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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.

Evan Carroll
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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 '22 at 20:50
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    Are you also going ask a Q about his proof that 1 x 5 = 6 ? https://youtu.be/ca1vIYmGyYA?t=2428 – Fizz Jul 19 '22 at 22:46
  • This thread is one of the reasons why I think skeptics.SE's policy on self-answering can be abused far too easily. Step 1: Post a question in which you doubt a claim that isn't really controversial and which can easily be shown to be true, even though you know that the claim is true. Step 2: Post your answer that shows the claim to be true, something that you knew all along. Step 3: Enjoy free internet points, because even a heavily downvoted question will probably result in a net positive due to how the reputation system works. – Schmuddi Jul 20 '22 at 07:14
  • The sun is getting hotter too, so the 'Goldilocks zone' will also move away from the sun. – Weather Vane Jul 20 '22 at 08:40
  • IIRC Earth is already outside the Goldilocks zone. It's only comfortably habitable due to the Greenhouse Effect. – gerrit Jul 20 '22 at 09:20
  • @Schmuddi I don't get your gripe. (1) the claim was shown to be false, you're simply wrong (2) who cares about rep anyway, _if_ you care do more work and comment/complain less. (3) what makes you think I know _anything_ about astronomy? This isn't my subject: 18% of Americans think the Sun revolves around the Earth. There is obviously a large spectrum here in "public knowledge". Self-answering is a good thing when you can, but the majority of my questions here are not self-answered. I have 124 questions, 39 answers. Out of that 124 Questions, 37 are "Famous Questions" on the site. – Evan Carroll Jul 20 '22 at 15:52
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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?) – LangLаngС Jul 20 '22 at 15:56
  • @LangLаngС I don't get all this controversy about Goldilocks Zone, NASA certainly disagrees: _**"The 'Goldilocks Zone,' or habitable zone, is the range of distance with the right temperatures for water to remain liquid"**_ https://exoplanets.nasa.gov/resources/323/goldilocks-zone/ How much more clear cut can you get then "will earth have the right temperatures for liquid water in 500 million years." Pretty basic from my perspective. I've updated the question to eliminate this confusion, but you're really stretching to push an invalidity argument (imho). – Evan Carroll Jul 20 '22 at 16:00
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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. – LangLаngС Jul 20 '22 at 16:15
  • @LangLаngС I'm not sure what you're on about. It's above my paygrade. But I'm content if the term has enough meaning for NASA, it has enough meaning to be used here. And it doesn't _sound_ like the nuance even comes into play when we're being liberal with the numbers and we come out 7500 km, a basically negligable amount to move the planet in or out of the zone regardless of whatever nuance you're trying to introduce. – Evan Carroll Jul 20 '22 at 16:49
  • "even with that apparently simple definition it is not straightforward to calculate the width of the HZ", how much mass is lost from the sun, how does radiation evolve, is Earth moved away, spiralling inwards, engulfed or survives… Too many variables (including _your_ 7.5 kkm) involved for one shot answer on such a timescale. Models! One attempt at an overview, partial: ["Distant future of the Sun and Earth revisited"](https://academic.oup.com/mnras/article/386/1/155/977315) – LangLаngС Jul 20 '22 at 16:54
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    Using the NASA defnitionof the Goldilocks zone mightnot be accurate. According to this: https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates there are many different estimates of the outer edge of the Sun's habitable zone. The nearest one is 1.004 AU, which would be 598,391.4828 kilometers farther thant the Earth's orbit. And perhaps the actual outer limit is even closer than that. – M. A. Golding Jul 20 '22 at 17:36
  • Answers that assume specific definitions different from the ones assumed by the quote in the question aren't too well received here. Yeah, in this case it was probably a simple data error by the "math genius" actor. But you could have focused your Q on that datum, instead of the whole Goldilocks theory, which is a lot more hairy. – Fizz Jul 20 '22 at 18:00
  • @Fizz we don't attack people for their credentials, we address the claim which is about the Goldilocks zone which has a well accepted definition given us to by NASA. – Evan Carroll Jul 20 '22 at 18:29
  • @EvanCarroll: do you have proof Howard was using that definition? – Fizz Jul 20 '22 at 18:30
