Possible Explanations for Earth's Unique Characteristics

[Loren Booda]

What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?

 

[Andre]

The large moon, stabilizing it's spin axis, preventing Earth from getting into the chaotic zone

 

[Integral]

Since we have observed such a tiny percentage of planets in the universe we can only speculate about how unique we are.

 

[Xnn]

There's roughly 10^24 planets in the universe.

No reason to suspect that ours is unique.

 

[turbo]

There's roughly 10^24 planets in the universe.

No reason to suspect that ours is unique.

And some reason to suspect that all of them are unique in some way. We have a very small sample to observe here in our neighborhood, and there are some vast differences between them in many metrics. There is no reason to assume that other planetary systems would not have significant differences, as well. We have discovered extra-solar planets by lots of means, not the least of which is tracking light-curves to discover large planets partially occulting their stars when their orbits are aligned with our line-of-sight. Are we going to find a "Mercury" orbiting a large star that way? Probably not for a very long time.

 

[SW VandeCarr]

There's roughly 10^24 planets in the universe.

No reason to suspect that ours is unique.

Are you sure this number isn't a substantial overestimate? First start with the visible universe. Take 10^11 stars per galaxy and say 5x10^11 galaxies (which would be much more than the ones we can see). So that's 5x10^22 stars. How many of those stars have planets? Most stars we know of are multiple star systems which are unlikely to have planets. It seems you might be over-estimating by a factor of at least 10^3. We really don't have enough information to estimate the number of planets in our galaxy, let alone the universe. The inner galaxy is a very different neighborhood compared to our location out in one of the spiral arms. I grant you, there are very likely many, many planets in the universe, but I wouldn't try to put a number on it. Estimates for Drake's equation are deliberately kept very conservative.

 

[Loren Booda]

The large moon, stabilizing it's spin axis, preventing Earth from getting into the chaotic zone

Might such a large moon - having its genesis in the coalescing debris of a planetary impact - be more likely suited to avoid chaos by stabilizing the spin axis of the resultant planet?

 

[Xnn]

You've missed the point...

Consider it 10^20+

 

[mgb_phys]

What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?

Earth has both cyrus and evo - that must be pretty unique!

 

[Andre]

Might such a large moon - having its genesis in the coalescing debris of a planetary impact - be more likely suited to avoid chaos by stabilizing the spin axis of the resultant planet?

Good one. Would every spinning (teristrial?) planet without substantial moon eventually enter the chaotic zone or not? This is caused by -as Laskar proposes- resonance between the obliquity cycle and the precession cycle. The moon speeds up the frequency of the precession cycle, preventing resonance, but what does it do to the obliquity cycle?

A good thesis question?

Second question of course is, how unique is that moon? Regardless of which hypothesis is true for the forming of Earth's moon, how feasible are these mechanims for forming similar planet - moon pairs elsewhere?

 

[Integral]

I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing. Still we simply do not have the knowledge to talk about what is common or uncommon in the universe in general.

 

[D H]

I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing.

Read section 2.2 of the paper that Andre cited. It is Earth's obliquity rather than its orbit that the Moon acts to stabilize. Lasker is claiming that Venus' rather anomalous rotation is due solely to gravitational effects by the other planets. The Moon acts to stabilize the Earth's rotation.

 

[negitron]

I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing

Without a large, stabilizing moon, Mercury, Venus and Mars all experience much larger variations in axial obliquity--Venus has turned itself nearly completely upside down! It's unclear as to whether a relatively stable axis is necessary for life but what is clear is that things here would be very different without our Moon. For one thing, seasonal variations would be far more extreme.

 

[Andre]

I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing.

