PDA

View Full Version : Why can't our telescopes see the surfaces of planets orbiting the nearest stars...



Ross PK81
2008-Oct-21, 05:47 PM
if they're powerful enough to see galaxies which are billions of light years away?

Thanks.

alainprice
2008-Oct-21, 06:01 PM
The mirror isn't big enough. The size of the mirror and the wavelength you're measuring conspire to limit the resolution. You're going to need a VERY big mirror and use some VERY short wavelengths.

Those galaxies take up more space in the night sky than you might think.

Heck, we haven't even imaged a star properly. I think there's only one star that we can see is not perfectly round in photos. Regulus, maybe?

I wan't to add a follow-up question:
How do we calculate the usable resolution of an array of telescopes?

cjl
2008-Oct-21, 06:13 PM
I think we've seen Betelgeuse resolved to more than just a point...

astromark
2008-Oct-21, 06:20 PM
We have images of a planet orbiting a star. Only because its massive and close to its parent star. Many times the size of Jupiter. Alainprice is correct in that we have not imaged one properly. If you understand the distances involved thats not surprising. 'Criky' !... We do not yet now that there are or not planets orbiting the closest star neighbor Alpha Centaurus A and B.
Its just a simple mater of resolution. A Earth like planet is not so big as to be detectable with the equipment we have as yet.
Long exposure times and massive telescopes are the tools we have. The technology is advancing. Patients... Building a massive reflector on the moon is a wonderful idea. Who's going to pay?

PraedSt
2008-Oct-21, 06:32 PM
I want to add a follow-up question:
How do we calculate the usable resolution of an array of telescopes?

Simple concept, difficult maths. Basically you have to add all the bits up.
http://www.merlin.ac.uk/user_guide/OnlineMUG/newch0-node30.html

The wiki 'general' articles:
http://en.wikipedia.org/wiki/Aperture_synthesis
http://en.wikipedia.org/wiki/Astronomical_interferometer

Hope this helps.

Van Rijn
2008-Oct-21, 08:17 PM
if they're powerful enough to see galaxies which are billions of light years away?


There are two things: The amount of light a telescope can gather, and resolution. There are many galaxies that can be detected, but can't be resolved. An exoplanet is extremely dim, especially compared to the star it is orbiting, so it's quite a trick just detecting one at all.

Then there is angular resolution, mentioned earlier in thread. A galaxy can be hundreds of thousands of light years across, a planet only thousands of miles/kilometers. Add to that the issues of atmosphere in distorting an image for large telescopes and telescope arrays. (And Hubble has a fairly modest aperture compared to a number of ground based scopes.)

JohnD
2008-Oct-21, 08:58 PM
See: http://antwrp.gsfc.nasa.gov/apod/ap060927.html
"Earth from Saturn"

Saturn's never nearer than 1.2 billion kilometers to Earth.
That is about one 10,000th of a light year.
The closest star that is believed to have a planet (on Wiki's list - http://en.wikipedia.org/wiki/List_of_stars_with_confirmed_extrasolar_planets ) is epsilon Eridani, 10 light years away. That's one hundred thousand times further away than those few pixels beyond Saturn. All the others are at least four times further.

They're just a bl**d* long way away! Much further and much smaller than anything we can see with long distance telescopes. Yet.
John

Jeff Root
2008-Oct-22, 01:09 AM
Galaxies have large angular diameters compared to planets because galaxies
are large compared to planets:

Diameter of Milky Way: 600,000,000,000,000,000 km
Diameter of Earth: 12,740 km

-- Jeff, in Minneapolis

JustAFriend
2008-Oct-22, 01:15 PM
Also remember that we haven't imaged the moons of the outer gas giants or even Pluto
in this solar system with telescopes and they are MUCH closer and larger than the planets of other stars.

Abaddon
2008-Oct-22, 01:53 PM
http://exoplanet.eu/catalog-imaging.php

List 6 exoplanets discovered by imaging.

ETA: Mostly brown dwarf type objects.

John Mendenhall
2008-Oct-22, 05:06 PM
Its just a simple mater of resolution. A Earth like planet is not so big as to be detectable with the equipment we have as yet.
Long exposure times and massive telescopes are the tools we have. The technology is advancing. Patants... Building a massive reflector on the moon is a wonderful idea. Who's going to pay?



'Patience' vice 'Patants', right?

You are correct. But it may not be too long to wait. I never expected to see a black hole resolved in my lifetime, but the radio folks appear to have done it. Or even one extraterrestial planet discovered, much less 300+ and counting. Astronomy is in a wonderful period of discovery, right here and now.

Regards, John M.

astromark
2008-Oct-22, 06:11 PM
Just looking at that wonderful image of Earth from the moon. As interesting as that is ( and it is ) I can not see me! or you... Oops, thats you and I...
There are still places on this Earth we know very little about. Do not expect to much from our giant telescopes. Send out the robots...and wait. Yes Patients.:)thankyou.

