Is our solar system typical?

[Puppycow]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.

I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

[mikeyx]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.

I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

as opposed to what? Until we map out larger portions of the universe, the answer would be conjecture. Short answer, after clarification, we need more information than we have at the present time.

[Doubt]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.


Although quite a few Astronomers have spent time telling us that our sun is average, the bad astronomer pointed out in his last book that our sun is really larger than most. So we are not atypical but not really average either thanks to solar gravity.


I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

IIRC, the Kepler mission finds planets by looking for the dimming that takes place when a transit occurs. But that also means it is much more likely to find a gas giant with a fast orbit rather than something small and rocky like Earth or Venus.

[Puppycow]

Although quite a few Astronomers have spent time telling us that our sun is average, the bad astronomer pointed out in his last book that our sun is really larger than most. So we are not atypical but not really average either thanks to solar gravity.



IIRC, the Kepler mission finds planets by looking for the dimming that takes place when a transit occurs. But that also means it is much more likely to find a gas giant with a fast orbit rather than something small and rocky like Earth or Venus.

That bias is really what i'm trying to get at. The planets we find are biased by the methods we use to find them. The methods are much more likely to find a so called hot jupiter than a Venus, Earth or Mars.

[ginjawarrior]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.

I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

i watched this talk about keplers discoveries the other day
27.40mins to 39mins he talks about how many planets they found very cool
jLNO5nXZoUI

[GreyArea]

I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.
So far we have discovered about 200 candidate planetary systems using the transit method. And it's still early days yet.

Meanwhile, there are other methods of detecting planets. Results for the different methods can be found at the Extrasolar Planets Encyclopedia. This index page (http://exoplanet.eu/catalog.php) lists discoveries by method, including transits.


Although quite a few Astronomers have spent time telling us that our sun is average, the bad astronomer pointed out in his last book that our sun is really larger than most. So we are not atypical but not really average either thanks to solar gravity.
Earlier this year, Phil Plait wrote (http://blogs.discovermagazine.com/badastronomy/2012/01/12/exoplanet-news-part-1-i-shall-call-it-mini-solar-system/) about some small exoplanets discovered around a red dwarf. He wrote in that post:
[This] shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy.

Here (http://blogs.discovermagazine.com/80beats/2012/01/12/in-the-milky-way-there-are-as-many-planets-as-stars/) is a blog post with further links about just how common planets appear to be in our galaxy. There could be an average of one planet per star, and possibly an average of five per star. That means hundreds of billions of planets here, and possibly trillions.


IIRC, the Kepler mission finds planets by looking for the dimming that takes place when a transit occurs. But that also means it is much more likely to find a gas giant with a fast orbit rather than something small and rocky like Earth or Venus.
A smaller orbital period is also going to be noticed sooner. As Kepler is in space longer, it gathers longer runs of data on its target stars. We've had to wait to detect candidate planets that have orbits that take as long as Earth's. That's because we need to see the first two dimmings and then a third at the right time. Kepler started its search in May 2009, so those kinds of results should be coming in between now and 2013.

It's all very exciting.

[Toontown]

I think it is reasonable to assume that each solar system evolves according to it's peculiar initial conditions, which will vary widely based on small differences in the initial gas cloud. Thus, even if a sun similar ours is formed, the planetary system is likely to be quite different.

There are indications that many solar systems might feature one or two very large gas planets which have vacuumed up all the smaller planets which were initially formed. However, such gas giants are very likely to have multiple moons, and those moons might well be habitable to life of some kind.

[I Ratant]

There's no reason to expect anything significantly different... masses orbiting a primary in a more or less orderly manner.
It's all gravity.
Depends on what is at those other systems in terms of size, as to their detectability.

[Corsair 115]

Here (http://blogs.discovermagazine.com/80beats/2012/01/12/in-the-milky-way-there-are-as-many-planets-as-stars/) is a blog post with further links about just how common planets appear to be in our galaxy. There could be an average of one planet per star, and possibly an average of five per star. That means hundreds of billions of planets here, and possibly trillions.


Wait, I thought most stars in the galaxy were part of multiple star systems, and that planetary formation around binary (or larger) star systems was thought to be difficult due to the gravitational influences and complications involved with multiple suns.

[Toontown]

Some gas giants have been detected which are surprisingly close to their suns. I wonder what Jupiter's ice moon Europa would be like if Jupiter occupied an earthlike orbit? I'm assuming mighty Jupiter would make a great bodyguard for a little water moon like Europa.