But are Venus and Mercury anywhere near Earthlike? The only thing that Venus shares with Earth is the order of magnitude of its size

And about the (r)evolution of it's rotation:

Correia A.C.M., Laskar J., de Surgy O.N. (2003). "Long-term evolution of the spin of Venus: I. theory" (PDF). Icarus 163: 1–23
^ Correia A.C.M., Laskar J. (2003). "Long-term evolution of the spin of Venus: II. numerical simulations" (PDF). Icarus 163: 24–45

which elaborates extensively that the lack of rotation of Venus is due to the chaotic zone, resonance between the obliquity cycle and precession cycle, causing excessive tilting of the spin axis, ultimately bringing the rotation of the planet to a halt. It's supposed to be the moon preventing the same fate for Earth.

 

[negitron]

But are Venus and Mercury anywhere near Earthlike? The only thing that Venus shares with Earth is the order of magnitude of its size

No, but that wasn't at all my point.

 

[Andre]

But it is the subject of the thread.

 

[negitron]

No, it's not.

 

[D H]

But it is the subject of the thread.

No, it's not.

That was pithy.

The subject of this thread is the uniqueness of the Earth. Loren asked

What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?

That was not the best way to phrase the question (in fact, I don't quite know what question Loren is asking here). Suppose the Earth does have a near twin somewhere in the universe, right down bearing humaniform intelligent life capable of sending spacecraft outside the planet's gravitational sphere of influence. Something will certainly distinguish our Earth from that near twin, even if it is milligrams in difference in planet mass or centimeters in difference in orbital distance from the parent star. Suppose the Earth has lots of such near twins. One may differ in mass, another will orbit at a slightly different radius, another will orbit a star with slightly different characteristics. There might be a lot of planets out there, but it is a finite number. Our Earth will always differ from its near-twins in some regard.

I don't think that is what Loren was asking for. Asking for one single characteristic was part of the problem. Those who espouse the rare Earth hypothesis claim that the Earth's (relative) uniqueness results not from one characteristic but a whole slew of characteristics.That said, my answer to Loren's original question is that the Earth supports intelligent lifeforms capable of sending spacecraft outside the planet's gravitational sphere of influence.

Is this characteristic unique throughout the universe? I doubt it. Whether it is rare is an entirely different question. If the closest planet to Earth that bears this characteristic is hundreds of thousands of galaxies away, does it really matter that we aren't unique? We are utterly, utterly alone. Does it really matter if the closest intelligent life-bearing planet is a lot closer, say ten galaxies away? Even in that case, we are still utterly alone, even though the Earth is far from "unique".

 

[SW VandeCarr]

That was pithy.

The subject of this thread is the uniqueness of the Earth. Loren asked

The subject here is really whether there are any other planets having characteristics friendly to intelligent life as we know it. Any planet that we are likely to ever discover can be expected to be unique in some way. Key points:

.If the planet is beyond any future limit of human exploration or discovery (whatever that might be), the question is irrelevant. However, "they" might be able to find us, in which case they might be kind enough to tell us about their home planet. Then again, they might not be so kind.

.There might civilizations that have long since disappeared (if and when we find evidence of them) or that have flourished (or are flourishing) in the unseen void where the light has not yet reached us.

.We might well find planets with life, but not intelligent life; which I take to mean having some kind of technology that we would recognize. These (non-intelligent) possibilities exist in our own solar system: on the Jupiter's moon Europa (covered with water ice, but with a vast liquid sea below), or the Jovian atmosphere at a level where liquid water exists at earth-like temperatures and pressures.

.If we do discover a "twin earth" we might be way too early (or too late) to shake hands (or whatever) with an intelligent species. After all, intelligent life (as I defined it) is rare on earth. In the case of a technologically advanced species (which I define as being able to use electromagnetic devices for communication), this has existed on Earth a very short time; since about 1844 compared to the 4.6 billion years that Earth has existed.

Note: I've plagiarized much of this post from my own book. (see my member page if interested).

 

[D H]

The subject here is really whether there are any other planets having characteristics friendly to intelligent life as we know it.

I agree that that is the hidden agenda behind his original post. That is not what Loren asked in the original post. What Loren asked was "What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?"

