How many jellybeans are in this jar?

[BDanielMayfield]

I left out unbound planets because we really have no Earthly idea how many of them there might be.

I know what you mean Rob, and yes we (as in you and I, at the least) have no good idea how many unbound planets exist. However, these guys; Louis E. Strigari, Matteo Barnabe, Philip J. Marshall and Roger D. Blandford are from the Earth, and they did have an idea about this question. About two years ago they published a paper entitled “Nomads of the Galaxy” available at: http://arxiv.org/abs/1201.2687 Here’s its abstract:

We estimate that there may be up to ~10^5 compact objects in the mass range 10^{-8} -10^{-2} solar mass per main sequence star that are unbound to a host star in the Galaxy. We refer to these objects as nomads; in the literature a subset of these are sometimes called free-floating or rogue planets. Our estimate for the number of Galactic nomads is consistent with a smooth extrapolation of the mass function of unbound objects above the Jupiter-mass scale, the stellar mass density limit, and the metallicity of the interstellar medium. We analyze the prospects for detecting nomads via Galactic microlensing. The Wide-Field Infrared Survey Telescope (WFIRST) will measure the number of nomads per main sequence star greater than the mass of Jupiter to ~ 13%, and the corresponding number greater than the mass of Mars to ~25%. All-sky surveys such as GAIA and LSST can identify nomads greater than about the mass of Jupiter. We suggest a dedicated drift scanning telescope that covers approximately 100 square degrees in the Southern hemisphere could identify nomads as small as 10^{-8} solar mass via microlensing of bright stars with characteristic lightcurve timescales of a few seconds.

I wasn’t a member of this forum back when this news hit the fan, er, press, so I don’t know how y’all oldtimers here took this news, but surely some of y’all must have heard of it. The notion that up to 10^5 or 100,000 rogues per star roam the galaxy was viewed with great skeptism by commenters on S&T’s blog. Was this a topic rasied here?

Hey, the search feature is very handy. With it I was able to find that this “Nomads of the Galaxy” paper was a topic of discussion here:

http://asterisk.apod.com/viewtopic.php? ... 6&p=176215

I must admit that I was a bit surprised that no one here expressed any doubts about the possibility of there being a whopping 100,000 rouge planets for every star in our galaxy. Art was the only one of y’all to express any skepticism at all. Good for you Art.

But even he seemed to be more concerned about the threat that so many rogues would cause via disturbing Oort cloud objects into the inner solar system every so often.

The 10^5 rogues per star figure that Strigari et al came up with was an upper limit though, and I would expect that it would have to be less in actuality. But how much less? If this is overstated by a full order of magnitude we’d still have 10,000 rouges per star in this galaxy. That certainly blows the doors off a paltry 5 or 10 planets per star.

Any thoughts on this?

Bruce

[geckzilla]

I am skeptical that any rouge planets exist at all.

The problem is that we can't observe so many things which are out there. Their existence is mostly inferred. You've seen in just about any given Hubble image of a local Milky Way object, if there happens to be a few shots from some years apart, random, unidentifiable, nondescript stuff can be seen just floating around in the darkness with only the faintest light to shine on it and even Hubble barely picks it up. I am very excited that the James Webb seems to be really happening because infrared imagery is actually very exciting the more you learn about it. Have a look at Hubble 12. If you look at it without the IR camera, it just looks like this. And that's just a taste... the merest of morsels, and it's amazing! I think the JWST is going to really help out with regard to these especially faint, small objects such as rogues and any number of other things we just can't see without such a telescope. Oooh, I get goosebumps thinking about it.

[Chris Peterson]

I am skeptical that any rouge planets exist at all. ;)

You doubt the existence of la planète Mars?

[geckzilla]

If Mars is made of rouge then the Moon la Luna is certainly made of cheese fromage. Curiosity must feel very pretty with all that blush.

[Nitpicker]

Ragged rocky rogues rarely require rouge, since stray starlight seldom shines sufficient.

[geckzilla]

Hah!