[rjh01]

One thing that puzzles me about our solar system is that there are no planets within the orbit of Mercury. Many of the planets found have an orbit that is closer to the sun that Mercury is.

[Corsair 115]

One thing that puzzles me about our solar system is that there are no planets within the orbit of Mercury. Many of the planets found have an orbit that is closer to the sun that Mercury is.


My understanding is that is thought to be due to gravitational perturbations between the planets of the system eventually causing some planets to either be ejected from the system or pushed into the star, and others (the so-called "hot Jupiters") to end up in a very close orbit around the star. Our solar system apparently has a stable assortment of gas giant planetary orbits and such gravitational disruptions haven't happened here.

[rjh01]

My understanding is that is thought to be due to gravitational perturbations between the planets of the system eventually causing some planets to either be ejected from the system or pushed into the star, and others (the so-called "hot Jupiters") to end up in a very close orbit around the star. Our solar system apparently has a stable assortment of gas giant planetary orbits and such gravitational disruptions haven't happened here.

That means that there are at least two possible evolutions of solar systems. I wonder if there are any differences in the stars themselves between the two systems?

[WhatRoughBeast]

One thing that puzzles me about our solar system is that there are no planets within the orbit of Mercury. Many of the planets found have an orbit that is closer to the sun that Mercury is.

Yes, but.....

A very large, very close planet produces a (relatively) large doppler shift in the star's output, and these are (relatively) easy tp detect. So, using spectroscopic data for searching is most likely to find exactly these systems.

Early days, yet.

[quarky]

The last time I looked at our solar system, I found myself saying "That's so typical".
Now I hope I didn't hurt its feelings.

Maybe our solar system doesn't have feelings.



(Typical.)

[dasmiller]

The last time I looked at our solar system, I found myself saying "That's so typical".
Now I hope I didn't hurt its feelings.

Our solar system should take some solace from the fact that there's not another one like it within a trillion miles. Literally, a tillion miles! (technically many trillion miles)


On the other hand, our solar system might be understandably miffed that we don't actually have a name for it. Our planet is "Earth," our galaxy is "The Milky Way," but our solar system is just "our solar system."

I mean, if we really cared, wouldn't we at least give it a name?

[WhatRoughBeast]

Oh, come on.

It's the Sol system. I mean, it alliterates and everything.

[geni]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.

There is no particular reason to assume that. In fact our current theories of planetary formation have a really hard time explaining the outer gas giants. Oh and please define similar in this context.


I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

It doesn't really matter. We can look at a lot of stars:

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

[Corsair 115]

That means that there are at least two possible evolutions of solar systems. I wonder if there are any differences in the stars themselves between the two systems?


My understanding is that it has to do with the distribution of the type of and relative masses of the planets in a solar system. Certain combinations are simply not gravitationally stable over the lifetime of the system. For example, having three Jupiter-like gas giants is apparently quite bad (based on sophisticated computer modelling of the gravitational interactions). Those three big planets' gravity perturb each other's orbits, and eventually this results in the entire system become orbitally chaotic.

Our system appears fortunate in that it has one big gas giant (Jupiter, 318 Earth masses), one moderately-sized one (Saturn, 95 Earth masses), and two small ones (Uranus and Neptune, 14 and 17 Earth masses, respectively). The result appears to be a solar system which remains pretty much orbitally stable over its lifetime.

[JoeBentley]

Right now our understanding of extrasolar solar systems is sketchy at best and our understanding of our own solar system is still sorta half hazard. And trying to extrapolate information from a data set of one where your only about 80% of the value of the one piece of data you have and the only think you know about the other data sets is that they exist is... well not likely to be fruitful.

[quarky]

Our solar system should take some solace from the fact that there's not another one like it within a trillion miles. Literally, a tillion miles! (technically many trillion miles)


On the other hand, our solar system might be understandably miffed that we don't actually have a name for it. Our planet is "Earth," our galaxy is "The Milky Way," but our solar system is just "our solar system."

I mean, if we really cared, wouldn't we at least give it a name?

Sounds worthy of a poll.


For that matter, what is the moon's name? The Moon? Saturn's moons get names.

[Pope130]

Sounds worthy of a poll.


For that matter, what is the moon's name? The Moon? Saturn's moons get names.

Our sun's name is "Sol". Our solar system (no capital) is the Sol System (capitalised).