  • lol Do we _EVER_ have proof of the definition someone is using when making a claim? No: and this is tiring. We **always** look for an accepted definition and rock with it. In this case, NASA uses the definition to mean "can sustain liquid water." We can use that definition to answer the claim. If NASA isn't authoritative enough for you on space-definitions, then make the argument that he could have reasonably been referring to a different definition of the Goldilocks zone which makes a material difference to the claim and renders it true. As is, I've argue the claim is false with the NASA def. – Evan Carroll Jul 20 '22 at 18:33
  • @EvanCarroll: Yeah, but **you linked to Wikipedia which says in bold** "0.99 - 1.004 Tightest bounded estimate from above" and "0.38 - 10 Most relaxed estimate from above" in AUs. Debunking nonsense calculations is one thing, pushing your own/preferred theory is another. – Fizz Jul 20 '22 at 18:34
  • See previous meta-questions here about definitions https://skeptics.meta.stackexchange.com/questions/2177/how-to-handle-questions-that-lead-to-arguments-about-definitions In particular, the last para in Oddthinking's answer there, about competing definitions. – Fizz Jul 20 '22 at 18:47
  • Timescale (speaking of magnitudes) _alone_ imo makes this too wobbly to answer without an enormous umbrella of ranges & possibilities to account for: ["Due to the chaotic behaviour, the time of validity for a precise orbital solution of the Earth will be in practice limited to 35–50 Ma."](https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.917.6&rep=rep1&type=pdf) That's just for the 'chaos' & tiniest diffs in starting 'data'/_assumptions_, add to that celestial collisions and other general weaknesses of models: we cannot know the _exact_ position of Earth then, & don't know HZ now… – LangLаngС Jul 20 '22 at 18:50
  • Am still hesitant to even flag for migration. So let me suggest alternatives: either trim down this Q&A, and/or move to EarthSci — or: use the info by now found in comments to upgrade the A substantially to a much less 'certain' but just the more educational post? (Although not much will change for the foundational problem of theory/model/unprovable predictions…) – LangLаngС Jul 20 '22 at 18:54
  • @Fizz that's correct I did. And I gave you the definition I was operating on in the question: the NASA definition. This is still very **VERY** silly, as the conservative definition doesn't change the answer. In 500 million years, Earth is still in _THAT_ Goldilock's zone too, and thus the claim is incorrect: the claim is that we will **NOT** be the habitable zone in 500 M years. – Evan Carroll Jul 20 '22 at 18:55
  • You misunderstood the issue then. It doesn't matter what def you want to assume. It matters what the author of the claim assumed. Besides, as far as I can tell, debunking the claim doesn't require assuming the NASA definition. It's enough to refer to the planetary positions claimed and drift rate (claimed) in the quote. So you're making this more complicated than it should be. – Fizz Jul 20 '22 at 18:58
  • @Fizz I can't continue this any longer. Just read the meta link you provided, _" It is sometimes necessary to let the answer specify a definition where it isn't clear in the original claim. (e.g. "Is a tomato a fruit?" That's a question best answered by sharing several competing definitions.) **However, letting people stray away from the normal scientific/legal/dictionary definitions of words is likely to confuse more than it enlightens.**"_ I didn't come up with a definition. I used NASA's. I also said I used NASA's definition. Earth will not be further away then Mars in 500 M years. QED – Evan Carroll Jul 20 '22 at 19:00
  • You also linked to Wikipedia which provides a much wider range of estimates. I'm also done with this discussion, as there's not much point in repeating myself again. – Fizz Jul 20 '22 at 19:02
  • @Fizz because all of this controversy seems to be about taking the "tightest bounded estimate" (not used by NASA but mentioned on Wikipedia), I'll ask a question I won't be able to answer: https://astronomy.stackexchange.com/q/49978/46848 I would love to see your contribution there that this range has more value then the one I've provided from NASA. **This is _way_ of out my pay grade, I will not answer that question.** – Evan Carroll Jul 20 '22 at 19:19
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    One part of the general claim that is not in question but *should* be, is whether it is reasonable to expect whatever drift rate there is to stay constant over such a long period of time. I highly doubt the answer is yes. – Todd Wilcox Jul 20 '22 at 21:08
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    Questions about ongoing current affairs are off topic. :-) – DJClayworth Jul 21 '22 at 01:10

1 Answers1

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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".)

Fizz
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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.se]. The answer on [astronomy.se] 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."** – Evan Carroll Jul 20 '22 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. – Evan Carroll Jul 20 '22 at 22:37
  • @EvanCarroll: indeed, but I was trying to come up with a convincing way to render that. See edit. – Fizz Jul 20 '22 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… – LangLаngС Jul 20 '22 at 23:49