A press conference was held on 12 July to present the first images from the James Webb Space Telescope.

Along with a hundred scientists from the Harvard-Smithsonian Center for Astrophysics, I followed the live broadcast from the Phillips Auditorium of the same centre.

We already had the Hubble telescope in space. Now we also have the James Webb.

Why is Webb better than Hubble?

When you spend $9 billion on a new telescope, you don't want it to be just a little bit better than the old one.



So why is Webb so much better than Hubble? That was the first question that came to me. My neighbours in the audience answered very nicely.

• Webb is bigger. Hubble's light collector is 4 m2. Webb's is 25 m2. So Webb collects 6.2 times more light.


• Webb is better placed. Hubble is orbiting the Earth, 550 km above our heads. So the Earth itself hides part of the sky from it [1]. As shown in the figure below (not to scale), Webb is at the “Lagrange point” L2, about 1.5 million kilometres from the Earth. The result is that no part of the sky is hidden from Webb.


• Hubble is sensitive to visible light. Webb is sensitive to infra-red light. Here is a very clear comparison of the 2 sensitivity ranges. Infra-red light penetrates best through the gas and dust found in the universe. Furthermore, the redshift of very distant objects means that their light reaches us in the infra-red. Finally, “cold” objects, such as exoplanets, tend to emit infra-red light.


• Infra-red detection instruments require a low and stable temperature. The L2 point provides this naturally. An Earth orbit does not.

What is to be done with Webb?

Those strengths should make it possible to progress in the study of the above-mentioned objectives:

• Distant universe: first stars and galaxies formed after the Big Bang, formation and evolution of galaxies.


• “Cold” targets: star and planet formation, extrasolar planetary systems.

Meanwhile, the first images are spectacular. My neighbours, who would have every reason to be fed up, as they spend their lives studying these pictures, could not contain their “oh” and “ah” when they saw Webb's pictures.

The loudest exclamations were for the image below. For comparison, I put the same one, taken by Hubble. I don't have to add anything else.

Antoine Bret, Professor in Physics at the Castilla-La Mancha Univerity (Spain).

Notes

1. If the Earth were a basket ball, Hubble would be 2 cm from its surface. This means that almost half of the sky is permanently hidden from it.