[BDanielMayfield]

I am skeptical that any rouge planets exist at all.

I reckon rarely recent rogues really resemble reddish rouge,
but as they cool they loose that rosy glow,
fading toward a dark magenta.

The problem is that we can't observe so many things which are out there. Their existence is mostly inferred. You've seen in just about any given Hubble image of a local Milky Way object, if there happens to be a few shots from some years apart, random, unidentifiable, nondescript stuff can be seen just floating around in the darkness with only the faintest light to shine on it and even Hubble barely picks it up.

Yes, but inferences can be based on pretty solid evidence. Chris has told us that astronomers infer the existance of about 120 planets in this solar system alone. Some of the unidentified objects you see in the the data you process could very well be distant planets of the Sun.

I am very excited that the James Webb seems to be really happening because infrared imagery is actually very exciting the more you learn about it. Have a look at Hubble 12. If you look at it without the IR camera, it just looks like this. And that's just a taste... the merest of morsels, and it's amazing! I think the JWST is going to really help out with regard to these especially faint, small objects such as rogues and any number of other things we just can't see without such a telescope. Oooh, I get goosebumps thinking about it.

That planatary nebula was wonderful geckzilla. Seeing that led me to your flicker page. Wow. You do good work. Thanks.

Bruce

[Ann]

Bruce wrote:
I must admit that I was a bit surprised that no one here expressed any doubts about the possibility of there being a whopping 100,000 rouge planets for every star in our galaxy. Art was the only one of y’all to express any skepticism at all. Good for you Art.

I don't think I paid any attention at all to that particular paper, and I don't know if I would have reacted if I had read about it. What I'm so terribly impressed at when it comes to space is that it is so totally, completely, utterly, unfathomably, unimaginably big. I once made a model of the inner solar system using cotton balls two centimeters in diameter and a table cloth two meters in diameter, and then I placed them at the more-or-less correct distances from each other. I gasped when I saw how far away from each other they were and how unbelievably tiny the "planets" were compared with all the empty vastness around them. And this, mind you, was the inner solar system, where things are relatively close to one another. The part of the outer solar system that is inhabited by Jupiter, Saturn, Uranus and Neptune is so much vaster, and then there is the Kuiper Belt that is vaster still, and then there is the Oort Cloud that is vaster still...

So if you told me that the disk that gave rise to the Sun also created 100,000 planets that have since been lost into the vastness outside the Oort Cloud, I would probably shrug. Certainly, though, I see Art's point that so many unbound planets for every star might disturb the Oort Cloud.

So here's what I think, for what it is worth. Art said that our solar system consists of ~1.0014 solar masses. In other words, all but 0.0014 of that mass is inside the Sun, and most of the rest is in Jupiter (I think). Saturn, Neptune and Uranus also contain some of it. Clearly there isn't much mass left for a multitude of undiscovered bound planets in the solar system. The way I understand it, though, 100,000 planets might contain as much mass put together as a main sequence star. If there are 100,000 unbound planets for every main sequence star, doesn't that mean that the average main sequence star was born from a disk where only half of the matter went into making the star, and the rest of it turned into planets? If we assume that quite a lot of matter is contained in bodies that are too small to be defined as planets, then we must assume that only a small part of the average nebula that created the average main sequence star went into making the star, and most of it went into making an incredible amount of comets and asteroids as well as 100,000 planets. Does that sound reasonable?

Can't say that it does to me, but I'm the complete amateur, remember.

Ann

[Nitpicker]

I had an uncle who was a sheep farmer. He had great fun teaching me that the way to count sheep was to count the legs and divide by four. What a silly uncle (probably a Seinfeld fan, too).

