ElI5: how do they know what other planets are made off, how can the be so sure? Jupiter could be made of a completely unknown gas?
In: 96
In addition to what other have said, we know everything in the solar system came from the same proto-planetary disk so it’s all made of the same stuff. The inner planets are rocky because the denser material sunk towards the center due to gravity.
Hydrogen and helium are also generally the most common elements in the universe and go into the making of all gas planets. So it’s not really possible for any gas planet to be made of an entirely unknown gas.
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I have nothing to add beyond what the others have said, but I find this topic particularly interesting because I actually performed this analysis as a NASA scientist years ago for Saturn. We used IR data collected from the Cassini probe and compared the spectra to simulations using forward solutions to the radiative transfer equation.
Basically, using some assumptions of the basic atmosphere composition (hydrogen, helium, and methane), you can match the ambient IR intensity (there are long portions of the spectrum which are noise-free). From there, you’ll see extraneous bumps and dips in the spectrum which are caused by absorption and emission of IR radiation due to molecular vibrations. We then look up the wavelengths of those perturbations in a spectral database (HITRAN is the main one), and we can figure out what gas is causing it based on experimental measurements. If you then run your radiative transfer equations backwards, you can solve for the amount of that gas needed to produce the observed blip. If you’re lucky, you can find multiple isolated signatures of the same gas to verify your solution.
We quantified measurements for ammonia, carbon dioxide, a special carbon dioxide isotope, and water. As far as I’m aware, no one has made a more precise measurement of Saturn’s water composition than I have (or at least no one has published it).
I haven’t thought about this work in a long time. I almost miss the academic life… too bad it didn’t pay well.
I want to break your question in two.
First:
Gasses.
There are a lot of unknown gasses. Almost as many as there are unknown molecules.
That’s because like a lego set, there are an almost infinite number of ways to put the bricks together. yes only some of them are stable or even connect properly but there’s still a HUGE number of combinations possible.
It’s currently impossible to analyze those gasses remotely and get their shapes and what they could do.
So for that, you are correct, they can’t be sure.
However:
Second:
Those molecules are made of a very finite number of atoms.
Like the lego set, there are only so many types of individual bricks.
And we have a LOT of very good ways to identify those bricks.
Once we’ve identified those bricks we can make some very good guesses as to what LEGO set was built with them.
Obviously we could be wrong but we’ve also sent probes and got on the pot analyses which show the deductions were pretty on the spot.
So for for the second we can be as sure as going there and having actually checked.
Through spectroscopy! When the sunlight passes through a planets atmosphere certain wavelengths are absorbed by different compounds present in the atmosphere and will leave corresponding gaps that show up on the spectrometer, aptly called special gaps. And each element and compound uniquely absorb different wavelengths of light. Although it can occasionally be a bit ambiguous, you get get very precise data, with regards to a planets atmospheric composition using this method! I’d look into it it’s rather fascinating!
To understand this, first you have to understand how light interacts with atoms. Atoms are made of protons, neutrons, and electrons. Protons and neutrons are in the center, and the electrons are on the outside. The electrons exist in something called an “orbital,” which is hard to explain but you can think of it as certain shapes and distances that they are more likely to be found in. Those orbitals have certain energy levels, and we number them like 1,2,3.. and so on. Light is made of something called photons. These are like tiny little packets of energy. When a photon hits an electron, the electron will absorb it and move up to a higher-energy orbital. When an electron moves down to a lower-energy orbital, it will emit a photon.
It turns out that the energy of a photon is also related to another property called its _wavelength_. The wavelength describes how much a photon is “spread out” in space as it travels. Shorter wavelengths mean less spread, and also higher energy. We can use a device called a spectrometer to measure the wavelength of photons.
If you collect light from a star or from a planet using a telescope, and then shine that light through a spectrometer, you find that there are lines at very specific wavelengths. For a star, these will be bright lines, and they are there because they come from the photons that were emitted by electrons falling down from higher energy levels into lower energy levels. For a planet, they will be dark lines, and they come from the photons that got _absorbed_ by gases in the planet’s atmosphere when light from a star shone on them and was absorbed by electrons increasing their energy levels.
Since we can measure the wavelengths of these photons, we can figure out what the energy level must have been of the photons that were emitted or absorbed. The last piece of the puzzle is to figure out what kinds of atoms those photons came from. It turns out that every different atom has different separations in energy levels between the electron orbitals. These are like fingerprints for elements. If you know what the pattern of wavelengths is for each type of atom, you can look at a spectrum of light with lots of different patterns on it and figure out what kinds of atoms must have made it. You can even figure out how much of each type of atom there are, based on the brightness of the lines.
Also I’d like to point out that there are no “unknown” gasses. We know every element in the periodic table and “unknown” ones would not be stable enough to exist in a steady state.
Planetologists study the atmospheres of other planets to try and understand what gases make them up. By doing this, they can rule out certain gases as being present on other planets, and therefore make an educated guess as to what those planets are made of.
When we want to know what things are made of, we perform tests. So if we want to know whether there’s too much CO2 in the air, we break out a CO2 detector, which has a way of checking for CO2. There are lots of ways to test, so we tend to use the easiest one we can. So while testing things on Earth, we can build a detector and drop it into the stuff we’re testing. But there are also ways to test from further away.
For example, we can test temperature by sticking a thermometer into the substance. We also have thermometers that work at a distance. We only use those if we need to, since they’re more difficult to use.
Likewise we can test for gases by putting a detector in a room, or scanning from a distance. We only use the distance scanners when we need to, and we can check that they work by doing controlled experiments and comparing them to in-the-room scanners. We then just build them bigger and point them at objects in space.
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