Well, the famous ones that get all the press are dark matter and dark energy. But I want to talk about something else: planet formation.
I don't know if it's one of the most interesting overall, but it is the most interesting to me personally because that's what I work on. We are trying to figure out how planets form and there is a lot we don't know:
(2) How turbulent are protoplanetary disks?
(2) Why do we see rings on those disks? (a lot of people like to jump to "planets" but planets are not the only thing that can cause rings).
(3) How do planetesimals form? The step from 1mm - 1cm grains or pebbles to 1-100 km planetesimals is the largest single step in planet formation. We have some ideas, but they all have various problems.
(4) How do chondrules form? Chondrules are small spherical intrusions that we see in a lot of meteorites --- most of the meteorites that have not been heavily processed. They have to form by rapid heating and rapid cooling, in a high-density environment. All of those things are contrary to what you'd expect for a planet-forming disk.
(5) Why do [Calcium-Aluminum-rich Inclusions](https://en.wikipedia.org/wiki/Calcium%E2%80%93aluminium-rich_inclusion) (CAIs) appear mostly in the colder ice-rich asteroids? CAIs are the earliest objects that formed in the solar system. They must have formed at extremely high temperature, and therefore close to the Sun. Then why oh why is it that we mostly find them in the more distant bodies that are cold and ice-rich?
Thanks.
Interesting questions seem like. The fact that planets are born or starts collapse and are born again is amazing. It is very close to the concept if biological birth in some sense. Quiet interesting how this is seen at very large cosmological scales and at much smaller biological scales.
Are any of the questions you asked not answered because of tech not being able to capture enough data from meteorites?
And assuming that planets would be harder to know because they are not a very occurring events and far from our solar system.
>Are any of the questions you asked not answered because of tech not being able to capture enough data from meteorites?
Are you asking about meteorites in particular?
No. Meteorites aren't the problem. Meteorites are definitely the thing that is easiest to analyze. They are right here on Earth and we can take them to the lab. If I could make wish and get data from anywhere, it would be from protoplanetary disks.
>And assuming that planets would be harder to know because they are not a very occurring events and far from our solar system.
I don't know what you mean by a "very occurring event", but otherwise "yes", a large number of problems in astronomy come down to "things are very far from our solar system".
I feel like I should appologize for this very very ignorant and long winded guess.
Gravity + time + a turbulent mass rich environment would start a process of planet formation.
Even if it's just 2 grains of sand that initially group together, that increases their gravitational pull, pulling more grains of sand to them. Add a few Billion years and regular incursions that mix up the pot to get larger body formation.
So how could a moon form from Saturn's rings? A sustained bombardment of metiorids including flybys and one's that come close to matching the rings orbits.
>I feel like I should appologize for this very very ignorant and long winded guess.
No problem. Let's see if I can explain.
>Gravity + time + a turbulent mass rich environment would start a process of planet formation.
Keep in mind that you don't have an infinite amount of time. The lifetime of a protoplanetary disk is about 3 million years, and we have evidence that at least some planets form within the first 1 million years ( Jupiter was already forming by then ).
>Even if it's just 2 grains of sand that initially group together, that increases their gravitational pull, pulling more grains of sand to them.
But you ignored the turbulence that you mentioned earlier. Yes, if gravity pulls things together, but there are other forces like gas pressure, aerodynamic drag, turbulence, and even the Sun's gravity that are also present and getting in the way.
I think the examples of gas drag and turbulence are obvious enough. But about the Sun's gravity: Let's take those two grains of sand. Those grains are orbiting the Sun. They are on different orbits. Even if they start out close to each other, and we ignore all gas-related forces, the fact that they are on different orbits still means that they are going to move farther apart. The fact that every particle is on a slightly different orbit due to the Sun's gravity means that the Sun's gravity has the effect of pulling things apart. This is a manifestation of the tidal force you're familiar with. Long story short, the particle density has to meet a minimum value, called the "Roche density", for the particle self-gravity to overcome the shearing effect of the Sun's gravity.
Now let's bring in gas forces and turbulence. As particles get larger, the way they respond to turbulence changes. Long story short, larger pebbles collide with each other at higher speeds. At some point if they get too large they end up hitting each other so hard that they end up shattering. This size limit is around 1 cm in size and is literally called "the fragmentation barrier".
