Will the James Webb Space Telescope find the most distant, oldest, youngest, or first galaxies? An astronomer explains

We have now seen the first data from the James Webb Space Telescope. It has observed the atmospheres of distant planets, groups of nearby galaxies, galactic light diffracted by invisible dark matter, and clouds of gas and dust in stellar nurseries.

We’ve also seen headlines claiming Webb “found the oldest galaxies we’ve ever seen,” but what does that mean?

I’m a professional astronomer studying ancient galaxies, and even I find this a bit puzzling.

Look far, look back

One of Webb’s main scientific goals is to look into the past and observe the early universe. Webb can do this because, like all telescopes, it’s a time machine.

Light travels at 300,000 kilometers per second, so when we look at the moon we see it as it was a second ago. Because the planets in our solar system are millions or billions of kilometers away, we see them as they looked minutes or hours ago.

To go even further, when we look at distant galaxies with telescopes, we often see light that has taken millions or billions of years to reach us. That means we see these galaxies as they were millions or billions of years ago.

Read more: When you look up, how far back in time do you see?

What did James Webb see?

The James Webb Space Telescope is able to see more distant galaxies than other telescopes, including the Hubble Space Telescope.

Like Hubble, it sits above Earth’s fiery and turbulent atmosphere. However, while Hubble has a 2.3 meter mirror for focusing light, Webb has a huge 6.5 meter mirror made up of 18 hexagonal segments. Finally, Webb is optimized for detecting infrared light, which we see from the farthest galaxies, as the expansion of the universe has stretched ultraviolet and infrared light into the infrared.

Among the first data Webb obtained were infrared images looking down on a galaxy cluster called SMACS 0723.

The light from SMACS 0723 took 4.6 billion years to reach us, so we see it as it was 4.6 billion years ago. That’s slightly older than the Sun and Earth, which formed just 4.56 billion years ago.

In recent weeks, galaxies far beyond SMACS 0723 have attracted attention. Webb has spotted a number of galaxies toward SMACS 0723 and other regions that may be so distant that it took their light 13.5 billion years to reach us.

I say “might” because more data is needed to absolutely confirm their distances, but some of these galaxies are very convincing candidates (other fewer).

Because it took light 13.5 billion years to reach us, we see these galaxies as they looked 13.5 billion years ago. The Universe itself is 13.8 billion years old, so we could see galaxies as they looked just a few hundred million years after the Big Bang.

Young, old or early?

While these very distant galaxies have been advertised as the “oldest galaxies,” I find that somewhat confusing. We actually see these galaxies as they looked when they were very young, maybe a hundred million years old or so.

It is true that these galaxies will be old now, but our own Milky Way is also very old now. While our sun is 4.56 billion years old, many stars in our galaxy are 10 billion years old, and some stars in the Milky Way are 13 billion years old.

Also, the very distant galaxies that Webb discovered will look very different today. Galaxies grow by absorbing gas and dark matter, forming new stars, and merging with other galaxies.

A small galaxy that started star-forming vigorously shortly after the Big Bang might have ended up being the seed of a galaxy that is very massive today and stopped forming stars long ago. This small galaxy and its ancient stars could also just have been part of a larger galaxy that formed relatively recently through merging of galaxies.

A record that will fall

So should we call these most distant galaxies young or old? Maybe not.

James Webb sees them earliest galaxies still observed – some of the First Galaxies formed shortly after the Big Bang.

I threw in one final caveat – “still observed”. Webb has only just begun his mission, and current analyzes are based on data collected over hours.

With days’ worth of data, Webb will turn his gaze to fainter and more distant objects, and see even more distant galaxies. The record for the most distant and thus earliest observed galaxy will probably tumble a few times this year.