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According to American actor Terrence Howard in his address at the Oxford Union Society:

Our planet is moving away from our sun at six inches a year [...] 15 centimeters a year our planet is pushing away from the sun. In less then half a billion years our planet will be out of the Goldilocks zone. We will somewhere near where Mars is, somewhere halfway between it.

I think, given the context, there are a few claims here that one can extract that are worth considering,

  • Does the planet face any other source of certain astronomical demise that would alter a Goldilocks paradigm from being useful like the Sun going red-giant and destroying the planet, over the next half-billion years. If so, this concern is moot.
  • Are we moving away from the sun at 6 inches per year? Can we measure to this degree of precision our distance from the sun?
  • If we are, does Earth's trajectory and speed place the planet outside of our star's habitable (Goldilocks) zone in the next half a billion years?

For the purpose of this question let's use the NASA definition of the "Goldilock's Zone",

"The 'Goldilocks Zone,' or habitable zone, is the range of distance with the right temperatures for water to remain liquid."

The claim itself mentions Mars, so that makes sense as that's also part of the 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.

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    One thing that not many people consider is that the goldilocks zone is not an absolute. There are many conditions yet for liquid water to exist on the surface of a planet which is well outside the zone. The zone itself is even a subject of debate today, so an answer on imperial numbers may not be valid next week depending on the opinions of the community.
    – tuskiomi
    Jul 19 at 20:50
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    Are you also going ask a Q about his proof that 1 x 5 = 6 ? youtu.be/ca1vIYmGyYA?t=2428
    – Fizz
    Jul 19 at 22:46
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    This seems better housed at EarthSci? It doesn't fit well here: Theory based, model driven, most probably no concrete empirical evidence ever available for the foreseeable future life-span of this site, or any of its users. Further: this Q(&A) seems to assume 'Goldilocks-zone' as a set & tight piece of measured numbers, while it is just the general idea that's accepted, with definitions varying quite a lot, & consequently proposed distances for our system being much more variable than either question or claim (0.38–10AU)? Exploring such theories is better served elsewhere (religious sites?) Jul 20 at 15:56
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    That quote shows exactly what I meant to convey: 'a general idea'. Which, how many, how important the factors influencing the modelled calculation for this are debatable. The reverse of the 0.3–10AU from Mercury to almost Saturn, would be 0.99–1.01 AU, putting even Venus (often included in zone) and Mars outside. [Has Europa the right distance for fluid water?] If we entertain theories, we should strive to give a complete picture of the sci-discussion, with all these uncertainties & problematic definitions. But that still would fall outside of the stricter empiricism this site imo requires. Jul 20 at 16:15
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    Questions about ongoing current affairs are off topic. :-) Jul 21 at 1:10

2 Answers 2

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Ignoring debates about what constitutes the "Goldilocks zone", the issue of the Earth-Sun (AU) distance change is not terribly settled either.

Howard appears to be using data from a somewhat cited 2004 paper by Krasinsky & Brumberg, who estimated 15 ± 4 cm / year. (There were other papers around that time that found comparable figures, using similar methods, e.g. 7 ± 2 cm/year.) However, some years later (2012), other researchers (Pitjeva & Pitjev) from the same (Russian) institute have cast doubt on that Krasinsky & Brumberg figure and its method of estimation. According to the latter, the margin of error is greater than the estimate, i.e. 1.2 ± 3.2 cm / year.

As far as I can tell the 1.5 cm figure given by another astrophysicist in Forbes, which is cited as the true figure in the self-answer, originates from a pretty simple model (that is probably not publishable as such in a peer-reviewed venue, nowadays), and lacks an estimate for the error margin.

More on this at https://astronomy.stackexchange.com/questions/49979/is-there-something-close-to-a-consensus-on-earth-sun-annual-distance-increase-s


Even if we grant the 0.15 m/year drift, rounding AU to 150 million km = 0.15 x 10^12 m, i.e. 0.15 trillion meters, means that the Earth-Sun distance would increase by one trillionth in a year, or it would take a trillion years for the distance to double.

But Mars sits at about 1.5 AUs. So to get that kind of increase (50%) it would take half a trillion years, not half a billion. I suspect Howard made this off-by-a-factor-of-1,000 error in his calculation.