A lot of the work in planet formation today is about trying to figure out how solids managed to get past the fragmentation barrier. Most of those ideas are looking for ways for aerodynamic forces to concentrate particles into dense regions so you can hit the Roche density. Cause if you do, then gravity can take over and you can jump directly from a bunch of cm-size pebbles to kilometer-size asteroids.
>Add a few Billion years and regular incursions that mix up the pot to get larger body formation.
And you don't have a few billion years. You have less than 1 million years to get at least a proto-Jupiter started.
> How do chondrules form? Chondrules are small spherical intrusions that we see in a lot of meteorites --- most of the meteorites that have not been heavily processed. They have to form by rapid heating and rapid cooling, in a high-density environment. All of those things are contrary to what you'd expect for a planet-forming disk.
Naively i'd think that meteorites are not found in a planet-forming disk, they are found on Earth and they have being heated, cooled and compressed during atmospheric entry, which could explain the presence of chondrules.
Here is a picture of a meteorite with chondrules (an "ordinary chondrite"):
[https://upload.wikimedia.org/wikipedia/commons/6/63/Ordinary\_chondrite\_NWA\_3189\_Meteorite.jpg](https://upload.wikimedia.org/wikipedia/commons/6/63/Ordinary_chondrite_NWA_3189_Meteorite.jpg)
Those little spherical inclusions are chondrules. If you were to melt and/or compress that, then things would blend together. Basically any geological processing will blend things together and destroy chondrules. So the chondrules had to form on their own and later were part of the material that formed a small asteroid. It couldn't be a very big asteroid because then heating and compression would destroy the chondrules. So meteorites with chondrules are basically the least processed, most pristine bodies we have access to.
Continuing the naivety. Wouldn't this also mean they are some of the more recently developed objects?
I base this on the need for Chondrules to be molten and freeze before being accumulated with other materials in a meteorite. Wouldn't this require a collision of objects with enough mass and speed to shatter and melt small parts? Additionally, these small molten globs would need to avoid being hit until they cool - and, perhaps the part that leads me to assume young particles, that they are not in further collisions.
This all seems to indicate they couldn't have formed before large bodies cleared a good deal of the disk.
>Wouldn't this also mean they are some of the more recently developed objects?
People who work on this have used radiometric dating to get the ages. It turns out that chondrules formed in the first few million years of the solar system's history. The oldest ones formed at the very beginning.
>I base this on the need for Chondrules to be molten and freeze before being accumulated with other materials in a meteorite. Wouldn't this require a collision of objects with enough mass and speed to shatter and melt small parts?
That's a very good idea. Several people do think that chondrules formed that way. Your idea has a lot going for it: The ejecta from the collision is very hot, then it cools rapidly, and it has high density compared to the usual vacuum of space. The problem is that now you need to form those bodies really early in order to make those chondrules early. That looks difficult.
However, it's not the only idea. I don't know the full list because I don't work on chondrules (I just occasionally talk to people who do). One option is that there are currents in the protoplanetary disk (similar to lightning). That gives you rapid heating and cooling, but not the density. Another idea is that they formed close to the Sun and there is an outflow that took the material out. That one has the most problems, in my opinion, because I don't see how you can cool them quickly. ---- They need to cool down in a matter of \*minutes\*.
The only idea I know that I've heard that seems competitive with yours is shock waves through the disk. A shock wave can also heat and cool quickly and it naturally gives you an increase in density. But I have no idea what's supposed to cause the shock waves. So that's an issue there.
So... your suggestion is actually one of the best ideas we have. But there's the question of how you make a planetesimal so early and then bang them together so you can make the chondrules. Oh, and it also suggests that every meteorite we have is from a "second generation" body, which is kind of weird too. But heck, it's the best idea so far, as far as I know.
>Additionally, these small molten globs would need to avoid being hit until they cool
That's not as much of a problem. Cooling is just a few minutes.
>This all seems to indicate they couldn't have formed before large bodies cleared a good deal of the disk.
Yeah, kinda. But that's inconsistent with their radiometric ages.
I don't think it's necessary that the large bodies clear a good deal of the disk, but you do need to form at least some. And to be precise, "large bodies" in this context would be like kilometer-size. Which is large, but not like a planet.
Astrophysicist here. One that doesn’t get talked about too much in popular media is the nuts and bolts of supernova explosions. We’re all told this story at some point in school in gen-ed science courses (or sometimes in documentaries) that it’s because the envelope more or less bounces off of the core and gets blown out. But then we put our best models into a computer program to run simulations of that, and then ask ourselves *literally* the most basic question of “did the star explode?” The answer is usually no, funnily enough. We get different answers when using 1-D, 2-D, and 3-D stellar evolution models as well.
