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when I look up into the night sky I see planets, not twinkling like the other stars, but still illuminated. However, when the Hubble or James Webb telescopes approach planets we see the local features instead of an illuminated sphere in space. why is that?

Qmechanic
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Franklin Montez
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  • Related, possible duplicate. – rob Jun 03 '23 at 17:30
  • thank you but all posts linked above don’t answer my question. i’m more questioning the optics behind it, not the photography. for example, even from the ISS they see a non-illuminated earth. im sure a distant earth would appear star-like though – Franklin Montez Jun 03 '23 at 17:36
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    HST is in low-Earth orbit, JWST is at L2. Neither, "approach planets" (which planets?). Please clarify your question. – ProfRob Jun 04 '23 at 11:31
  • I suspect you're using the word "illuminated" in some way differently from its usual definition ("lit up, exposed to light, not dark"), which is causing others not to understand what you're asking. Could you please try to clarify the distinction you're trying to make in other words, preferably in as plain and simple terms as you can? (FWIW, I do have a guess as to what you may mean, and if so, the answer comes down to the fact that any small or far-away non-dark object on a dark background looks like a "point of light". But I'd prefer if you clarified your question in your own words first.) – Ilmari Karonen Jun 04 '23 at 15:07
  • Ilmari Karonen I suppose what I meant is that we see white light from a distance, meanwhile seeing distinct coloration with a larger view of the planet. the others did a great job at answering my question – Franklin Montez Jun 04 '23 at 18:09

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The nearest star (Proxima Centaury) is hundreds of thousands of times farther away from us than the nearest planet (Mars). So even though stars are much larger than planets (the sun is roughly hundred times larger than Earth and many stars are much larger than the sun), they still appear much smaller than the largest visible planet (Jupiter). Our current telescopes have high enough resolution to resolve the planets in the solar system, but are still falling a little short of resolving other stars.

The angular size of an object in space is usually measured in seconds of arc, which is a 3600th part of one degree (and there are 360 degrees in a full circle). An arc second is a small quantity. The angular diameter of the moon is a little over 1800 arc seconds. The angular diameter of Jupiter is between 30 and 50 seconds of arc, depending on where Earth and Jupiter are in their orbits. Saturn appears with an angular diameter of between 14-20 seconds of arc.

The visually "largest" star, R Doradus, has an angular diameter of less than 0.06 seconds of arc.

The resolution of our best optical imaging telescopes, like the Hubble space telescope and the James Webb space telescope is on the order of 0.03-0.1 seconds of arc, so it's barely enough to resolve R Doradus as more than one pixel. There are interferometric telescopes that can measure the angular diameter of stars and reveal some surface details, but we are just beginning to build optical telescopes that will actually show the largest stars as more than just single dots. We also have radio telescopes that can make "images" of stars (and even black holes) with much higher resolution, but they don't show the visual appearance but merely the radio frequency emissions of these objects.

The twinkling of stars is directly related to their small angular size: you are seeing the motion of warm and cold air in the atmosphere, which distorts the light path. It is not an optical feature of the stars themselves. That planets "twinkle less" is due to their larger angular diameter: their light "averages" over more of the atmosphere before it reaches our eye, which reduces the variations in the changes of light intensity.

Disclaimer: all data taken from Wikipedia, so I apologize for any discrepancy with more credible scientific sources.

FlatterMann
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    Just to add: sometimes, it may seem like the stars are twinkling less. This is due to good atmospheric conditions. When the air is very still and stable, and you are at higher altitude, Earth's atmosphere introduces less of the distortion that @FlatterMann mentioned above. This is one reason why major scientific telescopes tend to be built in these places. – Izzy Jun 04 '23 at 13:52
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For objects with significant angular size, optical systems (eyes, classical or digital cameras) adapt the image exposure to get the image brightness of the distant object within their working range, not being too bright nor too dark, losing detains. In such cases, the objects do not seem to be illuminated much.

For objects with negligible angular size on dark background, the same optical systems adapt to the background with near point-like objects as very illuminated. Note that their sensibility is set to be much higher.

In contrary to the former case, the optical systems cannot adapt properly in latter case, measuring proper exposure of the objects. And if they could, such objects would be much harder to notice in the image.

Poutnik
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The planets reflect the radiation from the sun. At the night sky of the earth the planets look like point sources, all the reflected radiation seems to be coming from one point. If one approaches the planet details of the reflection are seen, depending on the terrain. We see details of the moon because it is close to earth.

The pictures taken on the moon itself do not show a shining landscape.

anna v
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