[rstevenson]

... So here's what I think, for what it is worth. Art said that our solar system consists of ~1.0014 solar masses. In other words, all but 0.0014 of that mass is inside the Sun, and most of the rest is in Jupiter (I think). Saturn, Neptune and Uranus also contain some of it. Clearly there isn't much mass left for a multitude of undiscovered bound planets in the solar system. The way I understand it, though, 100,000 planets might contain as much mass put together as a main sequence star. If there are 100,000 unbound planets for every main sequence star, doesn't that mean that the average main sequence star was born from a disk where only half of the matter went into making the star, and the rest of it turned into planets? If we assume that quite a lot of matter is contained in bodies that are too small to be defined as planets, then we must assume that only a small part of the average nebula that created the average main sequence star went into making the star, and most of it went into making an incredible amount of comets and asteroids as well as 100,000 planets. Does that sound reasonable?

It sounds to me like you're assuming that any unbound planet must have been created in a stellar system and then flung out of it somehow. That idea stretches my credulity to the limit. If there are really about 100,000 unbound planets for every star in the Milky Way, a good many of them must have been formed outside of stellar systems, in turbulent gas and dust clouds that didn't have enough gas/dust in one spot to create a star, but did have enough turbulence to concentrate the gas and dust in multiple little clumps, each one big enough to form a planet.

I've never read a theory of planetary formation which allowed for this kind of planet formation, but we're only now learning how many unbound planets there are, so it wouldn't surprise me to find that our current best theories of planet formation don't cover the situation.

Rob

[geckzilla]

Presumably if one of those dusty, star-forming nebulas didn't quite have enough mass to form a star there could be entire rogue planetary systems out there with a large planet taking the place of a star and the smaller planets orbiting in this dark system. Or is this a bunk idea for some reason?

[rstevenson]

I think there would not be enough mass to keep them bound in any kind of system. Once formed each new planet would just wander off in the direction that the center of mass of the bit of cloud from which they formed was moving, and because of the assumed (required?) turbulence, that would be different for each planet.

Rob

[geckzilla]

Why would the newly formed planets drift apart from one another but not the dust cloud itself? It would seem that if the planets cannot remain bound that the dust cloud itself could not have remained bound to itself and planets would have never had a chance to form.

[BDanielMayfield]

These are fascinating comments on the rogues forming without a star question. But I have this on the number of planets inside our system front:

A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun[1] that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects.[2][3] The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[1] brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, whereas those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown,[5][6][7] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,[8][9] who had coined the term dwarf planet in 1990.[10]

It is estimated that there are hundreds to thousands of dwarf planets in the Solar System. The IAU currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris.[11] Brown criticizes this official recognition: "A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct."[12] It is suspected that another hundred or so known objects in the Solar System are dwarf planets.[13] Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.[14] Individual astronomers recognize several of these,[13] and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets.[12] Brown currently identifies eleven known objects – the five accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, 2002 MS4 and Salacia – as "virtually certain", with another dozen highly likely.[13] Stern states that there are more than a dozen known dwarf planets.[14]

However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1)[15] are to be named under the assumption that they are dwarf planets.[16] The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.

At first I had thought that Chris’ total solar system planetary body estimate of about 150 less about 20 moons might be a little on the high side, but in view of the above …

I now have more respect for the IAU’s Dwarf Planet definition. The uncertainty in the true numbers of these bodies makes me rethink where the lower limit of “planet” for the purposes of this discussion should be placed.

[Ann]

Presumably if one of those dusty, star-forming nebulas didn't quite have enough mass to form a star there could be entire rogue planetary systems out there with a large planet taking the place of a star and the smaller planets orbiting in this dark system. Or is this a bunk idea for some reason?

To me that sounds quite plausible. Jupiter and Saturn, and even Uranus and Neptune, can be regarded as bodies that would have been stars if they had been more massive, and they are indeed surrounded by their own system of satellites.

But should we assume that these orphan (=starless) systems are typically born in little clouds that don't have enough mass to make a star? Isn't it more likely that these systems are born in places where a lot of normal star formation is taking place? We know that when a star cluster is formed, there will be only a few massive stars in it and a lot of low-mass ones. An example of this is this (large) Hubble picture of NGC 602 in the Small Magellanic Cloud. And consider the Pleiades. They are dominated by a small number of hot blue B-type stars, but according to http://en.wikipedia.org/wiki/Pleiades#Composition, the Pleiades contains over 1,000 statistically confirmed members, and up to 25% of the members may be brown dwarfs. It is not hard to imagine that many of these brown dwarfs may have been born from their own little eddies in a much, much larger region of star formation. Is it so hard to imagine that even smaller local disturbances in a big starforming nebula may have given rise to orphan planets?