This is a silly question that verges on being a straw man. Only the most vehement of rare Earth proponents say the Earth is "unlike any other planet in the universe." Does it really matter whether there are hundreds of thousands versus just one Earth-like planets spread around the universe? The end result is the same: We are alone. The only way we are not alone is if there are hundreds of billions of Earth-like planets spread around the universe.

 

[SW VandeCarr]

The end result is the same: We are alone. The only way we are not alone is if there are hundreds of billions of Earth-like planets spread around the universe.

You are probably right in terms of accessibility at least. But you can't assert this as fact. You need to inject just a little bit of uncertainty into your statement.

 

[Loren Booda]

Please allow me to amend my original question. Replace

"What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?"

with the more feasible

"How might Earth's physics be most unlike that of any practically observable planet?"

 

[Andre]

which brings us back to my original answer: the moon

 

[D H]

How might Earth's physics be most unlike that of any practically observable planet?

That's a better question, but you are still eliminating a lot of explanations for why some say the Earth is rare. The word "physics" pretty much eliminates geological, chemical, and biological concerns (assuming you don't consider geology, chemistry, and biology to be branches of physics). The words "Earth's physics" pretty much eliminates attributes of the parent star and perturbing effects by other planets in the star system.

How about "What scientific characteristics might make the emergence of Earth-like intelligent life on a planet rare or common?"


Requiring the planet to be observable eliminates the vast majority of the universe. At present, our ability to observe planets is very limited. While this ability will certainly improve a lot, will it ever improve to the point of being able to observe a planet in Andromeda, Messier 87, or beyond?

The Moon is one possible answer. The most widely accepted hypothesis regarding the Moon is that a Mars-sized object formed in the vicinity of one Earth's triangular Lagrange points. Jupiter pumped energy into the Earth-Theia system, eventually making the two bodies collide. This hypothesis solves several issues surrounding the giant impact hypothesis. A collision with a non co-orbiting planetoid would most likely have imparted too much energy and would have destroyed both the Earth and the body.

A collision with a non co-orbiting planetoid would constitute a freak event. A collision with a body that formed in the vicinity of one of the triangular points is not so far fetched. The Moon might not be quite the unique feature that some think.


A better answer is that the culmination of many different features make the Earth relatively rare. Asking for one feature is akin to asking what one feature most distinguishes Obama (or Einstein, or you) from everyone else.

Among the distinguishing characteristics:

• Location of the star. A star that is too close to the galactic core will have a greater chance of close encounters with other stars, possibly disrupting the orbits of the planets about the star and will have a greater chance of exposure to calamities such as supernovae. Stars far from the galactic core have low metallicity, possibly too low to form rocky planets.
• A single star. Most stars are in binaries or even larger groups.
• A stable star. Some stars exhibit a lot greater variability than does the Sun.
• Planet location. Venus formed too close to the Sun; Mars, too far.
• Gas giants. Our Jupiter stopped its sunward migration. This is not the case everywhere; many stars have hot Jupiters. Jupiter helped clear out the junk from the Solar system and may have helped the Earth form.
• Size. Too small and the planet will cool off too quickly. There are problems with being too big as well.
• A large Moon. This has been covered already.
• Water. The Earth's surface is 2/3 water. Could an otherwise Earth-like world that is 100% covered with water have spawned intelligent life capable of moving off of the planet? Could an otherwise Earth-like world that barely has any water have developed life?
• Life and complex life. We do not know whether life is a fluke or an inevitability. We do know that life was very simple for a long, long time. Even if simple life is a highly probable event, complex life might be not be.
• Metals. Intelligent life might arise on a planet that has a very limited amount of metal. Could it proceed beyond stone age capabilities?
• Fuel. Our fossil fuels arose when life went bonkers. A planet on which life just gets by may not have produced the fuels needed to sustain a modern civilization.

