Jump to content
Search In
  • More options...
Find results that contain...
Find results in...
Hellbent

Astronomy Discussion

Recommended Posts

Planets formed from the congealing of dust and rocks. The solar system has 8 planets, 4 rock planets on one side of the asteroid belt and 4 gas giants on the other side. The two sets of planets are separated by the asteroid belt. I just had the tickling thought that maybe at that distance from the sun the asteroid belt was not quite close enough to form a rock planet and not quite far enough to form a gas giant and not being quite able to form some intermediary, the rocks and dust at this distance from the sun were unable to form a planet, and so instead just remained as an asteroid. Anyone know if this is a plausible hypothesis? Are the types of planets in our solar system determined by the distance from the sun or is this organization of rock planets separated from gas giants coincidental?

Share this post


Link to post

Almost there, it never managed to form a single entity because of Jupiter's strong gravitational field rather than distance to the sun. The Oort cloud is filled with spherical trans-Neptunian objects. Don't forget Pluto/Charon, Eris/Dysnomia, etc.

Share this post


Link to post
Hellbent said:

Are the types of planets in our solar system determined by the distance from the sun or is this organization of rock planets separated from gas giants coincidental?

I think the general consensus is that rocky planets form closer to their star while gaseous ones form further out, but many observed extraterrestrial solar systems don't seem to conform to that pattern. This has led scientists to theorize that planets, particularly gas giants, can actually "wander" closer to their parent star during formation, possibly pushing other planets out of the system. I haven't kept up with planet formation theory in a while, so this might have changed.

What Zaldron mentioned is the current theory on the asteroid belt.

Share this post


Link to post

There isn't enough mass in the asteroid belt to form another planet, even a small one. People vastly overestimate how "dense" the asteroid belt is. NASA doesn't even bother trying to avoid it when sending out space probes as the odds of actually running into one are astronomical (pun intended :) ).

In general, all the planets have swept their orbits clear of asteroids. That's now part of the definition of what is a planet: a body large enough to both form a spherical shape due to self-gravity, and clears its orbit of debris. Since there is no planet between Mars and Jupiter, there is some leftover debris, i.e., asteroids. There are also asteroids between the Earth and Mars, Jupiter and Saturn, Mercury and the Sun, yada yada yada. The asteroid "belt" as a huge expanse of thick asteroids is a quaint notion leftover from days when it was believed that Mars had canals and other such astronomical myths.

Share this post


Link to post

Yeah, I remember the bit in 2001 when they go through the Asteroid Belt. That's how I learned how very un-dense it is. It doesn't work like in Star Wars.

Share this post


Link to post
Chilly Willy said:

In general, all the planets have swept their orbits clear of asteroids. That's now part of the definition of what is a planet: a body large enough to both form a spherical shape due to self-gravity, and clears its orbit of debris.


and doesn't orbit another planet.

Thanks everyone for teaching me something!

So if Pluto meets the above criteria, why has it been demoted to a "dwarf planet"? It's still a planet since it's spherical and clear of debris. Although it is really small compared to the other plants and seems out of place amongst the gas giants. If it was the first planet from the sun it might fit in better.

BTW, how do you all know so much about astronomy? Granted I don't read astronomy books and only occasionally read a news article.

Share this post


Link to post
Hellbent said:

So if Pluto meets the above criteria, why has it been demoted to a "dwarf planet"? It's still a planet since it's spherical and clear of debris. Although it is really small compared to the other plants and seems out of place amongst the gas giants. If it was the first planet from the sun it might fit in better.

BTW, how do you all know so much about astronomy? Granted I don't read astronomy books and only occasionally read a news article.

Its orbit is not contained in the same plane as every other planet orbit is, there's a rather pronounced tilt and eccentricity, however, the real reason it is no longer considered a planet is because it hasn't actually cleared the neighborhood.

It's all just bits and pieces after Neptune. Pluto's mass is an insignificant portion of the total mass of trans-neptunian objects in its neighborhood (0.07 of total).

Share this post


Link to post
Snarboo said:

I think the general consensus is that rocky planets form closer to their star while gaseous ones form further out, but many observed extraterrestrial solar systems don't seem to conform to that pattern. This has led scientists to theorize that planets, particularly gas giants, can actually "wander" closer to their parent star during formation, possibly pushing other planets out of the system. I haven't kept up with planet formation theory in a while, so this might have changed.


This is true. The reason that gas giants tend to form further out as far as I understand is that, early in the formation of any solar system, any large rocky masses will have shells of hydrogen/helium or other gas mixtures, including the planets closer to the star; however, because hydrogen and helium are such light gases, they won't remain around warmer planets and so will quickly evaporate or be stripped away by solar radiation. But around planets more distant from the star, gas is cooler and therefore less energetic, and so will tend to settle more. (This is also why Saturn's moon Titan has such a thick atmosphere even though it's slightly less massive than Mercury.)

