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How can scientist determine planets structures only by observation?

• I have always wonder about this. How can scientist determine a far away planet or moons atmosphere, minerals in the surface, the inner planets structure, planets cores, gasses in its atmosphere and other stuff just by plain observation. Is all this done by x-ray, infrared, etc. and still how can they determine to get so much vast information. Example is the discovery of far away earth like planets that may contain in some way some elements like our planet. How do they determine all of this??

• Answer:

  There is no one single technique to use. This is kinda like detective work, so you have to look at different clues; having the appropriate instrumentation helps. If you have a magnetometer, you measure magnetic fields; and those are produced by moving electric charges. This isn't the only way to measure it, since you can also detect for example radio emissions; those can come from accelerated electric charges (electrons for example). "Accelerated" here isn't restricted only to increasing your speed; since velocity is a vector, an acceleration can change the direction of the velocity (even conserving the speed). If the magnetic field is strong enough, the acceleration can be very intense and if the particles' velocities are high they can emit "breaking" radiation. In very-high-energy particles, they can emit x-rays because of that deflection; that's called "brehmstrahlung" in nuclear physics, from the german expression for "breaking radiation" or "breaking light" (it has nothing to do with cars). If you have an infrared detector, with different filters for different wavelengths, you can measure the intensity at different wavelengths (filters) and assuming it behaves as a black body you might be able to measure its temperature. That's how star surface temperatures are measured. http://en.wikipedia.org/wiki/Black-body_radiation If you have a rather precise spectrometer, what it does is that it allows you to measure the intensity of the radiation arriving at very narrow wavelength "windows". If you know at which wavelengths different substances absorb radiation (e.g. oxygen, nitrogen, carbon dioxide, water vapor, iron, helium, silicon, etc etc) then you can look at specific wavelengths to determine if those substances are there and how much (by the "width" of the line). This technique is called spectroscopy http://en.wikipedia.org/wiki/Spectroscopy and can be used not only on the visible range but also on the infrared. One technique that is also used is computer simulation. In this, you "already know" what's happening, or what's supposed to happen, and you let it run to see what would the results be as you'd measure them. If it matches what you detect, then you "know" what is happening. It also matters to see how close is the match, to ensure that you get confident in the identification. There are a whole lot of techniques that astronomers, geologists and many other people use. Each one gives hints. One technique can point to many different causes, but when you use another technique it might help exclude some causes and limit the true cause of the phenomenon. That's why you don't use a single instrument. Sometimes, one technique is enough; but often, you may have to resort to others.