Can we expect all stars...

[Btodd]

...to have multiple planets orbiting them?

Recently, while watching The Universe on the History Channel*, an expert stated that there were more stars in the known universe than all of the individual grains of sand, on every beach, on the entire earth.

I just ran across a figure of 30 billion trillion stars in the known universe.

Since our sun has multiple planets in orbit around it, is there reason to believe that all stars can be expected to have planets orbiting them? I'm trying to determine if it's safe to assume that there are more planets in the universe than 30 billion trillion.


*I realize that being on the History Channel doesn't mean it's correct, so any corrections are welcome.

[godless dave]

We don't know enough about planet formation, and haven't imaged enough stars, to be able to answer that question.

[Third Eye Open]

We don't know enough about planet formation, and haven't imaged enough stars, to be able to answer that question.

But we have detected planets orbiting other stars, a couple hundred IIRC. So it is safe to say that there are a LOT of planets out there.

[Dr. Trintignant]

But we have detected planets orbiting other stars, a couple hundred IIRC. So it is safe to say that there are a LOT of planets out there.

Another thing is observation bias--we've only found the big planets so far, because that's all we can see. I believe the smallest extrasolar planet we've found is around 4x Earth-size, and the majority are larger than Jupiter size. Although we can't say for sure until we have more sensitive instruments, there's every reason to believe that we'll see a lot more planets when we can see down to, say, Mercury-size.

- Dr. Trintignant

[JoeTheJuggler]

We should get some more information before too long.

http://kepler.nasa.gov/

[MG1962]

Probably not, though as others have said, there will be a lot. If a star forms close to an A or O class star, it will have its disk of material blown away

Also only the later generation of stars can have planets (Population I stars) These need to wait for the early (Population III and II stars) to cycle through and produce the elements needed for planetary formation

Here is a good article on the subject

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

[sol invictus]

I'm trying to determine if it's safe to assume that there are more planets in the universe than 30 billion trillion.

I think that's a safe assumption.

First off, these estimates of the total number of stars refer only to the part of the universe we can see. The whole universe is all but certain to be bigger than that (if it wasn't we would notice finite size effects, and we don't).

Secondly, folding in the effects of observation bias (larger and closer planets are easier to see) it seems that many, if not most, solar systems have multiple planets. So I think it's fairly safe to assume there are at least as many planets as stars.

[Beerina]

If a grain of sand is 1/4 mm in length, then I calculate 40,000,000,000,000,000,000,000 stars (400 billion galaxies x 100 billion stars per), which would yield 625 cubic km of sand.

There's gotta be a hell of a lot more sand than that on Earth. And even if not, it's in the same ballpark within a magnitude or so.

[Btodd]

If a grain of sand is 1/4 mm in length, then I calculate 40,000,000,000,000,000,000,000 stars (400 billion galaxies x 100 billion stars per), which would yield 625 cubic km of sand.

There's gotta be a hell of a lot more sand than that on Earth. And even if not, it's in the same ballpark within a magnitude or so.


Thank you...I would love to see some more people weigh in on this, because even though I know the universe is enormous, there's still that little voice in my head that says, 'Wait a minute.....'.:)

[MG1962]

If a grain of sand is 1/4 mm in length, then I calculate 40,000,000,000,000,000,000,000 stars (400 billion galaxies x 100 billion stars per), which would yield 625 cubic km of sand.

There's gotta be a hell of a lot more sand than that on Earth. And even if not, it's in the same ballpark within a magnitude or so.

On the other hand 625 cubic km of sand spread only a metre thick would cover something like half the total land area of the earh

[Undesired Walrus]

First off, these estimates of the total number of stars refer only to the part of the universe we can see. The whole universe is all but certain to be bigger than that (if it wasn't we would notice finite size effects, and we don't).

And therefore far older? That's cool. Yet a Universe that turned out to be 800 Billion years older than it is currently thought to be would surely severely screw up many of calculations Scientists are now making about certain things in the Universe wouldn't it? I'm sure they must use the current 13.7 Billion year timeframe as a fundamental part of many important calculations?

(What's a finite size effect?)

As for the OP, it appears that most of the giant gas giants discovered in distant systems orbit extremely close to their Sun, so close that they are in a tidal lock. That's another thing that is cool, and weird.

[wollery]

We don't know enough about planet formation, and haven't imaged enough stars, to be able to answer that question.That's true, but we can make some good educated guesses.