And the thing about wandering gas giants is true. It's interesting that so many of the extrasolar planets to be discovered are gas giants extremely close to their parent stars, but I think that these planet-star pairs show up better using the Doppler method scientists used to discover them.

Hellbent said:

BTW, how do you all know so much about astronomy? Granted I don't read astronomy books and only occasionally read a news article.


I'd kind of noticed that too. :P I've been reading about all this since I was 5 or so, so that's my excuse.

Share this post


Link to post
Zaldron said:

Its orbit is not contained in the same plane as every other planet orbit is, there's a rather pronounced tilt and eccentricity, however, the real reason it is no longer considered a planet is because it hasn't actually cleared the neighborhood.

It's all just bits and pieces after Neptune. Pluto's mass is an insignificant portion of the total mass of trans-neptunian objects in its neighborhood (0.07 of total).


Umm.. we've gotten samples from Pluto?
Hasn't cleared the neighborhood in that there's still asteroids in its path?


Pluto, just has to be a renegade.

-------------------------------------------------

One of the things that really bothers me is that our universe is only 3 times as old as Earth! I dunno.. that strikes me as very odd and troublesome.

Share this post


Link to post

I don't know exactly how you define "neighborhood" in terms of planets. I suppose a custom rule applies to our rocky, giants and trans-neptunian so long as we don't have a whole lot more of data on extrasolar systems to draw a meaningful definition.

The thing is, there's not a whole lot of extra stuff in between Mercury and Mars, if you count every planet as a unit with its own moon(s). The gas giants are absolutely gigantic in terms of mass compared to the pitiful pieces of rock that sometimes pass as secondary moons, and their well known moons are all significant portions of the mass orbiting around the sun in between those radii. Pluto/Charon, on the other hand, are just two of thousands upon thousands of pieces of debris, not one of them significant enough to warrant the planet status. Some may be five hundred times larger than others, but this is not enough.

Laxer definitions on the third rule (debris clearing) do not necessarily convert Pluto into a planet without granting the same status to dozens of other objects. That's... inconvenient, don't you think?

Share this post


Link to post
Hellbent said:

One of the things that really bothers me is that our universe is only 3 times as old as Earth! I dunno.. that strikes me as very odd and troublesome.

The fusion reactors inside stars had not developed anything heavier than helium before that... we didn't "miss much".

Share this post


Link to post
Zaldron said:

The fusion reactors inside stars had not developed anything heavier than helium before that... we didn't "miss much".

so is our universe relatively young? what about our planet relative to the age of the universe and other stars? Our planet is in the middle of its life I believe.

Share this post


Link to post
Hellbent said:

so is our universe relatively young? what about our planet relative to the age of the universe and other stars? Our planet is in the middle of its life I believe.

If I recall correctly, it's almost 4.5 billion years old, with the Universe being more or less 13 billion years old. It's a third generation star, if I recall correctly. It's expected to live 10 billion years.

The Big Bang is supposed to have developed massive quantities of hydrogen and helium, and maybe an infimal amount of the two, three next elements (i forget their name). Stellar nuclesynthesis of older generation stars forged the elements we're made of right now.

As to how "old" our Universe is, I don't dare tell you without knowing what's the latest info in expansion/inflation models. You have to factor in dark energy/matter and maybe cosmological constants not being "constant" to figure out a "heat dead" age. The presence of metals and other somewhat heavy elements is a way to measure age of galaxies. An "Earth" could have reasonably formed a long time ago (I don't know exactly), but it sure wasn't possible all the way back to 13 billion years ago.

Share this post


Link to post

The formation of the universe with the Big Bang is usually put at between 13 and 18 billion years ago, depending on how much dark matter/energy is figured into the equation. Only 3 minutes into the universe's existence, according to most theories, all the free quarks had organized themselves into protons and neutrons, which organized into hydrogen, helium, and a few lithium nuclei. They didn't associate with electrons until about 300,000 years or so after the Big Bang, when the universe had sufficiently cooled for that to happen. (At this point the universe became transparent; that's where the cosmic background radiation comes from.)

As Zaldron said, no other elements existed until the first generations of stars went supernova. The universe was an interesting place in these first generations, with quasars and supermassive stars which could never form today, but with all the activity it probably wasn't very friendly to life.

The Sun has existed for roughly 5 billion years, and the Earth about 4.5 billion (again, as Zaldron said.) Most estimates suggest that the Sun will live for another 5 or 6 billion years before it runs out of hydrogen and swells into a red giant, then condenses into a white dwarf star after exhausting its helium.

Given any theories about the future of the universe, it is definitely relatively young. The open-universe theory suggests in fact that the universe will continue expanding forever, and that eventually there will come a point when no new stars have formed, and all the old stars have long since died and left behind white dwarfs (which, most theories suggest, eventually collapse into black holes), neutron stars (also unstable over very long periods of time), or black holes (which themselves will eventually evaporate into radiation). After enough eons, according to this theory, the only thing left in the universe will be sparse radiation and neutrinos. I'm not sure if this theory has been revised much since I last read about it, but that's my understanding.