Rob, you said:

I've never read a theory of planetary formation which allowed for this kind of planet formation, but we're only now learning how many unbound planets there are, so it wouldn't surprise me to find that our current best theories of planet formation don't cover the situation.

Are we really learning how many unbound planets there are? Do we have any sort of good idea at all?

Ann

[rstevenson]

Why would the newly formed planets drift apart from one another but not the dust cloud itself? It would seem that if the planets cannot remain bound that the dust cloud itself could not have remained bound to itself and planets would have never had a chance to form.

Think turbulence. The cloud I'm imagining could well be part of a larger complex of clouds, some of which are forming stars, but this smaller imaginary cloud is being pushed around by the activity around it. This reaches a point where smaller bodies can condense out of the cloud, but while they will certainly maintain the momentum of their small portion of the cloud, the cloud itself was never particularly cohesive, so the newly formed planets can wander off rather than form a system.

It's not a black and white, either/or type of situation. Rather this suggestion I've made is one possible outcome of a spectrum of outcomes when sufficiently large clouds of gas and dust coalesce.

Rob

[rstevenson]

...Rob, you said:

I've never read a theory of planetary formation which allowed for this kind of planet formation, but we're only now learning how many unbound planets there are, so it wouldn't surprise me to find that our current best theories of planet formation don't cover the situation.

Are we really learning how many unbound planets there are? Do we have any sort of good idea at all?

Perhaps "learning" is not quite the word. A few researchers did an extrapolation from a small amount of data, resulting in the much hyped number of 100,000 for each star. From a Sky & Telescope report of last February: "But the authors themselves acknowledge the large uncertainty in their result. The statistics remain so weak that the group’s lower limit is still one loose planet for each star in the Milky Way." In other words, their extrapolated estimate should be thought of as "from 1 to 100,000", not just as "100,000". But it doesn't hurt to think about how unbound planets might be formed, no matter how many of them are eventually found to exist.

Rob

[Chris Peterson]

Think turbulence. The cloud I'm imagining could well be part of a larger complex of clouds, some of which are forming stars, but this smaller imaginary cloud is being pushed around by the activity around it. This reaches a point where smaller bodies can condense out of the cloud, but while they will certainly maintain the momentum of their small portion of the cloud, the cloud itself was never particularly cohesive, so the newly formed planets can wander off rather than form a system.

All of these clouds are just hydrogen, slightly contaminated with a some other gases and a tiny, tiny bit of dust. It takes a huge amount of hydrogen to create enough self gravity to get the condensation required to form a star. I'm skeptical that something like a gas giant could form that way, and even more skeptical that a terrestrial planet could do so.

While I doubt very much that there are hundreds of thousands of rogue planets for every star, it isn't hard to imagine hundreds or thousands per star, which formed and were ejected quite early in the process. That notion is consistent with our current theory of star system formation and evolution.

[BDanielMayfield]

I want to thank everyone who has contributed to this discussion, even if due to time I’m unable to directly address many of the thoughtful points that have been made here.

On the question of whether rogue planets can form in an “orphan” fashion, I very much agree with Chris; that this should be a rare (if ever) occurrence that might only happen with the most massive gas giants. And Ann, if such a rare orphan gas giant does form, any planet sized bodies that form in orbit of it would by definition be moons, not planets. So rogues forming out on their own shouldn’t contribute much to the tally.

At one time it was thought by some that, just as each stellar class becomes more numerous as we move down the main sequence, brown dwarfs would be more numerous than red dwarfs. Evidence is accumulating that this is not the case however, and the trend of more numbers of less massive objects is halted before the transition down to the BDs. For example consider this which I came across thanks to Ann’s link about the Pleiades:

The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun's mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.[21] Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Yes, the Pleiads include “many brown dwarfs,” but not so many that they overwelm the other star classes, constituing only “up to 25% of the total population of the cluster.”