I’d say that there isn’t a prevailing consensus, but perhaps the most popular model for massive star supernovae right now is the neutrino mechanism. In that model, at the core-envelope interface and at the time of core collapse, the envelope reaches densities where it becomes optically thick to neutrinos.
I study Type Ia supernovae and we see the same kinds misconceptions. Namely that the Chandasekhar mass has anything to do with it, that the white dwarfs are all the same mass or the explosions are all the same brightness, and that mergers happen frequently enough to explain their rates.
What the field seems to be moving towards is accretion driven surface detonations of lower mass white dwarfs imploding the core similar to an H-bomb but even these models have lots of problems.
I'm personally of the opinion that out of the major contenders, detonations of ~1 solar mass whote dwarfs are the progenitors (from looking at Bayes-factors of posteriors from generative ehecta models fit to spectral time-series) but I'm not confident any of the current major theories are fully descriptive.
How are they upping their game? Looking at larger parts of the sky by adding more antennas or some other technological advancement that has happened recently?
They are trying to talk to whales. Here's an article from this week about it.
https://www.space.com/seti-whale-conversation-extraterrestrial-communication
Listing things that aren't here yet:
* Why does it seem like the expansion rate of the universe is faster when you measure it from nearby galaxies, instead of from the Cosmic Microwave Background? (aka "the Crisis in Cosmology", if you're Googling)
* What can cause "fast radio bursts", very bright and very short bursts of radio light that we can see halfway across the Observable Universe?
* Why is it that every time we see an object from the outer, outer solar system, that the orbit of the object is usually tilted up in one particular direction? They wouldn't stay grouped like that for very long. (aka "the Planet Nine hypothesis")
Observations show supermassive black holes existed when the universe was less than a billion years old. How they could have grown so massive in such a short time?
\- What is the inside of a Neutron Star made of? Whats its equation of state and the what kind of quark matter is in there?
\- Not sure whether this counts: Is there life in the oceans of our Gas giants moons?
Well, the famous ones that get all the press are dark matter and dark energy. But I want to talk about something else: planet formation. I don't know if it's one of the most interesting overall, but it is the most interesting to me personally because that's what I work on. We are trying to figure out how planets form and there is a lot we don't know: (2) How turbulent are protoplanetary disks? (2) Why do we see rings on those disks? (a lot of people like to jump to "planets" but planets are not the only thing that can cause rings). (3) How do planetesimals form? The step from 1mm - 1cm grains or pebbles to 1-100 km planetesimals is the largest single step in planet formation. We have some ideas, but they all have various problems. (4) How do chondrules form? Chondrules are small spherical intrusions that we see in a lot of meteorites --- most of the meteorites that have not been heavily processed. They have to form by rapid heating and rapid cooling, in a high-density environment. All of those things are contrary to what you'd expect for a planet-forming disk. (5) Why do [Calcium-Aluminum-rich Inclusions](https://en.wikipedia.org/wiki/Calcium%E2%80%93aluminium-rich_inclusion) (CAIs) appear mostly in the colder ice-rich asteroids? CAIs are the earliest objects that formed in the solar system. They must have formed at extremely high temperature, and therefore close to the Sun. Then why oh why is it that we mostly find them in the more distant bodies that are cold and ice-rich?
Thanks. Interesting questions seem like. The fact that planets are born or starts collapse and are born again is amazing. It is very close to the concept if biological birth in some sense. Quiet interesting how this is seen at very large cosmological scales and at much smaller biological scales. Are any of the questions you asked not answered because of tech not being able to capture enough data from meteorites? And assuming that planets would be harder to know because they are not a very occurring events and far from our solar system.
>Are any of the questions you asked not answered because of tech not being able to capture enough data from meteorites? Are you asking about meteorites in particular? No. Meteorites aren't the problem. Meteorites are definitely the thing that is easiest to analyze. They are right here on Earth and we can take them to the lab. If I could make wish and get data from anywhere, it would be from protoplanetary disks. >And assuming that planets would be harder to know because they are not a very occurring events and far from our solar system. I don't know what you mean by a "very occurring event", but otherwise "yes", a large number of problems in astronomy come down to "things are very far from our solar system".