Our moon is "Luna" (although she sometimes uses the name "Selene").

[I Ratant]

Sounds worthy of a poll.


For that matter, what is the moon's name? The Moon? Saturn's moons get names.
Selene, for one.

[quarky]

Our sun's name is "Sol". Our solar system (no capital) is the Sol System (capitalised).

Our moon is "Luna" (although she sometimes uses the name "Selene").

Sure, I guess. But when there's about to be an eclipse of "Luna", its never really called by that name. Whereas, Europa is.

Shucks, even some of the asteroids have cool names.

I'd like the sun to be called "Sunny", and the moon called "Judy" and the solar system be called "Big Al".

I hope everyone's ok with that, and sorry for the topic drift.

[Puppycow]

Exoplanets!

http://xkcd.com/1071/

[Agammamon]

It seems reasonable to me to assume that it is typical.

Not necessarily that life is typical, but that stars similar to our sun also have planetary systems that are similar to ours.

I was thinking that if a transit of Venus is so rare even though we are basically in the same plane as Venus, how rare it must be to see exoplanets transit in front of other stars.

Its completely unreasonable to assume that our solar system is typical. Mainly because we've seen a ton of other solar systems and they don't look anything like ours.

We see lots of exo-planets transiting in front of stars - that's pretty much the only way to detect them right now and is the main reason that all the other systems we've detected tend to have huge jupiter plus sized planets in inner solar system orbits (orbits that in ours sytem would be inside the orbit of Jupiter).

As resolution gets better what's "typical" may change, but right now the standard solar system is a couple of really large gas giants in close orbits around the primary.

[ginjawarrior]

Its completely unreasonable to assume that our solar system is typical. Mainly because we've seen a ton of other solar systems and they don't look anything like ours.

We see lots of exo-planets transiting in front of stars - that's pretty much the only way to detect them right now and is the main reason that all the other systems we've detected tend to have huge jupiter plus sized planets in inner solar system orbits (orbits that in ours sytem would be inside the orbit of Jupiter).

As resolution gets better what's "typical" may change, but right now the standard solar system is a couple of really large gas giants in close orbits around the primary.

kepler is finding that earth sized planets are much more common than jupiter sized planets
while youtube isnt considered proper evidence here the video from post 5 explains it better than i could and its a talk from someone from kepler
27.40mins to 39mins important part
"most are nearly earth size"
"galaxy is teeming with planets"
"170+ stars with multiple planets"
"smaller stars more likely to have smaller earth sized planets"

[Puppycow]

Its completely unreasonable to assume that our solar system is typical. Mainly because we've seen a ton of other solar systems and they don't look anything like ours.

We see lots of exo-planets transiting in front of stars - that's pretty much the only way to detect them right now and is the main reason that all the other systems we've detected tend to have huge jupiter plus sized planets in inner solar system orbits (orbits that in ours sytem would be inside the orbit of Jupiter).

As resolution gets better what's "typical" may change, but right now the standard solar system is a couple of really large gas giants in close orbits around the primary.

You have to use common sense though. We know that there is a very strong bias to detecting hot jupiters because of the limitations of our methods. It is illogical to assume that the exoplanets we have detected are a representative sample.

[Puppycow]

kepler is finding that earth sized planets are much more common than jupiter sized planets
while youtube isnt considered proper evidence here the video from post 5 explains it better than i could and its a talk from someone from kepler
27.40mins to 39mins important part
"most are nearly earth size"
"galaxy is teeming with planets"
"170+ stars with multiple planets"
"smaller stars more likely to have smaller earth sized planets"

Thanks for linking that video, btw. I'm watching it right now. Didn't get around to it last time.

[Beerina]

There's no reason to expect anything significantly different... masses orbiting a primary in a more or less orderly manner.
It's all gravity.
Depends on what is at those other systems in terms of size, as to their detectability.

Well, "We happened. It is statistically unlikely we are an oddity rather than common." itself is statistically questionable.

[Corsair 115]

kepler is finding that earth sized planets are much more common than jupiter sized planets ...


My first thought there is to ask for a precise definition of what is meant by "Earth-sized".

[Trakar]

It seems reasonable to me to assume that it is typical...

What leads you to believe this?


