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Question, given the sun's wobble would result from rotating around the solar system's center of gravity, how feasible is it to actually know with certainty what

Question, given the sun's wobble would result from rotating around the solar system's center of gravity, how feasible is it to actually know with certainty what the solar system is made up of?

saagarjha 9y ago
Composition analysis is usually done using spectroscopy, which looks for the characteristic wavelengths of compounds that show up when analyzing spectra.
- wallace_f 9y ago
  This comment seems out of place.
  - saagarjha 9y ago
    I was answering your question in a roundabout way: usually "wobble" gives very little information about the chemical composition of an exoplanet (other than maybe that it's a large gas giant).
    - wallace_f 9y ago
      I see. I actually thought your comment might have been said in bad faith as an insult, but I now think it was focused on the context of the post, while mines was focused on the context of what I was replying to. I was replying to the aforementioned method of detecting planets via dopplar shifts in a star's light... I've always wondered about that--with how much precision can you actually know what is causing the shifts. How many gas giants, smaller planets, etc? For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star.
      - saagarjha 9y ago
        > For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star. Such orbits are extremely rare. Any such orbit would be highly unstable, as any perturbation would cause the planet to deviate from its location. (Strictly speaking, there are five points where objects can have an orbit that matches that of another planet. They're called the Lagrangian points, and the only ones that are stable are L4 and L5, 60˚ ahead and behind the planet in its orbit.)
elihu 9y ago
From the wobble you can determine the approximate mass of the planet and how long it takes to orbit it's sun. I suppose you might be able to deduce some things about the shape of its orbit as well. Knowing what a planet is made of would require some other technique. I think the best you could do from looking at the wobble would be to say things like "planets that orbit stars like this and having a mass of such and such with an orbital period of so many days are likely to be made of the same things that other planets with those characteristics are made of". On the other hand, we can know pretty easily quite a lot about what it's host star is made of by examining the light it emits.
- neom 9y ago
  If we can detect wobble, can we detect composition? (If someone observing us was more advanced, if they can detect wobble they can probably deduce composition?)
  - saagarjha 9y ago
    Probably, because we’ve been doing it for other exoplanets already. Wobble and composition are detected in different ways, though.
- wallace_f 9y ago
  I suppose two Jupiters on the same orbital plane, 180 degrees from each other, would cancel out? Is it possible for observers to really know we have 4 rocky planets, 4 gas giants, and what their mass is, etc? Or, with what degree of certainty could this be known, and from what distance?
  - animal531 9y ago
    I don't believe you can have 2 planets in the same orbit, and once they're in different orbits they'll take a different amount of time to orbit; so you should be able to get individual measurements. Edit: Well, as per the simulated stable thread, apparently you can have Trojan etc pairs of planets in the same orbit. Interesting.

Question, given the sun's wobble would result from rotating around the solar system's center of gravity, how feasible is it to actually know with certainty what the solar system is made up of?

saagarjha 9y ago

Composition analysis is usually done using spectroscopy, which looks for the characteristic wavelengths of compounds that show up when analyzing spectra.

wallace_f 9y ago
This comment seems out of place.
- saagarjha 9y ago
  I was answering your question in a roundabout way: usually "wobble" gives very little information about the chemical composition of an exoplanet (other than maybe that it's a large gas giant).
  - wallace_f 9y ago
    I see. I actually thought your comment might have been said in bad faith as an insult, but I now think it was focused on the context of the post, while mines was focused on the context of what I was replying to. I was replying to the aforementioned method of detecting planets via dopplar shifts in a star's light... I've always wondered about that--with how much precision can you actually know what is causing the shifts. How many gas giants, smaller planets, etc? For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star.
    - saagarjha 9y ago
      > For example, I suppose two planets of equal size, orbital plane, and 180 degrees from one another would mute their gravitational effect on their host star. Such orbits are extremely rare. Any such orbit would be highly unstable, as any perturbation would cause the planet to deviate from its location. (Strictly speaking, there are five points where objects can have an orbit that matches that of another planet. They're called the Lagrangian points, and the only ones that are stable are L4 and L5, 60˚ ahead and behind the planet in its orbit.)

elihu 9y ago

From the wobble you can determine the approximate mass of the planet and how long it takes to orbit it's sun. I suppose you might be able to deduce some things about the shape of its orbit as well. Knowing what a planet is made of would require some other technique. I think the best you could do from looking at the wobble would be to say things like "planets that orbit stars like this and having a mass of such and such with an orbital period of so many days are likely to be made of the same things that other planets with those characteristics are made of". On the other hand, we can know pretty easily quite a lot about what it's host star is made of by examining the light it emits.

neom 9y ago
If we can detect wobble, can we detect composition? (If someone observing us was more advanced, if they can detect wobble they can probably deduce composition?)
- saagarjha 9y ago
  Probably, because we’ve been doing it for other exoplanets already. Wobble and composition are detected in different ways, though.
wallace_f 9y ago
I suppose two Jupiters on the same orbital plane, 180 degrees from each other, would cancel out? Is it possible for observers to really know we have 4 rocky planets, 4 gas giants, and what their mass is, etc? Or, with what degree of certainty could this be known, and from what distance?
- animal531 9y ago
  I don't believe you can have 2 planets in the same orbit, and once they're in different orbits they'll take a different amount of time to orbit; so you should be able to get individual measurements. Edit: Well, as per the simulated stable thread, apparently you can have Trojan etc pairs of planets in the same orbit. Interesting.