The diameter of telescope for Gliese 581c?

Question

Gliese 581c is the first know exoplanet.

Its mass is 5 times that of Earth, and its diameter is the same as that of Earth. And Gliese 581 system is 20 light years away.

Q: What diameter of telescope would be needed to resolve the disk of Gliese 581c?

Answer 1

You will never see the disc of Gliese 581C (which, by the way, is a LONG way from being the first exoplanet discovered). It was discovered by other means, and since you can never even resolve the disc of the star it orbts you haven't a hope of resolving the disc of the planet. The telescope needed would be ridiculously large.

Gliese 581C is about 18000km in diameter, and about 20.4 light years (193,000,000,000,000km) away. With a bit of trigonometry that shows it subtends and angle of about 0.0000192 seconds of arc. For comparison, the full Moon subtends and angle of about 1800 seconds of arc.

The diameter D required to resolve an object of angular size R is given by dividing the wavelength of light in metres by the angular size in radians. 1 radian = 180/pi degrees, or about 206280 seconds of arc. 0.0000192 seconds of arc = 0.0000000000931 radians. Assume we use yellow light (about the middle of the spectrum) with a wavelength of 580nm, or 0.00000058m.

0.000000058/0.0000000000931 = 6230m

So just to see Gliese 581c directly you'd need a telescope over 6km in diameter!

Answer 2

Gliese 581c is NOT the first known exoplanet. There are over 260 others that we already know of. (http://vo.obspm.fr/exoplanetes/encyclo/encycl.html) Gliese 581c is the first extrasolar planet we've found that appears to have conditions similar to Earth.

Telescopic resolving power is not the answer to getting a peek at 581c. The big problem is the brightness of the star it's associated with because its glare hides the planet. If you could put the 200" Palomar telescope in space even it wouldn't be able to see Gliese 581c

Answer 3

You don't want to know.