Okay, the reasoned argument goes something like - we should assume that our star is one of around 100 Billion stars in our galaxy out of an estimated 100 Billion galaxies in the universe; and the assumption should be that closer to the norm for that group of objects. That would seem reasonable, if all stars were the same. Looking out at the universe in a bit more detail, however, it seems that, depending upon how much detail we include in our inspection, that our star is fairly distinct. Not that I would qualify it as "unique," but not "typical" either.

[Trakar]

Sounds worthy of a poll.


For that matter, what is the moon's name? The Moon? Saturn's moons get names.

Technically, Saturn doesn't have a Moon, merely natural satellites. Moon is a proper noun that has been colloquially adopted to speak of the "Moon-like" natural satellites of other planets.

Saturnians might similarly speak of Mar's titans. (If there were Saturnians given to similar conventions - ;))

[Trakar]

My first thought there is to ask for a precise definition of what is meant by "Earth-sized".

10Me stripped cores of migrated Neptunes around red dwarf flare stars,...apparently.

[Agammamon]

You have to use common sense though. We know that there is a very strong bias to detecting hot jupiters because of the limitations of our methods. It is illogical to assume that the exoplanets we have detected are a representative sample.


I know there's a bias - that was the point of my saying that they are typical. Typical is based on what you see most often and that's what we see most often.

Except now it looks like detection is getting good enough to start seeing large numbers of smaller planets which is shifting the typical to that end.

[Skeptic Ginger]

Binary stars the size of our Sun are more common than singles so in that respect our solar system is not typical. They have more recently discovered that there are more single stars but this was only discovered when we started detecting more of the smaller cooler stars.

Astronomers Had it Wrong: Most Stars are Single (http://www.space.com/1995-astronomers-wrong-stars-single.html)Stellar surveys found that more than half of all Sun-like stars were part of multiple systems. For more massive stars, like O- and B-type stars, the number was estimated to be as high as 80 percent...

...The catch, however, is that most stars in the Milky Way are not bright stars like our Sun, but dim, low-mass stars called red dwarfs.

[Agammamon]

Something to keep in mind - when astronomers say "earthlike" or talk about multiple stars in a system, what they mean can be vastly different than what we as layman visualize based on our usage of those terms.

"Earthlike" can be the burned out hunk of a gas giant that's had its atmosphere boiled off because it orbits its primary closer than Mercury.

And Alpha Centauri is a trinary system but one of the stars orbits .2 ly from the other two.

[Puppycow]

I know there's a bias - that was the point of my saying that they are typical. Typical is based on what you see most often and that's what we see most often.

Except now it looks like detection is getting good enough to start seeing large numbers of smaller planets which is shifting the typical to that end.

Well that is not what I meant by 'typical.'

I mean typical of what is actually out there, not just what we can see.

[Mark6]

Well that is not what I meant by 'typical.'

I mean typical of what is actually out there, not just what we can see.
Problem is, there is no evidence for it yet.

Worse, the known planetary systems without "hot Jupiters" -- which are a minority of known systems, but a large minority, -- are not similar to our solar system either. Their planets tend to have significant eccentricities, unlike "our" planets. So my answer to OP is: No, it is not reasonable to assume not our solar system typical. If anything, it is wishful thinking.

[Mark6]

Let me phrase it a bit more strongly: The whole notion of Copernican Principle -- we live on a typical planet of a typical star in a typical galaxy, and the way things are here is the way things are (mostly) out there, -- is completely unsubstatiated wishful thinking. In fact, the middle part of the above statement is blatantly untrue despite having been repeated in countless textbooks -- the Sun is bigger and brighter than at least 96% of the stars in the Galaxy; it is most certainly not a "typical star". (Typical star is an M-class dwarf.)

Either life/multicellular life/intelligence require highly unusual conditions, or they do not. If the latter is the case, we will see many solar systems similar to ours, and Copernican Principle will be vindicated. If the former is the case, we will not see many systems similar to ours -- the fact that we are here only proves life-bearing conditions are possible, not that they are common.

In the Middle East occures a bizarre terrain called sabka. It is essentially a tar pit, covered by compressed sand. The sand crust is strong enough for people and camels to walk on, but collapses under trucks or tanks, miring them in tar like sabertooth tigers in La Brea tar pit. Sabka occures nowhere else on Earth. Some insect living on/in sabka could apply Copernican Principle to say "We are not special. Everywhere should be like it is here". And would be utterly wrong.