George
2008-Oct-22, 06:12 PM
To see and Earth-sized planet in a nearby star system, say 50 lightyears distance, would be like trying to observe a grapefruit [correction: oops, make that an object smaller about 0.1mm in dia.] in New York by using a telescope in LA (ignoring Earth's curvature and atmospheric effects).

Digix
2008-Oct-22, 07:51 PM
To see and Earth-sized planet in a nearby star system, say 50 lightyears distance, would be like trying to observe a grapefruit in New York by using a telescope in LA (ignoring Earth's curvature and atmospheric effects).

is that really so impossible?
as I know military spy satellites can see what time shows your hand clock.
and all further problems to increase resolution are because atmospheric effects.

alainprice
2008-Oct-22, 08:23 PM
Spy satellites are a few hundred kms away.

NY to LA is a few thousand miles.

PraedSt
2008-Oct-22, 08:24 PM
as I know military spy satellites can see what time shows your hand clock.
and all further problems to increase resolution are because atmospheric effects.

How do you know that Digix? I'm not sure whether the grapefruit analogy is correct, but NY to LA is around 4,000 km. Military satellites don't fly so high.

Military satellites are also limited by diameter, not just atmospheric effects. Taking an enormous mirror into space is difficult, if not impossible at the present time, which is why ground based telescopes are much larger.

cjl
2008-Oct-22, 08:41 PM
is that really so impossible?
as I know military spy satellites can see what time shows your hand clock.
and all further problems to increase resolution are because atmospheric effects.

Military spy satellites don't have anywhere near enough resolution to read your watch. They're good, yes, but not that good.

Jeff Root
2008-Oct-22, 09:02 PM
I disbelieve rumors that spy satellites can get a resoution better than a
couple of inches on the ground. Recognize the rectangular shape of a car
license plate, yes; Recognize the big letters on the plate, maybe; Read the
date on the annual renewal tab, no. And always, only if the sky is clear,
the light is right, and the contrast is high.

-- Jeff, in Minneapolis

pzkpfw
2008-Oct-22, 10:10 PM
Even if a spy sat could read a watch on Earth, that doesn't mean it could read the same watch on, say, Mars, if it were pointed that way.

(Let alone a planet orbiting another star.)

It's still all about angular resolution.

(And similar to the reason you can block the view of a mountain with your thumb - if you are standing far enough away from it.)

Galaxies are big.

George
2008-Oct-22, 10:10 PM
How do you know that Digix? I'm not sure whether the grapefruit analogy is correct, but NY to LA is around 4,000 km.
That's very close. The map service shows 3905 km and the analogy would produce an object 4.1 inches in diameter (~ grapefruit) as the comparative size. [Correction again: not a grapefruit but ~ 0.1 mm in dia.]



Military satellites don't fly so high. Yes. Let's use this analogy, too. At 150 km altitude, the object on the ground would be only about 4 mm in diameter [correction: about 4 microns instead] if we compare this to seeing a planet 50 lyrs. away.


Taking an enormous mirror into space is difficult, if not impossible at the present time, which is why ground based telescopes are much larger.
Interferometry from space using two or more smaller telescopes will do the trick, assuming the stellar glare problem is solved.

nauthiz
2008-Oct-22, 10:26 PM
Recognize the rectangular shape of a car
license plate, yes; Recognize the big letters on the plate, maybe; Read the
date on the annual renewal tab, no. And always, only if the sky is clear,
the light is right, and the contrast is high.
And only if the satellite looking at the ground from a rather shallow angle, which I presume it isn't - wouldn't that produce all kinds of atmospheric distortion?

Digix
2008-Oct-23, 02:51 AM
Well maybe that is myth about these satellites i am not so sure now.


Yes. Let's use this analogy, too. At 150 km altitude, the object on the ground would be only about 4 mm in diameter if we compare this to seeing a planet 50 lyrs. away.
not sure, but isn't nearest start just about 5Ly away?
after all each star should potentially have few planets of earth-mars size

Jens
2008-Oct-23, 04:22 AM
if they're powerful enough to see galaxies which are billions of light years away?

Thanks.

I think others have answered. But an analogy might be easy to understand. Why can you see a mountain hundreds of miles away with your bare eyes, but you can't see the detail on the surface of a BB if you hold it just a meter from your eyes?

Also, just as a small aside: it is possible to see at least one galaxy (M31) with the naked eye. It's kind of a blur in the sky, but you can definitely see it if the sky is dark enough.

Hornblower
2008-Oct-23, 01:31 PM
Here is an analogy. With our unaided eyes we can see M31, but not Neptune or Pluto.

nauthiz
2008-Oct-23, 03:04 PM
We can see Betelgeuse with our unaided eye, but even our most powerful telescopes have failed to detect the Betelgeusian Destructor Fleet that is headed this way.

Though that's not really our scopes' fault. The fleet is huge and close, plenty large enough to be visible at this point, but they have really good stealth technology.

Logger
2008-Oct-23, 03:47 PM
To see and Earth-sized planet in a nearby star system, say 50 lightyears distance, would be like trying to observe a grapefruit in New York by using a telescope in LA (ignoring Earth's curvature and atmospheric effects).

