Why does James Webb see distant galaxies better than planets?

The difference in quality between certain images obtained by James Webb may be surprising. Why does the telescope see nearby planets so blurry (like Mars) and distant galaxies so sharp?

The very first images of Mars obtained by James Webb are a bit confusing. While the JWST Space Telescope has already accustomed us to stunning images of nebulae and galaxies, its visuals of the Red Planet are almost disappointing. We can have a similar reaction in front of the image of Neptune taken by James Webb (even if it is much less blurred than those of Mars). How is it that the telescope sees extremely distant galaxies more clearly than planets that are closer to it?

Galaxies certainly very distant, but also gigantic

It may seem odd that the JWST’s images of galaxies appear more detailed than its images of nearby planets., pointed out Corey S. Powell, American science journalist on Twitter on September 19, 2022. The explanation: the galaxies are far away, but they are huge. To illustrate this fact, the journalist compares the size of Mars today, to that of the Stephan Quintet, a grouping of galaxies, which is one of the first formations that James Webb officially imaged.

The size of Mars, compared to Stephan’s Quintet. // Source : Via Twitter @coreyspowell (cropped image)

This comparison helps to take the measure of the sizes of the various celestial objects that James Webb observes. We cannot expect the same results from the space observatory, depending on the size of the structures and bodies it sees and whose portraits it transmits to us. In addition, in the case of Mars, it should be noted that the planet appears extremely bright to it. The telescope and its instruments, very sensitive, were above all designed to detect the weak light emitted by very distant galaxies. They can be “blinded” by the glare of a nearby planet.

And, by the way, James Webb also manages to send us breathtaking images of planets. The JWST demonstrated this with its stunning views of Jupiter. But, not all his images of planets are (and will be) so detailed. That said, we were warned. While the telescope was still traveling through space, toward its final location, scientists had stressed that you shouldn’t expect to be blown away by absolutely every image of James Webb. Which doesn’t mean that we shouldn’t be enthusiastic: the scientific progress made possible by these shots certainly promises to be exciting.

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