Does it mean laws of physics -- or even laws of geology, -- are different in Middle East than elsewhere on Earth? Of course not -- it just means that for sabka to exist, a set of rather unlikely conditions (each of them individually entirely within the realm of these laws) must come together. OTOH, sand in various forms exists just about everywhere. Wether our planetary system is akin to sand or akin to sabka is yet to be determined. And so far (weak) evidence points to sabka.

[Corsair 115]

... the Sun is bigger and brighter than at least 96% of the stars in the Galaxy; it is most certainly not a "typical star". (Typical star is an M-class dwarf.)


My understanding it's also unusual in that it contains a higher proportion of 'metals' (elements heavier than helium) than comparable stars found in our local neighbourhood.

[Senex]

We are all remnants of a supernova.

Typical or not -- it doesn't suck to be alive.

It's amazing and statistically improbable.

[MG1962]

My understanding it's also unusual in that it contains a higher proportion of 'metals' (elements heavier than helium) than comparable stars found in our local neighbourhood.

The thing to remember, the stars we are currently traveling with are not the stars the sun started life with back in the day. In terms of the metal our star contains, it is not that unusual for the time frame the Sun formed, stars with a lot of metal are known as population I stars.

[MG1962]

Let me phrase it a bit more strongly: The whole notion of Copernican Principle -- we live on a typical planet of a typical star in a typical galaxy, and the way things are here is the way things are (mostly) out there, -- is completely unsubstatiated wishful thinking. In fact, the middle part of the above statement is blatantly untrue despite having been repeated in countless textbooks -- the Sun is bigger and brighter than at least 96% of the stars in the Galaxy; it is most certainly not a "typical star". (Typical star is an M-class dwarf.)

But you leave out the bit about formation rates. The galaxy is populated with smaller dwarf class stars simply because of their longevity.

[Mark6]

So what?

[Puppycow]

Let me phrase it a bit more strongly: The whole notion of Copernican Principle -- we live on a typical planet of a typical star in a typical galaxy, and the way things are here is the way things are (mostly) out there, -- is completely unsubstatiated wishful thinking. In fact, the middle part of the above statement is blatantly untrue despite having been repeated in countless textbooks -- the Sun is bigger and brighter than at least 96% of the stars in the Galaxy; it is most certainly not a "typical star". (Typical star is an M-class dwarf.)

Either life/multicellular life/intelligence require highly unusual conditions, or they do not. If the latter is the case, we will see many solar systems similar to ours, and Copernican Principle will be vindicated. If the former is the case, we will not see many systems similar to ours -- the fact that we are here only proves life-bearing conditions are possible, not that they are common.

In the Middle East occures a bizarre terrain called sabka. It is essentially a tar pit, covered by compressed sand. The sand crust is strong enough for people and camels to walk on, but collapses under trucks or tanks, miring them in tar like sabertooth tigers in La Brea tar pit. Sabka occures nowhere else on Earth. Some insect living on/in sabka could apply Copernican Principle to say "We are not special. Everywhere should be like it is here". And would be utterly wrong.

Does it mean laws of physics -- or even laws of geology, -- are different in Middle East than elsewhere on Earth? Of course not -- it just means that for sabka to exist, a set of rather unlikely conditions (each of them individually entirely within the realm of these laws) must come together. OTOH, sand in various forms exists just about everywhere. Wether our planetary system is akin to sand or akin to sabka is yet to be determined. And so far (weak) evidence points to sabka.

Interesting post. Thanks.

[Craig B]

That means that there are at least two possible evolutions of solar systems. I wonder if there are any differences in the stars themselves between the two systems? It has been argued that while a gas giant in the outer part of a solar system protects the inner planets from colliding comets - we have seen Jupiter performing this service - a star-hugging "hot Jupiter" would be useless, or worse. So perhaps life, or at any rate complex life, appears only in one of these kinds of systems.

[Craig B]

Let me phrase it a bit more strongly: The whole notion of Copernican Principle -- we live on a typical planet of a typical star in a typical galaxy, and the way things are here is the way things are (mostly) out there, -- is completely unsubstatiated wishful thinking. In fact, the middle part of the above statement is blatantly untrue despite having been repeated in countless textbooks -- the Sun is bigger and brighter than at least 96% of the stars in the Galaxy; it is most certainly not a "typical star". (Typical star is an M-class dwarf.) The school of thought in opposition to which the Copernican Principle was formulated asserted not merely that the earth was not an average place, but that it was a singular and transcendentally special place, with a divinely-ordained central position in space and time. In relation to that the CP is powerfully substantiated. The sun is not an average star in mass, but it belongs to a reasonably normal category of stars.

