How can they tell what a planet is made of?

[spurius]

How can they tell if an exoplanet is a gas or solid type? If most of em are detected by measuring a stars wobble, or by a planets transit across the star then what sign do these methods show to suggest what type of planet it is?

[rfdesigner]

They can know the mass from the spectral measurments. I think just from this we can guess really really heavy = big gas giant. But if you've got an eclipsing case then you can measure the light dimming and so get a good estimate of the size of the planet... which means we can work out the density.

Derek

[JulianO]

It's basically a measurement of density. If you know the size of the planet from it's radius (which you can get from the light curve), and you know how heavy it is (which you can get from radial velocity measurements) then you can work out the density. (mass/volume).

If the density comes out less than that of water - it is probably a gas giant, if it comes out at about the density of rock or greater - its a rocky planet!

That said - there are now even cleverer techniques to look at spectroscopy to detect elements in the planets atmosphere - it's an interesting time. A pity we lost Kepler, but Kepler only finds them - the hard work is done by follow up telescopes.

[rfdesigner]

That said - there are now even cleverer techniques to look at spectroscopy to detect elements in the planets atmosphere - it's an interesting time. A pity we lost Kepler, but Kepler only finds them - the hard work is done by follow up telescopes.

I'd heard that was being proposed (by trying to see the spectral effect of the 'plume' comming off a planets atmoshpere) I didn't realise it had been made to work.

Derek

[JulianO]

Yes - I think its a comparison of the spectrum when the planet is transiting the star, and when it is behind the star (when you know there is no planetary light involved). If you subtract one from the other, what you're left with is probably a huige amount of noise - but if you're careful you can find some signal in it.

[spurius]

It's basically a measurement of density. If you know the size of the planet from it's radius (which you can get from the light curve), and you know how heavy it is (which you can get from radial velocity measurements) then you can work out the density. (mass/volume).

If the density comes out less than that of water - it is probably a gas giant, if it comes out at about the density of rock or greater - its a rocky planet!

That said - there are now even cleverer techniques to look at spectroscopy to detect elements in the planets atmosphere - it's an interesting time. A pity we lost Kepler, but Kepler only finds them - the hard work is done by follow up telescopes.

Cheers, I think I got it now.

Didnt know kepler went out of order. Thats what they get for buying parts from halfords.

[JulianO]

Here's a recent article on measuring exoplanetary atmospheres which is worth a read.

http://astrobites.org/2013/06/19/determining-the-minimum-heights-of-atmospheres-on-exoplanets/

[xarcane]

Hi I was trawling through some posts when I saw this one. Basically the guys above are right but the three main techniques used are Radial Velocity, Transit and Gravitational Microlensing.indirectly, mainly by the radial velocity technique, although

transit and gravitational microlensing search programmes are starting to report

discoveries.discoveriesdiscovered indirectly, mainly by the radial velocity technique, although

transit and gravitational microlensing search programmes are starting to report

discoveries.discoveries .discovered indirectly, mainly by the radial velocity technique, although

transit and gravitational microlensing search programmes are starting to report

discoveries.discoveries .discovered indirectly, mainly by the radial velocity technique, although

transit and gravitational microlensing search programmes are starting to report

discoveries.discoveries .

[xarcane]

what the hell happened to my post lol!