Searching Space: UNLV astrophysicist explains key differences between James Webb and Hubble

As children awoke on Christmas morning to presents under the tree, a new NASA telescope began its journey into outer space. 

When the batteries wear out on those shiny new toys, the James Webb Space Telescope (JWST) will be the gift that keeps on giving — at least for 10 or so years — helping astronomers around the globe gain a clearer understanding of the young planets and stars forming beyond our solar system.

“We have a good idea of what’s going on in our own solar system because we have good data,” said UNLV astrophysicist and exoplanet expert Jason Steffen. “But we don’t know, for example, if it's common for planets orbiting very distant stars to have carbon dioxide in their atmospheres, like Venus. Until we actually understand how varied different planets' atmospheres are, there’s no way for us to compare how common or rare the Earth’s atmosphere is to others.”

That’s where James Webb comes in.

It’s the latest NASA telescope to launch from Earth and will serve as a complementary observatory to Hubble, which has been operating and collecting groundbreaking space data for over 31 years. Webb was fully deployed on Jan. 8, and will reach its final orbital position in about two weeks. 

There’s a huge difference between the two observatories, however, and it makes for exciting possibilities, said Steffen, who previously served as a member of the science team for NASA's Kepler mission. While Hubble primarily looks in the visible part of the light spectrum, Webb was designed to look in the infrared. 

The longer wavelengths of infrared light enables the telescope to pierce through the clouds of swirling dust — evidence of star and planet formation — that visible light captures, giving scientists a sharper-than-ever-before look into the atmospheres of distant celestial bodies. 

“Up to this point, our insight has been inhibited by the fact that we’re looking in the visible and we need a more sensitive and sophisticated instrument in the infrared,” Steffen said. “We need to be able to see through the dust into the places where the physics is happening.”

It’s similar to lifting a tablecloth to reveal the cups, plates, and bowls beneath.

“You can tell that there’s objects there because you see the outlines, but you don’t know exactly what those objects are until you remove the tablecloth or look through it,” Steffen said. “The infrared light allows you to look through it.”

As JWST continues its journey through space to reach its orbital location, Steffen shared insights into how the newest telescope will broaden our understanding of planetary systems beyond us.

How will the JWST impact the study of exoplanets?  Will it help pinpoint if there’s life beyond Earth?

JWST will give a lot of new insights into the composition of the atmospheres of some exoplanets. It isn't really a life-searching instrument, but the more we learn about the atmospheres of other planets, the more we will understand the conditions that might exist on their surfaces. There are two early programs to look at exoplanets: one is for transiting planets and the other for direct imaging. The direct imaging technology should help scientists find light being emitted by young planets that have just formed.

For the transiting planets, JWST will look at two main signatures: the phase curve (mostly reflected light, but some emission from the planet) and the transit spectrum. The phase curve will tell us how dark or shiny the object is. The transit spectrum will tell us some of the chemical composition. 

That’s what Kepler looked for — transiting planets. To date, most exoplanets have been discovered using this method, which occurs when a planet passes between a star and its observer.

Transiting planets give you a lot of information. One, you’re sure that it’s a planet because you can see it — it tells you how big the object is. You can tell if it’s Jupiter-sized or Neptune-sized based on how much light it blocks. Also, when the planet transits you’re going to be able to see the sunset. The light from the star is going to pass through the atmosphere, so you’ll pick up a signature of what the atmosphere is made of.

How will JWST collect this data, and what are some advantages over Hubble and retired observatories?

JWST is primarily designed to work in the infrared. At 1 micron (about two times the wavelength of green light) the two are similarly sensitive. At 2 microns, Hubble can't see anything and JWST is just getting started. Comparisons with the Spitzer Space Telescope are a bit more straightforward. For the most part, JWST will be about 10 times more sensitive than Spitzer.

This will allow it to detect several larger molecules in exoplanet atmospheres including water, carbon dioxide, methane, and ammonia.

Some exoplanets are going to be shinier in the infrared because they emit carbon dioxide, for example, in the atmosphere. Carbon dioxide interacts with infrared light, which is kind of the whole problem, and is what makes it a greenhouse gas. The Earth emits infrared light, and then it bounces off of the carbon dioxide in the atmosphere and gets reflected back to the surface. So by looking in the infrared, we’ll be able to see the signatures of these different atmospheric molecules that are going to be very interesting and expand our understanding of what’s happening beyond Earth. It’s really the next instrument that needed to be built.

What exoplanets are on the list for study? Will any exoplanets from your time on the Kepler team make the list?

There is a list of early exoplanet targets that JWST will look at, with sizes ranging from Earth-mass to Jupiter-mass, 300 times the size of Earth. The list may continue to evolve, but as of January 2020 it had, for example, the TRAPPIST-1 system, five that were observed by TESS (Transiting Exoplanet  Survey Satellite), several from the WASP (Wide Angle Search for Planets) survey, and some from the HATNet (Hungarian-made Automated Telescope Network) survey.  

You likely won't see many Kepler planets, at least early on, because most Kepler targets were dim. Most of the systems we discovered were pretty far away — 1,000 light years away, which is not in the solar neighborhood. 

The other surveys were looking at nearby, bright stars and will give better signals for JWST.

Are there any other interesting differences between JWST and Hubble?

Yes — its orbit is weird compared to Hubble and Kepler. Hubble goes around the Earth but JWST is in an orbit that’s fixed relative to the Earth. A combination of the sun’s gravity and the Earth’s gravity keep it in the same position even though it’s farther away from the sun. 

If you were to remove the Earth, Webb would probably orbit once every 380 days, which means that over time it would drift away from us. And eventually it would be so far away from the Earth  — on the other side of the sun — that we wouldn't be able to communicate with it. But the additional influence of the Earth’s gravity will keep the observatory in a one-year orbit.

Kepler was in a different orbit and in a region where Earth’s gravity wasn’t able to have this same effect. As a consequence, Kepler over time is falling behind, and in another 20 years, Kepler will be on the opposite side of the solar system. 

Right now, Kepler is just tumbling in space. There’s no resurrecting it. In 50 years there might be some announcement from NASA that if you look in this direction of the sky you might see Kepler coming back into our field of view. Once Webb is done, they’ll probably move it out of its orbital position — second Lagrange point, or L2 — so that we can put something else in that place.