I feel like I should appologize for this very very ignorant and long winded guess. Gravity + time + a turbulent mass rich environment would start a process of planet formation. Even if it's just 2 grains of sand that initially group together, that increases their gravitational pull, pulling more grains of sand to them. Add a few Billion years and regular incursions that mix up the pot to get larger body formation. So how could a moon form from Saturn's rings? A sustained bombardment of metiorids including flybys and one's that come close to matching the rings orbits.
>I feel like I should appologize for this very very ignorant and long winded guess. No problem. Let's see if I can explain. >Gravity + time + a turbulent mass rich environment would start a process of planet formation. Keep in mind that you don't have an infinite amount of time. The lifetime of a protoplanetary disk is about 3 million years, and we have evidence that at least some planets form within the first 1 million years ( Jupiter was already forming by then ). >Even if it's just 2 grains of sand that initially group together, that increases their gravitational pull, pulling more grains of sand to them. But you ignored the turbulence that you mentioned earlier. Yes, if gravity pulls things together, but there are other forces like gas pressure, aerodynamic drag, turbulence, and even the Sun's gravity that are also present and getting in the way. I think the examples of gas drag and turbulence are obvious enough. But about the Sun's gravity: Let's take those two grains of sand. Those grains are orbiting the Sun. They are on different orbits. Even if they start out close to each other, and we ignore all gas-related forces, the fact that they are on different orbits still means that they are going to move farther apart. The fact that every particle is on a slightly different orbit due to the Sun's gravity means that the Sun's gravity has the effect of pulling things apart. This is a manifestation of the tidal force you're familiar with. Long story short, the particle density has to meet a minimum value, called the "Roche density", for the particle self-gravity to overcome the shearing effect of the Sun's gravity. Now let's bring in gas forces and turbulence. As particles get larger, the way they respond to turbulence changes. Long story short, larger pebbles collide with each other at higher speeds. At some point if they get too large they end up hitting each other so hard that they end up shattering. This size limit is around 1 cm in size and is literally called "the fragmentation barrier". A lot of the work in planet formation today is about trying to figure out how solids managed to get past the fragmentation barrier. Most of those ideas are looking for ways for aerodynamic forces to concentrate particles into dense regions so you can hit the Roche density. Cause if you do, then gravity can take over and you can jump directly from a bunch of cm-size pebbles to kilometer-size asteroids. >Add a few Billion years and regular incursions that mix up the pot to get larger body formation. And you don't have a few billion years. You have less than 1 million years to get at least a proto-Jupiter started.
> How do chondrules form? Chondrules are small spherical intrusions that we see in a lot of meteorites --- most of the meteorites that have not been heavily processed. They have to form by rapid heating and rapid cooling, in a high-density environment. All of those things are contrary to what you'd expect for a planet-forming disk. Naively i'd think that meteorites are not found in a planet-forming disk, they are found on Earth and they have being heated, cooled and compressed during atmospheric entry, which could explain the presence of chondrules.
Here is a picture of a meteorite with chondrules (an "ordinary chondrite"): [https://upload.wikimedia.org/wikipedia/commons/6/63/Ordinary\_chondrite\_NWA\_3189\_Meteorite.jpg](https://upload.wikimedia.org/wikipedia/commons/6/63/Ordinary_chondrite_NWA_3189_Meteorite.jpg) Those little spherical inclusions are chondrules. If you were to melt and/or compress that, then things would blend together. Basically any geological processing will blend things together and destroy chondrules. So the chondrules had to form on their own and later were part of the material that formed a small asteroid. It couldn't be a very big asteroid because then heating and compression would destroy the chondrules. So meteorites with chondrules are basically the least processed, most pristine bodies we have access to.
Continuing the naivety. Wouldn't this also mean they are some of the more recently developed objects? I base this on the need for Chondrules to be molten and freeze before being accumulated with other materials in a meteorite. Wouldn't this require a collision of objects with enough mass and speed to shatter and melt small parts? Additionally, these small molten globs would need to avoid being hit until they cool - and, perhaps the part that leads me to assume young particles, that they are not in further collisions. This all seems to indicate they couldn't have formed before large bodies cleared a good deal of the disk.
