by Chris Peterson » Thu Feb 17, 2011 12:40 am
dougettinger wrote:I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?
If recent exoplanet discoveries have taught us anything, it's that there's a huge variability in the structure of planetary systems. One possibility is that most stars don't have Oort clouds. Another is that we can only detect the ice (for now, anyway) during the early stages of the system, when everything is warmer. Objects in the Oort cloud of a mature stellar system are likely to be very cold, and have a weak IR signal compared with the background.
Since many comets have high inclinations it then is assumed that the comet sink or source is spherical like the Oort Cloud is envisioned. Comets could not have been perturbed into such high inclinations by the outer massive planets. However, high inclinations and hyperbolic (non-periodic) comets suggests to me that they could have been captured from interstellar space. As these interstellar comets spiral around the Sun some become perturbed by the outer planets as claimed by Wikipedia.
An interstellar object wouldn't spiral around the Sun. It would be in a hyperbolic orbit, and would only pass through the system once. If it was out of the ecliptic, it would probably not be perturbed significantly by Jupiter, and would therefore not be captured. It is certainly possible for a body to enter on the ecliptic, lose some momentum to Jupiter, and then be captured by the Sun (meaning it would end up in an elliptical orbit around the Sun). But again, you have to consider the statistics. If you assume that all (or most) comets were captured in this way, you have to explain where the vastly greater number of comets are that should be passing through the system all the time in very hyperbolic orbits.
I quote from Wikipedia, "If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun ( a few tens of kilometers per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have geniunely hyperbolic trajectories." The main caveat is the assumed absolute velocity of these comets. They could have been propelled by the blasts of supernovae and easily have velocities of 250 km/s, close to the Sun's speed. If the trajectories are closely parallel then the capture process would almost imitate an Oort Cloud being perturbed.
If they are propelled by a supernova, they will have even greater energies. The point of the article is that interstellar bodies orbiting the galaxy at the same radius as the Sun will have a similar orbital velocity. But what is "similar"? You need to understand that at Oort cloud distances, or a fraction of a light year, the solar escape velocity is measured in centimeters per second. That's compared with an orbital velocity of 250 km/s. So in order to have an orbit that looks like a typical long period comet, the interstellar debris would have to be matched in velocity with the Sun by something like one part in 25 billion. There is no mechanism that could explain that. Even if there were a debris field close to this, there would still be a statistical distribution, and we should see strongly hyperbolic cometary orbits- but we don't. Virtually all long period comets have an eccentricity very close to 1.
The interstellar densities of objects could be much higher than expected or measured.
I don't think so. The density required to produce the observed cometary flux would be high enough that we'd seem to be in a nebula, with stars more than a few light years away showing significant extinction. In addition, that much mass, heated by all the stars in the area, would raise the background well above the observed 3 K. I don't believe that the regional mass density required to explain comets could go unobserved with our current technology.
It is also worth noting that we have captured and analyzed dust from comets, and it all dates to the beginning of the Solar System. So if comets are interstellar, they are coming from a source almost exactly the same age as the Sun, which seems extremely unlikely. Any interstellar material co-orbiting the galaxy with the Sun would not be associated with the region of our Sun's formation- the perturbations associated with our numerous galactic orbits over 4.7 billion years have thoroughly scrambled things up (which is why the stars that formed with the Sun are no longer around us).
[quote="dougettinger"]I also recall that APOD. Why cannot we see this ice ring or ice signature around stars nearby to the Sun?[/quote]
If recent exoplanet discoveries have taught us anything, it's that there's a huge variability in the structure of planetary systems. One possibility is that most stars don't have Oort clouds. Another is that we can only detect the ice (for now, anyway) during the early stages of the system, when everything is warmer. Objects in the Oort cloud of a mature stellar system are likely to be very cold, and have a weak IR signal compared with the background.
[quote]Since many comets have high inclinations it then is assumed that the comet sink or source is spherical like the Oort Cloud is envisioned. Comets could not have been perturbed into such high inclinations by the outer massive planets. However, high inclinations and hyperbolic (non-periodic) comets suggests to me that they could have been captured from interstellar space. As these interstellar comets spiral around the Sun some become perturbed by the outer planets as claimed by Wikipedia.[/quote]
An interstellar object wouldn't spiral around the Sun. It would be in a hyperbolic orbit, and would only pass through the system once. If it was out of the ecliptic, it would probably not be perturbed significantly by Jupiter, and would therefore not be captured. It is certainly possible for a body to enter on the ecliptic, lose some momentum to Jupiter, and then be captured by the Sun (meaning it would end up in an elliptical orbit around the Sun). But again, you have to consider the statistics. If you assume that all (or most) comets were captured in this way, you have to explain where the vastly greater number of comets are that should be passing through the system all the time in very hyperbolic orbits.
[quote]I quote from Wikipedia, "If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun ( a few tens of kilometers per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have geniunely hyperbolic trajectories." The main caveat is the assumed absolute velocity of these comets. They could have been propelled by the blasts of supernovae and easily have velocities of 250 km/s, close to the Sun's speed. If the trajectories are closely parallel then the capture process would almost imitate an Oort Cloud being perturbed.[/quote]
If they are propelled by a supernova, they will have even greater energies. The point of the article is that interstellar bodies orbiting the galaxy at the same radius as the Sun will have a similar orbital velocity. But what is "similar"? You need to understand that at Oort cloud distances, or a fraction of a light year, the solar escape velocity is measured in centimeters per second. That's compared with an orbital velocity of 250 km/s. So in order to have an orbit that looks like a typical long period comet, the interstellar debris would have to be matched in velocity with the Sun by something like one part in 25 billion. There is no mechanism that could explain that. Even if there were a debris field close to this, there would still be a statistical distribution, and we should see strongly hyperbolic cometary orbits- but we don't. Virtually all long period comets have an eccentricity very close to 1.
[quote]The interstellar densities of objects could be much higher than expected or measured.[/quote]
I don't think so. The density required to produce the observed cometary flux would be high enough that we'd seem to be in a nebula, with stars more than a few light years away showing significant extinction. In addition, that much mass, heated by all the stars in the area, would raise the background well above the observed 3 K. I don't believe that the regional mass density required to explain comets could go unobserved with our current technology.
It is also worth noting that we have captured and analyzed dust from comets, and it all dates to the beginning of the Solar System. So if comets are interstellar, they are coming from a source almost exactly the same age as the Sun, which seems extremely unlikely. Any interstellar material co-orbiting the galaxy with the Sun would not be associated with the region of our Sun's formation- the perturbations associated with our numerous galactic orbits over 4.7 billion years have thoroughly scrambled things up (which is why the stars that formed with the Sun are no longer around us).