This constant speed model, however, is pretty bad, because it ignores a bunch of things, like the accelerated rate at which the Sun will shed mass as it expands to the red giant stage. Wikipedia cites one 2008 paper, according to which the Earth-Sun distance will reach that 1.5 AUs much sooner, but the habitable zone (HZ) will also have moved outwards much faster...

Certainly, with the 10% increase of solar luminosity over the next 1 Gy [...], it is clear that Earth will come to leave the HZ already in about a billion years time, since the inner (hot side) boundary will then cross 1 AU. By the time the Sun comes to leave the main se- quence, around an age of 10 Gy (Table 1), our simple model predicts that the HZ will have moved out to the range 1.29 to 1.86 AU. The Sun will have lost very little mass by that time, so the Earth’s orbital radius will still be about 1 AU – left far behind by the HZ, which will instead be enveloping the orbit of Mars.

By the time the Sun reaches the tip of the RGB, at 12.17 Gy, the Earth’s orbital radius will only have expanded to at most 1.5 AU, but the habitable zone will have a range of 49.4 to 71.4 AU, reaching well into the Kuiper Belt!

I can't say how much consensus that paper has (beyond the fact that the Sun will move to red giant stage), but it shows that such things aren't simple to calculate. (The paper uses the NASA-like def of habitability "conditions on it allow the presence of liquid water on its surface".)

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  • Disappointed you did all that talking about the Goldilocks zone and ignored it in your answer, and didn't comment on the thread on Astronomy. The answer on Astronomy you cited provides concrete reason to believe that the models you're citing doesn't fit with observation. But let's play devil's advocate and reiterate my problem here. Even if you grant 15 cm / yr from the one refuted model that doesn't meet observations, you have 75,000 km of drift in 500,000 million years. Earth is still very much habitable and within all definitions of the Sun's Goldilocks zone." Jul 20 at 22:34
  • Let me put it to you differently, you can always find a different model, or a different (less authoritative and well adopted) definition. You've spent a lot of time making that point. But you're still NOT connecting it to the claim. How does this matter? I want an answer that says I'm wrong under whatever circumstances because of the drift rate. Not just that someone pushes a more aggressive drift rate that still renders the claim in question HIGHLY inaccurate. Jul 20 at 22:37
  • @EvanCarroll: indeed, but I was trying to come up with a convincing way to render that. See edit.
    – Fizz
    Jul 20 at 23:13
  • Please also include the direct quote I posted in Qcomments & took from your now included 'revisited' paper link & then feel free to flag that comment as NLN… @EvanCarroll all papers collected on this Q&A so far show two things: you may be 'right', and your answer is way from conclusive evidence, as all real papers strongly caveat multiple times about the unsatisfactory theories, models & unaccounted or unaccountable factors, presenting more limitations than firm ground overall. Any answer here needs a few models to model all the other basic models' outcomes… Jul 20 at 23:49
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Nevermind (7500 km) being no where near as far out as Mars (67M km), which is regarded as within the Goldilocks zone, this is far less then the diameter of Earth. At any point in time we have someone on Earth further away from the sun then Earth will drift in the next 500 million years.

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    There is 100 cm in a meter.
    – TimRias
    Jul 19 at 22:16
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    ...and 1.5 cm in 0.6 inches.
    – Penguino
    Jul 19 at 22:58
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    "there is no reason to suspect doom from the sun in that time frame" You only considered the Sun's expiration, and not other astronomical factors. The Wikipedia page (!?) you cite says "At present, it is increasing in brightness by about 1% every 100 million years. It takes at least 1 billion years from now to deplete liquid water from the Earth from such increase" What will be the consequence in half of that time?
    – Oddthinking
    Jul 19 at 23:30
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    We've seen the issue of whether maths needs to be cited raised repeatedly. This answer is a good illustration of why it isn't good enough to let answerers come up with their own mathematical models. You made at least 2 errors in the original calculation, and we don't know if there are more. Added to that, you made a (I believe false) claim that there are no other astronomical threats; this is again your own model, and not backed by references.
    – Oddthinking
    Jul 20 at 1:09
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    @Oddthinking The claim is NOT about whether or not there is life on Earth with half the water. It's about whether or not Earth has drifted outside the Goldilox Zone and that's still a useful paradigm. I could see you arguing that because the sun goes red-giant before that point, there is no planet in the Goldilocks zone before Jupitor or something. But if there is a Goldilox zone in 500 million years, and if Earth can still sustain any liquid water it would satisfy that definition, no? Jul 20 at 1:42

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