How the James Webb Space Telescope Will Look Back in Time

Published: 2022-03-29

On Christmas Day of last year, one of the most pivotal moments for space exploration was launched: the James Webb Space Telescope.

Its mission is to orbit nearly 1 million miles away from Earth and literally look back in time (more on how that's possible in the next paragraph) to understand how our solar system has developed and whether or not there is life on other planets. I was a bit confused about the claims "look back in time" so did some research on how that's possible. Here's a brief illustration:

The Moon is about 239,000 miles away from Earth, which means it takes 1.3 seconds for the light to get from the moon to earth. Therefore, when you look at the moon, you're looking 1.3 seconds back in time (Yeah, you're a wizard Harry).

The distances between objects in space are so vast, that we measure them in terms of how many years it will take light to travel to get there. The nearest star is over four-light years away, so when you see that star, you're seeing it for what it was four years ago.

The Virgo Cluster of galaxies (galaxies at the center of the Virgo constellation) is about 60 million-light years away from the Milky Way. Therefore, the light we see from that galaxy is from around the end of the dinosaur age.

But that's not all. As the universe expands, light gets stretched into longer and longer wavelengths, beyond the point which we can see -- this is called infrared light. Hubble can see some infrared light, but The Webb observes infrared light exclusively. This allows Webb to see much older galaxies than Hubble, because their light has been stretched beyond that of Hubble's ability to see.

If Hubble showed us a galaxy at the toddler stage, Webb can show us it at a newborn stage, and that is how Hubble will "look back in time" to see how the universe and other galaxies have expanded.

So, Webb will show us planets like an actual movie, but billions of years ago? Well, not exactly.

The Spectra

Webb looks at planets through their spectra -- the wavelengths they emit, absorb, transmit, or reflect -- through a technique called Spectroscopy. Scientists use that method to determine what things are made of, how hot they are, how dense they are, and how fast they are moving in space.

Since different materials give off and interact with different colors of light in different ways depending on what the materials are made of, scientists can use what they know about other planets' spectra in our solar system to determine the composition of the planets Webb captures.

Webb will first observe the star exoplanets (a planet that orbits a star outside of our solar system) are orbiting and gather its spectra. Then, Webb will wait until the planet it wants to observe passes in front of the star it's orbiting and measure the spectra again. At this point, the exoplanet will then be backlit, like an eclipse on earth, so the planet will absorb some of the light emitted from the star, making its spectra differ from the spectra Webb originally captured.

Researchers will then compare the two data sets: the data originally collected about the star and the data collected while the planet blocked the star to see which type of light was absorbed by the planet and which wasn't. Finally, they will use the data from the two spectras and compare it with data about how certain molecules absorb light. When they put it all together, they are able to determine which molecules are present on the exoplanet.

For example, we know what Jupiter's Spectra looks like and we know what Jupiter is made out of. So we can use its spectra and compare it with spectra from the exoplanet and say, "Oh wow, this looks a lot like Jupiter's so there must be a lot of Hydrogen and Helium on that planet." (Just an example, not actual data)

And that is some of the basics of how this very big, very expensive, and very cool piece of technology will explore the most distant universes and look back in time. It can't be long until we understand just exactly how the universe came to be and answer that crucial question we've all been wondering: does the chicken or the egg come first?