Another thing is observation bias--we've only found the big planets so far, because that's all we can see. I believe the smallest extrasolar planet we've found is around 4x Earth-size, and the majority are larger than Jupiter size. Although we can't say for sure until we have more sensitive instruments, there's every reason to believe that we'll see a lot more planets when we can see down to, say, Mercury-size.Finding Mercury size planets will be a very tricky proposition. You can't detect them from doppler effects of the parent star, because the star's own surface variation (helioseismology) is larger than the effects a Mercury size planet would produce. The Earth would only just be above the noise created by the Sun's surface seismology. Transits would also be extremely difficult to detect, because the size of Mercury relative to the Sun is so small that the photometric change from a transit would be dwarfed by the Sun's natural variability (all stars are slightly variable). The problem isn't instrument sensitivity, but the inherent noise.

Probably not, though as others have said, there will be a lot. If a star forms close to an A or O class star, it will have its disk of material blown awayAlso, many low mass stars form in tight clusters, and will lose their disks to gravitational interactions with more massive stars.

Also only the later generation of stars can have planets (Population I stars) These need to wait for the early (Population III and II stars) to cycle through and produce the elements needed for planetary formation

Here is a good article on the subject

http://en.wikipedia.org/wiki/MetallicityNot entirely true. You need high metallicity to form rocky planets, but there's no reason you can't form gas giants from a low metallicity disk.

And therefore far older? That's cool. Yet a Universe that turned out to be 800 Billion years older than it is currently thought to be would surely severely screw up many of calculations Scientists are now making about certain things in the Universe wouldn't it? I'm sure they must use the current 13.7 Billion year timeframe as a fundamental part of many important calculations?No, the Universe being bigger than we can see would have no effect on our age estimates of the Universe. One of the things that sets the age limit is the maximum distance that we can observe out to. This is set by the age of the Universe, because light from the most distant observable objects started towards us when matter and photons decoupled, shortly after the Big Bang. If the Universe were older we'd be able to see further.

As for the OP, it appears that most of the giant gas giants discovered in distant systems orbit extremely close to their Sun, so close that they are in a tidal lock. That's another thing that is cool, and weird.Again, that's purely a selection effect. Those planets are the easiest to detect, and due to their short orbital periods, take very little time to confirm. For instance, Jupiter would be relatively easy to detect, but its orbit takes 12 years, which means that we'd need to observe the Sun for about 36 years to know for certain that it was in a stable planetary orbit. We've been looking at other stars for about 10 years. We'd also be able to pick up Saturn's signal, but that would require about 90 years!

Still wonder why we haven't found any Solar System twins?

[sol invictus]

And therefore far older? That's cool. Yet a Universe that turned out to be 800 Billion years older than it is currently thought to be would surely severely screw up many of calculations Scientists are now making about certain things in the Universe wouldn't it? I'm sure they must use the current 13.7 Billion year timeframe as a fundamental part of many important calculations?

What wollery said. Bigger doesn't mean older.

(What's a finite size effect?)

Well, unless the only way for the universe to have a finite size is if it "wraps around" on itself, either like the surface of the earth, or like a piece of paper rolled into a tube and glued (or in various more complex ways). If we lived in a universe like that and could see anything close to most of it, we'd notice it. For example - look as far as you can see in that direction, then look as far as you can see in the opposite direction, and.... see something similar?

We don't see that, which means if the universe is finite, it's still a fair amount bigger than the part we can see.

[Undesired Walrus]

No, the Universe being bigger than we can see would have no effect on our age estimates of the Universe. One of the things that sets the age limit is the maximum distance that we can observe out to. This is set by the age of the Universe, because light from the most distant observable objects started towards us when matter and photons decoupled, shortly after the Big Bang. If the Universe were older we'd be able to see further.


Is it possible we just don't have the technology to see further yet?

What wollery said.

?

But you just said that the Universe may be far larger than the observable universe? Surely that means that the original expansion (The Big Bang) started longer ago than we currently believe?

[Dr. Trintignant]

Finding Mercury size planets will be a very tricky proposition. You can't detect them from doppler effects of the parent star, because the star's own surface variation (helioseismology) is larger than the effects a Mercury size planet would produce. The Earth would only just be above the noise created by the Sun's surface seismology. Transits would also be extremely difficult to detect, because the size of Mercury relative to the Sun is so small that the photometric change from a transit would be dwarfed by the Sun's natural variability (all stars are slightly variable). The problem isn't instrument sensitivity, but the inherent noise.