Also something astronomer Sergio Dieterich wrote in another blog concerning the anouncement of the smallest RD stars relates to this. I had asked if stars at the very bottom of the stellar main sequence would be numerous, and he wrote:

Right now the consensus is that the trend of more stars with fainter luminosities peaks at slightly more massive stars, and that there is a rather sharp fall-off after that. So while we expect to find a few more nearby stars like 2ma0523, they are certainly not a lot of them. We know form the WISE survey that stars appear to outnumber brown dwarfs by about 6 to 1, and it could be that a similar ratio is also representative for very low mass stars. We don't really know the shape of the drop-off yet. Several groups, including ours, are working on it.

So in the Pleiades stars outnumber BDs by about 3 to 1, while in the Sun’s vicinity BDs are about half as numerous as that. This fall off in the numbers of these lighter objects likely continues down toward the lightest BDs, and even the most massive (<13 Jupiter) rogue planets.

[BDanielMayfield]

So resuming the inquiry, our solar system alone may have not just a paltry eight planets, but, if we include dwarf planets, more than 100, 1,000 or even 10,000. Also during the Solar system's formation there were likely many planetary sized bodies ejected from this system, with estimates ranging widely from 1 to 100,000.

On the number of rogues per star part of the question I’m inclined to be very skeptical about the high side of that range. Art questioned this on the grounds that many rogues passing through our system would frequently perturb Oort cloud objects, which I think is a very valid argument.

Consider meteorites. I think I remember reading that no known meteorite has a chemical or an isotopic signature differing enough from solar system norms to have shown evidence of coming in from outside our system. Is this true? If so, wouldn’t this also suggest that rogues aren’t tremendously abundant?

Bruce

[Chris Peterson]

Consider meteorites. I think I remember reading that no known meteorite has a chemical or an isotopic signature differing enough from solar system norms to have shown evidence of coming in from outside our system. Is this true?

Yes.

If so, wouldn’t this also suggest that rogues aren’t tremendously abundant?

I don't think so. As far as we know, all meteorites originated in the asteroid belt, which we understand to be a product of how our own system formed. Why would rogue planets end up producing meteorites on Earth? Even if there are enough to disturb Oort cloud objects, that only results in occasional collisions with bodies that don't produce meteorites.

[BDanielMayfield]

Consider meteorites. I think I remember reading that no known meteorite has a chemical or an isotopic signature differing enough from solar system norms to have shown evidence of coming in from outside our system. Is this true?

Yes.

Thanks Chris.

If so, wouldn’t this also suggest that rogues aren’t tremendously abundant?

I don't think so. As far as we know, all meteorites originated in the asteroid belt, which we understand to be a product of how our own system formed. Why would rogue planets end up producing meteorites on Earth? Even if there are enough to disturb Oort cloud objects, that only results in occasional collisions with bodies that don't produce meteorites.

I was thinking that a certain percentage of rogues and lesser rubble from the formation of other systems would be rocky, and thus possibly able to have reached Earth. But maybe lack of such evidence isn't telling due to such remote odds?

[BDanielMayfield]

I’d like to now address the question of how many planets would be in orbit of the typical star. Can any tentative conclusions be drawn yet, based on exoplanet findings?

The vast distances between stars and the vast brightness ranges between stars and their planets makes planet finding hard, especially in what could be called the middle areas of planetary orbits. Radial velocity and transit detection methods are good at finding close in planets, while direct imaging can find planets in very wide orbits. So exoplanet discoveries have proven that solar systems often cover areas differing from what exists in our system. For instance several multiple planet systems have been found in which all the planets would all be inside of Mercury’s orbit here, while exoplanets have been imaged at much greater distances from their star than Neptune or Pluto are from the Sun.

To me this suggests that many systems could have far more classical planets than the eight now recognized in this system. Rob has put out 5 as an initial estimate for the average number of planets per star, not counting rouges, (nor dwarfs). Do we know enough yet to perhaps adjust this number upward?