This is not to say that there are lots of things like us out there - nobody has the slightest idea if there are or not - merely that there may be other things comparable with us somewhere else, and that we are not central, or favoured, however rare we may be.

[Corsair 115]

It has been argued that while a gas giant in the outer part of a solar system protects the inner planets from colliding comets - we have seen Jupiter performing this service - a star-hugging "hot Jupiter" would be useless, or worse..


There is also the problem that, under the current understanding of planetary formation, a Jupiter-like gas giant can't form close to its star. So the theory is that such gas giants formed further out and then gravitation interactions with other planets, especially other gas giants, eventually resulted in the planet ending up close to the star.

[Craig B]

There is also the problem that, under the current understanding of planetary formation, a Jupiter-like gas giant can't form close to its star. So the theory is that such gas giants formed further out and then gravitation interactions with other planets, especially other gas giants, eventually resulted in the planet ending up close to the star. Which would presumably destroy any rocky inner planets, or eject them from that stellar system!

[Mark6]

The school of thought in opposition to which the Copernican Principle was formulated asserted not merely that the earth was not an average place, but that it was a singular and transcendentally special place, with a divinely-ordained central position in space and time. In relation to that the CP is powerfully substantiated. The sun is not an average star in mass, but it belongs to a reasonably normal category of stars.
True, and in a way Copernican Principle was a natural reaction -- especially back when there was no way to tell if it went overboard. However nowadays a) there is evidence it went overboard, and b) there is an example of very similar thinking which turned out to be quite wrong. I am talking about theory of "gradualism" in paleontology. Gradualism was a reaction to/denial of Cuvier's "catastrophism": Cuvier did not know about evolution, thought all species existed unchanged since Creation, and that every extinct species died out when some catastrophe (volcano, flood, whatever) encompassed that species' entire range.

When paleontogists realized that species do change with time, they rejected Cuvier completely. Through first half of 20th century it was a matter of faith that ALL extinctions occur gradually, over geological time. And when Alvarez realized that asteroid impact did in the dinosaurs, paleontologists resisted the idea very strongly. You could say he went against their religion -- the anti-Cuvier religion. Guess what -- Cuvier was not 100% wrong. Catastrophes can cause extinctions. Assumption of gradualism went overboard.

[Mark6]

Which would presumably destroy any rocky inner planets, or eject them from that stellar system!
Unless they formed after the gas giants migrated.

[Craig B]

True, and in a way Copernican Principle was a natural reaction -- especially back when there was no way to tell if it went overboard. However nowadays a) there is evidence it went overboard, and b) there is an example of very similar thinking which turned out to be quite wrong. I am talking about theory of "gradualism" in paleontology. Gradualism was a reaction to/denial of Cuvier's "catastrophism": Cuvier did not know about evolution, thought all species existed unchanged since Creation, and that every extinct species died out when some catastrophe (volcano, flood, whatever) encompassed that species' entire range.

When paleontogists realized that species do change with time, they rejected Cuvier completely. Through first half of 20th century it was a matter of faith that ALL extinctions occur gradually, over geological time. And when Alvarez realized that asteroid impact did in the dinosaurs, paleontologists resisted the idea very strongly. You could say he went against their religion -- the anti-Cuvier religion. Guess what -- Cuvier was not 100% wrong. Catastrophes can cause extinctions. Assumption of gradualism went overboard. I was thinking of exactly that as an analogy of your CP example! Gradualists did overdo their opposition to the idea of catastrophes as the cause of extinctions or other significant events. And they did so, not because such individual catastrophes contradict gradualist evolution - for most certainly they don't - but because they were over sensitive to any idea that savoured of the "catastrophic principle" to which they rightly counterposed evolution. But essentially the gradualists were right and the catastrophists wrong.

Example. Traditional Christianity holds that languages were created all at once at the Tower of Babel, catastrophically. Modern scholars believe that languages evolve over time. But of course a language can disappear very quickly indeed if its speakers are annihilated by war or epidemic, or overwhelmed by bearers of another culture. This catastrophic possibility is in no way to be taken as evidence for the truth of the Babel story, or other similar myths.

[Trakar]

Unless they formed after the gas giants migrated.

The only problem with this is that it really alters the dynamics of planetary formation; I'm not sure you could get (structurally or compositionally) a Solar type inner planetary system if you have an early formed gas giant that migrates through the inner system.