# Is there a physical limit to how close we can zoom in on other extra-solar planets?

1. Dec 3, 2011

### hiddenvariabl

I keep seeing reports about how all the planets similar to earth in other nearby planetary systems are now being discovered. Since we won't have the technology to send probes or go there ourselves for some time, all we can do is look at them. My question is: Could we ever get better resolution images of those planets with a telescope located in our solar system? In other words can we ever make telescopes that can zoom in on the surface of another planet in another planetary system or is it just physically impossible for us to collect that much light from something so far away? Wouldn't we need a telescope with a really big collection area to do so?

2. Dec 3, 2011

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Let's start with the easy questions - where are you going to find millions of square miles of glass for your lense?

3. Dec 3, 2011

### DrStupid

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

How about using the sun as gravitational lense? If we can't do that we could work with interferometers.

4. Dec 3, 2011

### cepheid

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Having a big collection area in order to be able to see fainter and fainter objects is always helpful, but it's not the primary reason why you'd need a REALLY big telescope. Due to diffraction, the fundamental physical limit on your angular resolution (the minimum angular separation of two points that you can resolve as being distinct from each other) is determined by your telescope aperture size. In fact, your diffraction limit scales inversely with telescope diameter (and it scales linearly with wavelength). At a wavelength of 550 nm, in order to have an angular resolution corresponding to a physical object size of 1 Earth radius at the distance to Alpha Centauri (the closest star at ~4.3 ly), you'd need a telescope aperture of >4200 m in diameter. That's more than 100 times larger in diameter than the largest optical telescope that's currently even being proposed to be built (I believe that TMT is supposed to be 30 m and E-ELT is supposed 40 m). And that's just to get an angular resolution such that the smallest feature you can see in your image is about 6400 km at the distance of the closest star (which is an angular resolution of about 1.5x10-10 radians or 32 microarcseconds. To see features of 10 km in size at that (unrealistically close) distance you'd need a telescope aperture of almost 3 million metres (3000 km or almost half an Earth radius). Since the diffraction limit size scales linearly with wavelength, the situation would be even worse in the infrared (where the contrast ratio of the planet's emission to the emission from its parent star is much more favourable).

Contrast ratios are another important point. A good analogy is that directly (visually) detecting a planet is like "looking for a firefly next to a searchlight." Its tiny emission is going to get drowned out in the much much larger emission from its parent star, which is at least 10,000 times brighter, but probably a few factors of 10 more. On those few occasions in which direct imaging of planets was achieved, the light from the parent star was blocked out using an occultation disk. But even then, there was all kinds of messy spillover light that (again due to diffraction and imperfections in the optics) looked like speckles. Guess what? Any planets that might have been there would have looked very much like these speckles. Very fancy image processing techniques had to be employed in order to even see which features were real objects and which were due to optical artefacts in the image. It was also necessary to observe the same system on multiple occasions that were widely separated in time (in order to try and discern the orbital motion of the planets). So, astronomers have enough trouble as it is identifying whether or not a planet is present in an image let alone resolving it into a disc, let alone resolving surface features on that disc.

5. Dec 3, 2011

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

You mean apart from the fact that the sun is in your way? Your next problem is that a gravitational lens doesn't have a true focal point. The one after that is that the image, such as it is, is out way past Pluto.

6. Dec 3, 2011

### Chronos

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Agreed, you would need a big chunk of glass. Our largest telescopes strain to view the nearest stars as anything more than a point source.

7. Dec 4, 2011

### DrStupid

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

This problem can simply be solved by an occulter.

This problem can be solved numerically: http://arxiv.org/pdf/astro-ph/0304162v1

This problem can be solved with swing by and a lot of time.

8. Dec 5, 2011

### twofish-quant

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Yes

http://www.oamp.fr/lise/publis/labeyrieAA96.pdf [Broken]

The problem isn't light collection. The problem is that since light is a wave, you have to have a big telescope to keep the light from spreading.

Last edited by a moderator: May 5, 2017
9. Dec 5, 2011

### twofish-quant

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

But it's only ten times larger than the largest interferometers that are now in operation. CHARA has a baseline of 300m and COAST has a baseline of 100m.

There's no engineering reason I can think of that we can't build a 10km space interferometer.

The problems are political and financial, which unfortunately trumps engineering.

http://en.wikipedia.org/wiki/Terrestrial_Planet_Finder
http://en.wikipedia.org/wiki/Space_Interferometry_Mission

The question is not can we image extrasolar planets. The real question is do we think it's important enough to try.

10. Dec 5, 2011

### Drakkith

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

I am unfamiliar with the details of an interferometer. Is it possible to have 2 relatively small telescopes set 10km apart as an interfermoeter?

11. Dec 5, 2011

### twofish-quant

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Yes.

Also with radio waves it's possible to have two telescopes on the different parts of the earth and then "electronically" combine the readings to give you a telescope that is effectively the size of the planet.

12. Dec 5, 2011

### Drakkith

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Any idea how difficult it would be to set up a small interferometer as an amatuer?

13. Dec 6, 2011

### NYNY

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Actually, we might be pretty close to having the technology to send a probe, although we'd be long dead before our distant descendants got any information from it. The Kepler system is out of reach, but a nearby star might make an interesting goal. (In reality, of course, only the most dedicated of visionaries would be willing to fund such a project.)

14. Dec 6, 2011

### NYNY

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Not at all easy. Radio dishes have to be pretty large ... and you need some very heavy-duty computing power.

(I don't think you can do optical interferometry, because the interference patterns would be much smaller than the sensor elements of a CCD. Two optical telescopes, even if many miles apart, will generate identical images.)

15. Dec 6, 2011

### Drakkith

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Yep, we can do optical interferometry. http://en.wikipedia.org/wiki/Astronomical_interferometer

16. Dec 7, 2011

### NYNY

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Well, yes ... but "we" doesn't include amateurs. Optical inteferometers require you to merge the light (not the images) from the two telescopes, with mind-boggling precision. It's quite amazing, really, that the professionals -- multi-million-dollar budgets notwithstanding -- can do it at all.

17. Dec 7, 2011

### Drakkith

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

Ah ok I see what you meant. Well, never say never, but you're probably right.

18. Dec 13, 2011

### DrStupid

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

e.g. something like this: http://www.spacedog.eu/astronomy/historic-astronomy/first-ligth-for-virtual-360-000km-telescope.html [Broken]

Last edited by a moderator: May 5, 2017
19. Dec 13, 2011

### Drakkith

Staff Emeritus
Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

That...is...awesome...

Last edited by a moderator: May 5, 2017
20. Dec 13, 2011

### twofish-quant

Re: Is there a physical limit to how close we can zoom in on other extra-solar planet

There are a lot of people doing amateur radio interferometer.