Whether these are truly applicable and make the Earth rare, we don't really know. We might have a better idea after the results from Kepler start coming in. Kepler is only going to answer a small number of these questions.

 

[Xnn]

Depends on how one describes Earth in the first place. Consider the following:

A planet within 10% of Earth's mass, a primarily nitrogen atmosphere with oxygen between 15 - 25%, mostly covered with water, average temperature between 10 to 20C.

By that definition, Earth is probably 1 in a 10 billion. Pretty rare, but in the visible universe, this would mean there would be billions of other planets that fit the same definition. However, the nearest similar planet would probably be at least a 1000 light years away. Likely too far for any type of communication as we know it due to energy and time requirements.

Of course, any such Earth like planet that does exist would likely have slightly different coastlines, different plants and animals. The intelligent creatures (if they were still alive) would not look exactly like ourselves, and would surely have a different set of languages and history. They would also have different names and lives than each of us.

However, consider that the visible Universe is not the only Universe in existence. Instead, it is just a single Universe of the greater Multiverse.

http://en.wikipedia.org/wiki/Multiverse

In the Multiverse, numerical constraints are no longer and it become possible for parallel universes and of course parallel planet Earth's. On the parallel Earth everything is the same. It’s inhabited by intelligent creatures just like ourselves. The coastline and cities and countries are just as we know them. The speak the same language, have the same names and history and are doing exactly what we are doing right now.

 

[D H]

By that definition, Earth is probably 1 in a 10 billion.

What does this number mean? That one out of every 10 billion stars has an Earth-like planet?

Where did you pull this number from? By all appearances, you just made it up. Different people have twiddled the multiple variations of Drake's equation, and have come up with answers that vary everywhere from Earth being very common (thousands, maybe millions of Earth-like planets in our galaxy) to rare to extremely rare. You stated this number definitively, as if it is a given. That is far from the case. The true answer is that we don't even have the foggiest idea of whether Earth-like planets are very common or extremely rare.

However, consider that the visible Universe is not the only Universe in existence. Instead, it is just a single Universe of the greater Multiverse.

You are once again stating highly hypothetical things as if they are fact. This is not the site for doing that.

 

[Richard111]

Is it not the SIZE of the moon in relation to the Earth that is unique?

 

[D H]

In our solar system, yes. Elsewhere, who knows?

Extrapolation is often a bad idea. Even with physical laws that have been extremely well validated over some domain, extrapolating outside that domain can be a bad idea. Example: Newtonian mechanics does not work very well when extrapolated to the realm of the very small, very large, or very fast.

Extrapolating from a sample size of one with only some rather crude ideas as to why things are the way they are is downright stupid.

We can hypothesize about features that might make the Earth common or rare. However, until we develop much better understanding of the planet formation process and much observational techniques, those will remain hypotheses only -- and hypotheses that are in essence extrapolations from a sample size of one.

 

[Andre]

In short the discussion is irrelevant, we can only compare Earth with observable planets. Beyond that is speculation. That would be fine if it could be tested and falsified. It can't. Hence the question withdraws itselfs from the scientific method.

But the dynamic role of the gravity of considerable moon on the Earth physics is worth mentioning.

 

[Richard111]

But the dynamic role of the gravity of considerable moon on the Earth physics is worth mentioning.

Yes. Were tides essential to the development and diversity of life we see in the fossil record?

 

[Andre]

The issue was not so much the tides as the spin axis stabilisation

 

[Richard111]

True. Without a moon of sufficient size there would be no stabilisation.
Without water in the liquid phase no tides and maybe no life.
All seem unique to me but as mentioned above how do you confirm this planet is unique in the observable universe?

 

[D H]

No tides? No. There will still be solar tides. With regard to uniqueness, we do not know. At present, the only confirmation regarding the uniqueness (or lack thereof) of the Earth is a sample size of one: Our own solar system. Assessing this hypothesis with respect to a tiny chunk of our galaxy but outside our solar system might be possible in the near future. Assessing it with respect to the observable universe at large? Never.