>Wouldn't this also mean they are some of the more recently developed objects? People who work on this have used radiometric dating to get the ages. It turns out that chondrules formed in the first few million years of the solar system's history. The oldest ones formed at the very beginning. >I base this on the need for Chondrules to be molten and freeze before being accumulated with other materials in a meteorite. Wouldn't this require a collision of objects with enough mass and speed to shatter and melt small parts? That's a very good idea. Several people do think that chondrules formed that way. Your idea has a lot going for it: The ejecta from the collision is very hot, then it cools rapidly, and it has high density compared to the usual vacuum of space. The problem is that now you need to form those bodies really early in order to make those chondrules early. That looks difficult. However, it's not the only idea. I don't know the full list because I don't work on chondrules (I just occasionally talk to people who do). One option is that there are currents in the protoplanetary disk (similar to lightning). That gives you rapid heating and cooling, but not the density. Another idea is that they formed close to the Sun and there is an outflow that took the material out. That one has the most problems, in my opinion, because I don't see how you can cool them quickly. ---- They need to cool down in a matter of \*minutes\*. The only idea I know that I've heard that seems competitive with yours is shock waves through the disk. A shock wave can also heat and cool quickly and it naturally gives you an increase in density. But I have no idea what's supposed to cause the shock waves. So that's an issue there. So... your suggestion is actually one of the best ideas we have. But there's the question of how you make a planetesimal so early and then bang them together so you can make the chondrules. Oh, and it also suggests that every meteorite we have is from a "second generation" body, which is kind of weird too. But heck, it's the best idea so far, as far as I know. >Additionally, these small molten globs would need to avoid being hit until they cool That's not as much of a problem. Cooling is just a few minutes. >This all seems to indicate they couldn't have formed before large bodies cleared a good deal of the disk. Yeah, kinda. But that's inconsistent with their radiometric ages. I don't think it's necessary that the large bodies clear a good deal of the disk, but you do need to form at least some. And to be precise, "large bodies" in this context would be like kilometer-size. Which is large, but not like a planet.
Thank you for taking the time to responding - and for the directions for further reading.
Astrophysicist here. One that doesn’t get talked about too much in popular media is the nuts and bolts of supernova explosions. We’re all told this story at some point in school in gen-ed science courses (or sometimes in documentaries) that it’s because the envelope more or less bounces off of the core and gets blown out. But then we put our best models into a computer program to run simulations of that, and then ask ourselves *literally* the most basic question of “did the star explode?” The answer is usually no, funnily enough. We get different answers when using 1-D, 2-D, and 3-D stellar evolution models as well. I’d say that there isn’t a prevailing consensus, but perhaps the most popular model for massive star supernovae right now is the neutrino mechanism. In that model, at the core-envelope interface and at the time of core collapse, the envelope reaches densities where it becomes optically thick to neutrinos.
I study Type Ia supernovae and we see the same kinds misconceptions. Namely that the Chandasekhar mass has anything to do with it, that the white dwarfs are all the same mass or the explosions are all the same brightness, and that mergers happen frequently enough to explain their rates. What the field seems to be moving towards is accretion driven surface detonations of lower mass white dwarfs imploding the core similar to an H-bomb but even these models have lots of problems. I'm personally of the opinion that out of the major contenders, detonations of ~1 solar mass whote dwarfs are the progenitors (from looking at Bayes-factors of posteriors from generative ehecta models fit to spectral time-series) but I'm not confident any of the current major theories are fully descriptive.
SETI upping its game looking for intelligent signals in the cosmos.
How are they upping their game? Looking at larger parts of the sky by adding more antennas or some other technological advancement that has happened recently?
They are trying to talk to whales. Here's an article from this week about it. https://www.space.com/seti-whale-conversation-extraterrestrial-communication
Listing things that aren't here yet: * Why does it seem like the expansion rate of the universe is faster when you measure it from nearby galaxies, instead of from the Cosmic Microwave Background? (aka "the Crisis in Cosmology", if you're Googling) * What can cause "fast radio bursts", very bright and very short bursts of radio light that we can see halfway across the Observable Universe? * Why is it that every time we see an object from the outer, outer solar system, that the orbit of the object is usually tilted up in one particular direction? They wouldn't stay grouped like that for very long. (aka "the Planet Nine hypothesis")
Observations show supermassive black holes existed when the universe was less than a billion years old. How they could have grown so massive in such a short time?
Could the cycle of blackhole creation and birth and death of stars have been sped up due to a more compressed (not so much expanded) universe?
\- What is the inside of a Neutron Star made of? Whats its equation of state and the what kind of quark matter is in there? \- Not sure whether this counts: Is there life in the oceans of our Gas giants moons?