Bruce

[rstevenson]

Hi Bruce,

I only chose 5 because I wanted a number to work with. It's just a way to say "somewhere between 0 and 10."

I don't think we have enough information yet to make more than a rough stab at an answer to your question. Our current efforts are designed to find planets most easily when they are large and in close orbits, and to the surprise of few, we're finding them. But that doesn't tell us anything much about the stars around which we haven't found any yet, nor even about what other planets may be orbiting those stars where we have found a few planets. Those other stars may have lots of planets but the plane of their orbits is tilted to our line of sight, so our best methods won't catch any of them. So we can't say that x% of stars have planets and the rest don't. We can only say we've found planets around x% of the stars we've looked at, and the others may have them too, or they may not.

That leaves us with little information as to the average number of planets around stars. If you check the Exoplanet page at Wikipedia for the number of stars with planets, they give estimates as low as a few percent, up to almost certainty. And they estimate that there would be something like 1.6 planets per star overall. But then they say that we don't really know, so I think you can read their 1.6 planets per star as similar to my 5 per star. Don't take it literally.

Rob

[Ann]

This is such a fun question to speculate on. As usual, I found Rob's post excellent. (And there are a lot of other people here who make excellent posts - and thank you so much for raising the question, Bruce!)

To me, some of the questions to ask are these:

1) How many planets are there in our own solar system?

2) Is our own solar system typical?

3) Are there any reasons to suspect that our solar system might be untypical?


Okay! Let's attempt to offer some answers. Conventional wisdom says that there are either eight or nine planets in our solar system (choose either number depending your Pluto-friendliness). Chris pointed out, however, that any Sun-orbiting body that is sufficiently large and massive to pull itself into a spherical shape should be considered a planet, in which case a number of moons and an unknown number of undiscovered Kuiper Belt objects must be considered planets. Chris estimated that there might be about a hundred spherical objects in our solar system.

So is our solar system typical? I'd say not. The solar system can't be typical since the Sun is not a typical Milky Way star, as it is much larger, brighter and more massive than the "average" Milky Way star. I suspect that the Sun is slightly more metal-rich than the typical Milky Way star, too.

So if the Sun is not a typical Milky Way star, does that mean that our solar system is also probably untypical? For example, do we have reasons to believe that the "typical" Milky Way star (an M-type star of perhaps half the mass of the Sun) has fewer planets than the Sun? What an interesting question. To even begin to answer it, let's consider the Sun's retinue of planets and compare it with the number of Jovian and Saturnian moons. Jupiter has its four large Galilean moons, of course, which all easily qualify as planets using Chris' definition. I think, however, that none of the lesser of Jupiter's moons do. According to wikipedia,

the remaining 63 moons and the rings together comprising just 0.003 percent of the total orbiting mass

So Jupiter apparently has only four planet-mass moons. Saturn, however, appears to have more. Saturn only has one really large satellite, Titan, compared with four for Jupiter, but it has five other spherical moons: Dione, Iapethus, Tethys, Enceladus and Mimas. On the other hand, by Chris' definition, all the spherical moons in our solar system should be considered planets of the Sun.

My question is this: Can we expect less massive stars to have fewer planets than the Sun? Maybe we can, on average. But the profusion of orbiting bodies around Saturn seems like a warning against assuming that light-weight stars should be short of planets. Clearly it all boils down to the characteristics of the disk that gave birth to the star and its planets.

But the Sun just might be a little more metal-rich than most stars in the Milky Way, and that might mean it has a more planets than average. Of course we have no observational evidence of how metal-poor disks affect the number of terrestrial planets in the solar systems they give birth to.

To me, it seems that we might just assume that the Sun could have a few more planets than the average Milky Way star. So if our Sun has eight or nine (large) planets, then maybe the average star has five. (But if the Sun should be considered having a hundred planetary mass satellites, then I don't know how to estimate the number of planets orbiting the average star in our galaxy.)

Ann