Share this post


Link to post

My recent geo courses taught that the earth is 4.54Ga, that the sun is 4.6Ga and the universe 13.7Ga (putting it 3x age of earth). I also think the current theory is that the universe's expansion is slowing and will eventually stop, and then contract again for the Big Crunch. I don't recall how far along in that cycle we are, though, or if astrophysicists even have solid theories on how long the universe will expand for.

It's also interesting to note how close the ages of the sun and earth are. 0.06Ga only equals 60 million years, less than the time the dinosaurs have been extinct and about 1/10th the period of time that has elapsed since the Cambrian Explosion event of the first small shelly fauna here on earth. Btw, an interesting book about this event is Wonderful Life by Stephen J. Gould.

It would be interesting to know at what point in the Universe's history were planets first able to form and whether our Solar System is relatively young, medium aged or old.

Just curious StupidBunny, when did you read or acquire that information in your last post?

Share this post


Link to post
Hellbent said:

So if Pluto meets the above criteria, why has it been demoted to a "dwarf planet"? It's still a planet since it's spherical and clear of debris.


It hasn't cleared its orbit of debris. That's the condition that Pluto failed given the new definition of "planet" adopted.

Share this post


Link to post
Chilly Willy said:

It hasn't cleared its orbit of debris. That's the condition that Pluto failed given the new definition of "planet" adopted.


... and that change in definition was made because otherwise we'd have 13 or 14 planets by now.

Share this post


Link to post
Graf Zahl said:

... and that change in definition was made because otherwise we'd have 13 or 14 planets by now.


Yep! It's amazing the number of Pluto-sized bodies they've found. If they didn't change the definition, memorizing the planets would be like trying to remember (all) the moons of Saturn. :D

Share this post


Link to post
Chilly Willy said:

It hasn't cleared its orbit of debris. That's the condition that Pluto failed given the new definition of "planet" adopted.


The real reason is because it doesn't dominate it's orbit with Charon. Since Charon takes up about 1/4th-1/2 of the mass in the system, the two worlds are flung around like the ends of a baton, with the two new, tiny worlds (Nix and Hydra) pulled along for the ride.

Share this post


Link to post

This does not bode well at all for my astrological influencings: my ruling planet is Pluto :(

Share this post


Link to post
Hellbent said:

This does not bode well at all for my astrological influencings: my ruling planet is Pluto :(


At least you aren't Sedna :)

"You are incredibly distant most of your life, except for one tiny, infinitesimal point."

Share this post


Link to post
Prince of Darkness said:

The real reason is because it doesn't dominate it's orbit with Charon. Since Charon takes up about 1/4th-1/2 of the mass in the system, the two worlds are flung around like the ends of a baton, with the two new, tiny worlds (Nix and Hydra) pulled along for the ride.


If there was a definition for it, they'd be considered binary dwarf planets, but until then, Charon is still considered Pluto's moon. It think wikipedia puts it best (with references to back it up):

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

1. The object must be in orbit around the Sun.
2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
3. It must have cleared the neighbourhood around its orbit.

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).

Share this post


Link to post

Scientists said they discovered the most distant object in space, according to a Space.com report on April 28. The gamma-ray burst, seen above inside the white circle, [AOL doesn't allow direct linking] is roughly 13 billion light-years away. "The burst most likely arose from the explosion of a massive star," said astrophysicist Derek Fox.

That's pretty far. That's the same distance that light travels in the amount of time our universe has been around for. O_O Which is 76,254,048,000,000,000,000,000 miles away or 819,936,000,000,000 astronomical units away. Astromical unit = distance earth is from the sun.

It's amazing how absolutely useless this information is.

Share this post


Link to post

Can gamma ray bursts still fry Earth from that far?

That's one of those doomsday scenarios. A gamma ray burst that just happened to be aimed straight at us could allegedly instantly fry us from across the galaxy.

Share this post


Link to post
Danarchy said:

Can gamma ray bursts still fry Earth from that far?

That's one of those doomsday scenarios. A gamma ray burst that just happened to be aimed straight at us could allegedly instantly fry us from across the galaxy.


I'm guessing a gamma ray 13 billion light years away can't. But if it could, what else is it frying closer to it? 13 billion light years is a ridonculously unfathomable distance.

Share this post


Link to post
Danarchy said:

Can gamma ray bursts still fry Earth from that far?

That's one of those doomsday scenarios. A gamma ray burst that just happened to be aimed straight at us could allegedly instantly fry us from across the galaxy.


No, because we are hit by them all the time- gamma rays from the birth of the universe are still flying around at the speed of light; and unless they get gobbled up by a black hole, they always will be (until the universe ends, of course, see this. It's as depressing as all hell).

Share this post


Link to post
Prince of Darkness said:

That would still have to be in the same galaxy, and would have to be "aimed" at us (if you could call it that), so...Maybe?

Hmm...though according to that, the Milky Way is possibly too metallic to sustain a gamma ray burst.

Share this post


Link to post

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
×