was used to substantiate his rare Earth hypothesis. Is this range still relevant today. Certainly you can argue that NASA doesn't accept this definition.

NASA definition of the Goldilocks zone,

In our solar system, Earth sits comfortably inside the Sun’s habitable zone. Broiling planet Venus is within the inner edge, while refrigerated Mars is near the outer boundary.

Mars's perihelion is far away at 1.3814 AU.


2 Answers 2


A number of contemporary researchers in this field consider Hart's model plain erroneous at the lower temperatures (i.e. upper range on the AU), among them David Catling who reasons that the main problem with Hart is that he ignores ways to break out of "Snowball Earth" conditions, like vulcanism accumulating CO2 or other greenhouse gasses, whereas the modern models would have to consider these.

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A similar assessment is found in Kasting's book.

Exactly why his climate model failed to recover from runaway glaciation is not clear. It was a highly simplified model, though, and its treatment of both radiation and convection left much to be desired. If he had used a more elaborate climate model, he might have gotten different results.

But fixing that won't necessarily give you something as wide as NASA ['s website] assumes. Most of these papers I've looked at don't dive below 0.95 AU, although there is more spread above.

Kasting et al (1993) would be typical of these, e.g. 0.95 - 1.37 AU, but that further narrows to 1.15 AU upper limit if habitability for 4Gy is also presumed necessary (as Hart did). Kasting's 1993 paper is about 6 times more cited than Hart's (in Google Scholar, 2.4K vs 400+ citations, for the 1978 paper, although his 1979 one has almost as many).

But if one considers a bi-dimensional model, the upper range is (almost) "triangularly" restricted, e.g. from one Catling's slides (on his own recent [2017?] work).

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In his book, Kasting calculates (nearly) the same 1.65 AU absolute upper limit (p. 178), as you see above, and so says "Mars might well be habitable today if it were able to recycle its atmospheric CO2".

There are models which come up with much wider estimates, such as Abe et al. but these somewhat more substantially relax what we know of climate models for Earth. And they don't talk much if at all about errors in Hart because their main target of criticism are models like Kasting's. E.g. Abe et al. spend some time attacking the latter, with the ultimate goal of claiming that life could have existed on Venus, as recently as 1 billion years ago. They generally widen the (hard) Simpson-Nakajima limit from 1.4 (Kasting, (1988)) to 1.7 or even 1.8 S0, so that allows them to claim that ocean evaporation would be much delayed on Venus-like planets.

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What Abe et al. essentially propose are Dune-like (hot) planets that could habitable due to a thin atmosphere limiting the greenhouse effect, while preserving some liquid water, possibly seasonal, at the poles, or on the dark side of a despun planet. (The don't really talk of the implications of such assumptions on other aspects of life as we know it, such as [not much] resistance to ionizing radiation etc.)

Funny addendum, perhaps: both Kasting and Hart worked for NASA at some point in their careers, as do a couple of co-authors of Abe et al.

  • $\begingroup$ Just a note on volcanism outgassing: it is part of the deep carbon cycle, where you also have carbon going back to the mantle through subduction. It is true that this cycle is not at equilibrium; since 200 Ma, it was estimated that carbon outgassing is net positive, but it was not necessarily always the case. doi.org/10.3389/feart.2019.00263 $\endgroup$ Commented Jul 21, 2022 at 13:23
  • $\begingroup$ Good answer. Would it be fair to say then that while the lower range of the Goldilocks zone is more disputable the consensus for the upper range seems to have settled around or above 1.3 AU? $\endgroup$ Commented Jul 21, 2022 at 16:00

There is habitable and then there is habitable. The biosphere of the planet Earth, for example, contains many regions where life flourishes but where unprepared and unprotected humans would rapidly die.

The only scientific discussion of the possiiblity of human habitabile exoplanets is Habitable Planets for Man, Stephen H. Dole, 1964.


And of course Dole was using the science of 60 years ago in his calculations and estimates.

Other scientific estimates of the habitable zone of the Sun are for liquid water using lifeforms in general, not for for humans or other oxygen breathing lifeforms in particular. Earth had life for at least one billion year, and possibly for several billion years, before the era hunderds of milions of year ago when it first had enough oxygen in the atmopshere for large multicelled land animals - like dinosaurs or humans, for example - to survive. In fact the vast amjority of oxygen in Earth's atmosphere was produced by life, photosynthic plants.

Scientists know that life forms can survive on world where humans couldn't survive - that was the case on Earth for billions of years, and is still the case in many parts of Earth.

So scientific disussion of the habitabiity of other worlds include worlds without atmospheric and other conditions necessary for human survival, if other Earth like lifeforms could survive there.

There are about a dozen different estimates of the inner or outer edges, or both, of the suns's "Goldilocks zone" in this list:


And they differ quite widely. I note that some of the estimates which extend the inner or outer edges do so by assuming specific atmospheric densities and compositons. Such atmospheres would probably not be breathable by humans or by animals with similar requirements, but life that evolved in such atmospheres should be able to tolerate them.

Thus the human habitable zone of a star is likely to be a smaller subset of the liqud water using life habitable zone of a star.

So if someone now, in the year AD 2022, wants to know the limits of the circumstellar habitable zone of a speciific star, they should use the luminosity of that star relative to the sun to adjust the inner and outer limits of the Sun's circumstellar habitable zone to fit the other star's luminosity. And for better accuracy, they should use the inner and outer edges of the Sun's circumstellar habitable zone as accepted in the year AD 2122.

And if they don't have a time machine, they will be stuck with studying all the estimations and calculations of the Sun's circumstellar habitable zone so far and decide which seems most accurate to them.

I also note that a number of worlds in our solar system are known to have vast subsurface oceans of liquid water below massive world wide icecaps, and other worlds are suspected to have such subsurface oceans.


And it has been speculated that possibly there could be liuid water using lifeforms in those subsurface oceans. Those worlds all have ice surfaces due to being too cold for liquid water on their surfaces, and so are beyond the conventional definition of the Sun's circumstellar habitable zone.

If life can develop and survive in such subsurface oceans of icy worlds there oculd potentially be liquid water using life within hundreds of as yet undiscovered icy worlds in the outer solar system far beyond the habitable zone of the Sun.

As Wikipedia says:

In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.[15] Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist within in lithospheres and asthenospheres of the Solar System.[16] Sustained by other energy sources, such as tidal heating[17][18] or radioactive decay[19] or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons.[20] Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars,[21] or ammoniates,[22] due to its different colligative properties. Thus, the term Goldilocks Edge has also been suggested.[23] In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.[24]


So at the present time there seems to be considerable uncertainty about the distances from the Sun where a world could possibly, under some circumstances, have liquid water on parts of its surface.


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