Agreed, but it's still just a matter of technology. Very-long-baseline space-based optical interferometers with statite starshades should do the trick, I think...

- Dr. Trintignant

[wollery]

Is it possible we just don't have the technology to see further yet?Well, since lots of other data points to the age of the Universe being 13.7 billion years, no.

But you just said that the Universe may be far larger than the observable universe? Surely that means that the original expansion (The Big Bang) started longer ago than we currently believe?No, it doesn't.

You might want to read up on inflationary theory.

[MG1962]

But you just said that the Universe may be far larger than the observable universe? Surely that means that the original expansion (The Big Bang) started longer ago than we currently believe?

Well see the situation is as we see further we see younger structures in the universe. One of the reason astronomers are so confident about the age is the fact these structures appear to meet what we predict we should be seeing at that particular age

This means that there is growing confidence that we understand the basic mechanism for the big bang, and what had to happen after the intitial expansion to allow star formation and the early galaxies to form.

Think of it in terms of the human skeleton. As we age things change in our bones. With no reference point forensic science can tell with reasonable accuracy the age of the person they are studying.

[Cuddles]

is there reason to believe that all stars can be expected to have planets orbiting them?

No. We're pretty sure that a lot, quite possibly a large majority, of stars have planets around them. However, "all" is far too strong a word. It would only take one exception for that "all" to be wrong.

[wollery]

Agreed, but it's still just a matter of technology. Very-long-baseline space-based optical interferometers with statite starshades should do the trick, I think...

- Dr. TrintignantI'm not sure, as I've never done the maths, but you may get a diffraction limit for close orbiting small planets if they're a long way away.

[Undesired Walrus]

Well, since lots of other data points to the age of the Universe being 13.7 billion years, no.

No, it doesn't.

You might want to read up on inflationary theory.

Asking questions about the Universe deserves responses like this? If my question is so beneath you, don't respond.

[JoeTheJuggler]

Is it possible we just don't have the technology to see further yet?

No, the limit is fixed by the age of the universe. Light from farther away has not had time to reach us.

[Ragnarok]

I think I'm with UW on this one. How exactly have scientists worked out this time scale, or is it beyond the scope of a layman's comprehension?

[sol invictus]

Is it possible we just don't have the technology to see further yet?

No technology could let you see further than light had time to travel.


But you just said that the Universe may be far larger than the observable universe? Surely that means that the original expansion (The Big Bang) started longer ago than we currently believe?

No, it doesn't.

Suppose there was a bright flash at time t=0, emanating from every point in an infinite and otherwise empty space. You live some amount of time later. You'll see light from a thin spherical slice of that t=0 volume coming from all directions - light that has just now arrived at your location. As time passes, the shell you're seeing grows. If it helps, imagine that the flash wasn't quite uniform - it has patterns in it - so you can see those patterns changing as time goes by.

The radius of that shell now is the radius of our observable universe, and also the time since the big bang. But that radius says nothing about how big the universe actually is - in my example above, the universe is infinitely big, but the radius and time are finite.

I think I'm with UW on this one. How exactly have scientists worked out this time scale, or is it beyond the scope of a layman's comprehension?

The most direct way is to measure the Hubble constant H- the proportionality factor between distance and recession velocity, as in v=d H. H has dimensions of 1/time, and (at least in a flat space) a time 1/H ago everything was on top of us.

In reality general relativity corrects that a little because spacetime is curved, but 1/H is still pretty close to the age of the universe.

[Ragnarok]

The most direct way is to measure the Hubble constant H- the proportionality factor between distance and recession velocity, as in v=d H. H has dimensions of 1/time, and (at least in a flat space) a time 1/H ago everything was on top of us.

In reality general relativity corrects that a little because spacetime is curved, but 1/H is still pretty close to the age of the universe.

So, that'll be a "Yes." then.

[Paulhoff]

If a grain of sand is 1/4 mm in length, then I calculate 40,000,000,000,000,000,000,000 stars (400 billion galaxies x 100 billion stars per), which would yield 625 cubic km of sand.

There's gotta be a hell of a lot more sand than that on Earth. And even if not, it's in the same ballpark within a magnitude or so.
It is to be compared to all the sand on all the beaches on earth, not all the sand on earth.

Paul

:) :) :)