 

[Richard111]

Ooops, yes. Solar tides. But would that have been sufficient to maintain spin stabilisation? Would the limited range of solar tides have encouraged life to adapt to less exposed land surfaces?

The geological time required for life, from first appearance in water, to adapt to dry(ish) land was far longer than the time to present.

Without the moon, life as we know it may not have been possible.

There is geological and paleontological evidence that the Earth rotated faster and that the Moon was closer to the Earth in the remote past.

http://en.wikipedia.org/wiki/Tidal_acceleration

 

[Ophiolite]

Plate tectonics. The stabilisation of atmospheric composition, the presence of terrestrial environments in addition to the marine, the maintenance of mean temperatures in the stability range of liquid water, all of these would not be possible without active plate tectonics. This in turn is contingent upon stochastic processes in the accretionary disc from which the Earth formed, in as much as this determined crustal thickness and established whether plate tectonics would be a long term affair as on Earth, a short term affair as on Mars, or a periodic catastrophic affair as on Venus.

 

[Andre]

Plate tectonics. ... or a periodic catastrophic affair as on Venus.

What evidence is there to substantiate that the apparent catastrophic affair on Venus was periodic, rather than unique, one time only?

 

[Ophiolite]

There are two principal options:
1. The resurfacing event that occurred around 600 million years ago and the stagnant lid period that preceded it, were themselves preceded by 'conventional' plate tectonics.
2. The resurfacing event etc, was the most recent in a series of such events.

You have to dispose of the internal heat. These are the two obvious mechanisms. I have a gut feel - unquantified - that continuous conventional plate tectonics should have maintained water content on the planet, so I lean to the possibility of periodic resurfacing. The evidence is consisten with either.

 

[D H]

Ooops, yes. Solar tides. But would that have been sufficient to maintain spin stabilisation? Would the limited range of solar tides have encouraged life to adapt to less exposed land surfaces?

AFAIK, the tides are not the mechanism by which the Moon aids in keeping the Earth's rotation stable. Tides are dissipative forces. The same gravity gradient forces that cause the tides also induce a conservative torque on the Earth as a whole. This torque, averaged out over the Moon's 18.6 year nodical period, results in the 26,000 year lunisolar precession.

Without the moon, life as we know it may not have been possible.

Emphasis mine. This is purely conjectural.

There is geological and paleontological evidence that the Earth rotated faster and that the Moon was closer to the Earth in the remote past.

I am not debating that the Moon slows the Earth's rotation rate. That is a fact. What is debatable is whether the Earth's day would be eight hours long if the Moon never existed. If indeed the Moon formed from a collision between the Earth and Theia, we do not know how much the collision itself changed the Earth's rotation rate.

 

[Andre]

There are two principal options:
1. The resurfacing event that occurred around 600 million years ago and the stagnant lid period that preceded it, were themselves preceded by 'conventional' plate tectonics.
2. The resurfacing event etc, was the most recent in a series of such events.

You have to dispose of the internal heat. These are the two obvious mechanisms. I have a gut feel - unquantified - that continuous conventional plate tectonics should have maintained water content on the planet, so I lean to the possibility of periodic resurfacing. The evidence is consisten with either.

but these are still suppositions following assumptions about heat and heat production in the core, which are also hypotheses. How many times did Venus lose its spinning energy for instance (due to the chaotic zone)? and how about possible relationships there?

 

[Ophiolite]

but these are still suppositions following assumptions about heat and heat production in the core, which are also hypotheses.

There are no assumptions here, only reasonable deductions leading to plausible hypotheses.
You will note in an earlier post I talk of two principal explanations. I do not rule out other possibilities, but simply find these two are the most probable based upon our current understanding.

Since I also think Venus suffered one or more catastrophic collisions akin to what formed the moon, or stripped mantle from Mercury, then issues of angular momentum cease to be insoluble problems.