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How do they know what it's made of? How do they even know the density?
by bioweek 17y ago
How do they know what it's made of? How do they even know the density?
- boredguy8 17y agoThe gist: spectrum analysis of light that passes through the planet's atmosphere from the system's sun compared to light that doesn't pass through the planet's atmosphere. They look at what light bands the planet's atmosphere absorbed. (IANA astrophysicist, learn more at http://en.wikipedia.org/wiki/Absorption_spectrum http://en.wikipedia.org/wiki/Absorption_spectrum )
- robryan 17y agoFrom what I understand though there are some assumptions made based off the readings to arrive at the description we have? As in the same spectrum differences could mean more than one thing?
- bioweek 17y agoI thought the article said they looked at how the stars light was dimmed by the planet. They also managed to do a spectrum analysis?
- hugh_ 17y agoNo, they didn't -- getting the absorption spectrum of a transiting exoplanet is an order of magnitude or two beyond the capabilities of present-day instruments. They just measured the density. From the density, though, you can get a pretty good idea of what it looks like. There are three basic ingredients which go into making planets: rock (including pure metals), "ice" (a catch-all term which includes all phases of water along with methane and ammonia) and hydrogen/helium mixture. Everything else is too rare to worry about. Knowing (basically) the density of each of these components, plus knowing the fact that H/He mixture won't accrete to a body until its mass goes beyond a certain point, you can put pretty good constraints on what the composition of a planet of a certain mass and radius should be. Of course we could be wrong -- it could equally well be a ball of pure argon or something; however it's hard to imagine how any such thing would form.
- tocomment 17y agoSo how do they know the mass?
- hugh_ 17y agoThis can be obtained by looking at the red/blue-shift of the star as it wobbles back and forth due to the planet's motion. Combine this with the mass of the star, and you can figure out the mass of the planet. I forget how you get the mass of the star, but I think it can be pretty well established from its brightness.
- adamfeldman 17y agoThey do spectroscopy - http://en.wikipedia.org/wiki/Spectroscopy http://en.wikipedia.org/wiki/Spectroscopy
- hugh_ 17y agoThey haven't yet done spectroscopy on it. It's mentioned as a future possibility, but right now it's not feasible to do spectroscopy on an exoplanetary atmosphere -- the signal-to-noise you get when you try to measure something as tiny as the effect of a transiting planet's atmosphere on the spectrum of a star is too damn small to be measured with today's instruments. (I just happen to have heard a talk on this very subject a few weeks ago). Maybe in a few decades, though.
- lssndrdn 17y agoTo find out what it's made of, they are thinking of using spectroscopy. To find out the density, it's mass over volume. They infer the mass by the planet's distance from its star and by its orbiting speed, and they get the volume by the amount of light it takes away from the star when it pases in front of it.
- bioweek 17y agoCouldn't any mass planet be going any speed at any distance around a star? I'm not sure why these factors would depend on mass.
- hugh_ 17y agoYou're correct -- the orbital speed and path tell you nothing about the mass. A grain of dust will follow exactly the same orbital path as, say, Jupiter. It's the redshift/blueshift of the actual star that you need to measure.
- bioweek 17y agoOh so when the planet is between us and its star, it pulls the star towards us which blueshifts the light? That sounds hard to detect.
- fh 17y ago> That sounds hard to detect. Well, obviously you need really precise spectrometers, but just because the effect is tiny, doesn't mean that you can't detect it.
- lutorm 17y agohugh_ is right. It's a mass/volume calculation.