I'm getting that the grapefruit would have an apparent diameter several hundred, or maybe a thousand, times larger than the planet.

Did I do the maths right?


Military spy satellites don't have anywhere near enough resolution to read your watch. They're good, yes, but not that good.

You haven't seen my watch :)

AndreasJ
2008-Oct-23, 04:43 PM
Also, just as a small aside: it is possible to see at least one galaxy (M31) with the naked eye. It's kind of a blur in the sky, but you can definitely see it if the sky is dark enough.

The poor Magellanic clouds just don't get no respect. :( They're plenty visible if you live south of ~15o N.

George
2008-Oct-23, 04:57 PM
not sure, but isn't nearest start just about 5Ly away? after all each star should potentially have few planets of earth-mars size
The Centauri system, so far, has revealed no planets. This is, likely, a trinary star system that makes planetary formation less likely, though multiple star systems have been found to hold planets.

50 lightyears is extremely close when we consider that the galaxy is about 100,000 lyrs across.

Nevertheless, using about 5 light years distance in observing an Earth-sized planet, the analogy would mean that an orbiting spy satellite at 150 km would compare in seeing a 4 cm [correction: 4 microns] diameter object.

PraedSt
2008-Oct-23, 05:01 PM
We can see Betelgeuse with our unaided eye, but even our most powerful telescopes have failed to detect the Betelgeusian Destructor Fleet that is headed this way.

Though that's not really our scopes' fault. The fleet is huge and close, plenty large enough to be visible at this point, but they have really good stealth technology.

I also heard that Betelgeusian spies keep sabotaging our scopes :(

George
2008-Oct-23, 05:14 PM
I'm getting that the grapefruit would have an apparent diameter several hundred, or maybe a thousand, times larger than the planet.

Did I do the maths right?
Surprisingly, the HST, with its 0.014 arcsecond resolution, should be barely able to see some surface features for a planet the size of Earth at 5 light years distance, assuming I did the math correctly.

Unfortunately, this includes the assumption that the planet would be bright enough for the Hubble to see it, as well as, not being affected by the horrific glare from the host star.

Being bright enough is a big deal. Imagine a Jupiter-sized planet 1/5 the way into the Oort Cloud region. At this distance, ~ 10,000 AU, this planet would be invisible to the HST as it would be less than 31 in magnitude.


You haven't seen my watch :) Ah yes, now I see you. Your watch is two minutes fast. ;)

cjameshuff
2008-Oct-23, 05:24 PM
Nevertheless, using about 5 light years distance in observing an Earth-sized planet, the analogy would mean that an orbiting spy satellite at 150 km would compare in seeing a 4 cm diameter object.

A scope with the angular resolution to image the disc of an Earth-sized planet at 5 light years distance would be able to image a 40 micron object at 150 km. You could count large bacteria with that spy sat. At that distance, the Hubble, with 0.014 arcsecond resolution, can only see details about 2.8 million km across.

Alpha Centauri is actually closer to 4.4 ly away, which bumps it up to 46 microns...not actually all that much help. It still may be practical in a limited way, but it will require a rather different sort of instrument: http://en.wikipedia.org/wiki/New_Worlds_Mission

George
2008-Oct-23, 05:30 PM
A scope with the angular resolution to image the disc of an Earth-sized planet at 5 light years distance would be able to image a 40 micron object at 150 km. You could count large bacteria with that spy sat. At that distance, the Hubble, with 0.014 arcsecond resolution, can only see details about 2.8 million km across.
I certainly could be wrong, but where?

5 lightyears is 47 trillion km. Using an object 3,211 km in diameter at that distance equates to 0.014 arcsecond (3.9e-6 deg.).

cjameshuff
2008-Oct-23, 05:42 PM
I certainly could be wrong, but where?

5 lightyears is 47 trillion km. Using an object 3,211 km in diameter at that distance equates to 0.014 arcsecond (3.9e-6 deg.).

atan(3211 / 47e12) = 3.9e-9 deg

Maybe you used billions of km instead?
Also, Earth's radius is 6378 km.

George
2008-Oct-23, 06:44 PM
atan(3211 / 47e12) = 3.9e-9 deg

Maybe you used billions of km instead? Yikes! Yep, that's it. Thanks, cjameshuff. [I had a hunch my calculation was too favorable.]

[I used convert-me.com and tried to copy the conversion, but had to type in the zeros in my spreadsheet but missed three. ]


Also, Earth's radius is 6378 km. Yes, but surface features require more than one pixel so I used about 5x for the minimum.

Correcting my numerous errors above...

Seeing and Earth at 50 lyrs. is like seeing something about 1/5th smaller than a tiny period at the end of this sentence in New York when looking from LA.

Seeing and Earth at 5 lyrs. is like seeing something about twice size of a tiny period at the end of this sentence (1 mm) in New York when looking from LA.

Seeing and Earth at 5 lyrs. is like seeing an object only 4 microns in diameter ( a human hair is roughly 90 microns in dia.) from a satellite that is 150 km in altitude looking onto the surface.