Where do you point the world’s most powerful space telescope? It’s not an easy question. The James Webb Space Telescope (JWST), launched in December 2021, has amazed astronomers since it began sending back its first science data in July 2022. It has seen galaxies breathtakingly close to the dawn of time, probed the atmospheres of exoplanets in unprecedented detail and provided stunning new views of worlds in our solar system. But it’s just getting started. Last week on May 10 astronomers learned if their proposals for the telescope’s second year of science were successful. Competition was fierce, and while there were plenty of winners and some incredible science set to be conducted, there were many more who missed out on JWST’s “Cycle 2,” which starts next month. “There was an extraordinary response from the science community,” says Nancy Levenson, interim director of the Space Telescope Science Institute (STScI) in Maryland, which runs JWST.
In total, astronomers submitted about 1,600 proposals to STScI for observing time on the NASA-led JWST. But only 249 were selected—meaning that JWST has an “oversubscription” of nearly 7 to 1, similar to that for the Hubble Space Telescope. To minimize the chance of bias, the process of selecting JWST’s programs is completely anonymous, with hundreds of astronomers from multiple subfields involved in the decision process. That said, there were clear winners and losers. Some astronomers, such as Nathan Adams of the University of Manchester in England, put forward multiple proposals that were rejected. “We had four proposals, and none of them got time,” Adams says. “Obviously we’re a bit disappointed.” Others, such as Mary Anne Limbach of Texas A&M University, were much more successful. Limbach had three proposals approved. “We’re excited about the time we got,” she says.
Limbach’s proposals are focused on white dwarfs, the remnant Earth-sized cores left behind after stars such as our sun swell into red giants and expel their outer layers. After this dramatic event, it’s thought these stellar corpses can still harbor intact planets—potentially offering us the chance to study them and learn more about the fate likely to befall Earth in five billion years when our sun enters its red giant phase. Limbach will attempt to confirm two suspected white dwarf worlds but will also search for up to a half dozen more elsewhere in the sky. “JWST can see if any of these nearby white dwarfs look brighter than they should be,” she says. “If they do, that could be an indicator there’s a planet there. JWST is really the only observatory capable of confirming them.”
A dominant area of JWST’s Cycle 1, which had about 1,200 proposals, was hunting for the earliest known galaxies in the universe, which were formed just a few hundred million years after the big bang. The same is true for Cycle 2, with both galaxies and exoplanets getting the most telescope time. An accepted proposal from Daniel Eisenstein of Harvard University is hoping to push JWST to its limits by hunting for galaxies perhaps up to just 200 million years post-big bang. Distances to faraway galaxies are measured in redshift—the degree to which light we see from a galaxy has been shifted to the red end of the spectrum by the universe’s expansion. Eisenstein will hunt for galaxies beyond redshift 15, farther than any others conclusively seen. “We don’t yet have a convincing case of a galaxy beyond redshift 15,” he says. “It’s really exciting to be able to continue the search that started in this first year.”
Rohan Naidu of the Massachusetts Institute of Technology will also be scouring the distant universe, but not for those highest-redshift galaxies. Instead, his program, which he co-leads with Jorryt Matthee of the Swiss Federal Institute of Technology in Zurich (ETH Zurich), will be using a giant cluster of galaxies called Abell 2744 to gravitationally magnify the light of some smaller objects up to 750 million years after the big bang. The goal is to look for clumps of primordial gas, which could contain clusters of Population III stars—the first stellar generation thought to have lit up the universe. These long-theorized objects have yet to be directly seen but are expected to be composed almost entirely of pure hydrogen and helium—which should allow them to be enormous, each weighing in hundreds of times heavier than our sun. “We’re really pushing JWST to the hilt,” Naidu says. “We’ll get back some very promising regions that might be hosting these clusters.”
A key target of interest for JWST’s Cycle 1 was the TRAPPIST-1 system, an arrangement of seven Earth-sized worlds—some of which might be habitable—around a red dwarf star about 40 light-years from Earth. While three TRAPPIST-1 programs were selected in Cycle 1, however, only one has been selected this time, led by Michaël Gillon of the University of Liège in Belgium. He will hunt for atmospheres on TRAPPIST-1b and c, the two innermost planets of the system. Early studies of TRAPPIST-1b suggest it has no atmosphere, but Gillon says his technique—measuring the temperature difference between the day and night side of the planet—will tell us for sure. That could have important implications for TRAPPIST-1’s other more temperate worlds that might conceivably support life. “If we can demonstrate that one of these two planets has an atmosphere, we will be in a very good position to ask for an ambitious program on JWST to dig into the other planets,” he says.
Closer to home, Christopher Glein of the Southwest Research Institute (SWRI) in Texas will use JWST to probe Saturn’s moon Enceladus, which may harbor a habitable ocean beneath its icy surface. Observations from NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017, showed that the moon occasionally ejects water from this ocean via a plume at its south pole. While no spacecraft currently orbits Saturn, JWST is the next best thing. Incredibly, it will be able to “look for evidence of ocean chemistry” on the surface of Enceladus, Glein says. It will even be sensitive to certain substances, such as ammonia and various organic molecules, that could tell scientists about the habitability of the moon’s hidden ocean. In 2040 Enceladus’s south pole will enter a long winter of darkness that will last until 2055, making a potential future landing there to hunt for life difficult. Glein, however, is hoping to show with JWST that the moon’s polar plume is depositing frozen sea spray all across the surface, perhaps all the way up to the sunlit equator, where a landing could be more feasible. “JWST can act as a bridge between the Cassini era and a lander on Enceladus,” he says.
Not all areas of research were so lucky. David Kipping of Columbia University submitted two proposals to use JWST to hunt for moons orbiting exoplanets, known as exomoons. JWST “is the first machine humanity has ever built that is actually capable of doing this experiment,” Kipping says. But both proposals were rejected. “We’re definitely disappointed,” he says. “We really felt like this was a slam-dunk argument.”
JWST should be able to find exomoons down to the size of Europa, Kipping says, but even if it can’t, the results “would be pretty profound.” A failure to turn up an expected population of exomoons “would mean the models we use in our solar system aren’t universal,” he says, and could be a clue that our local abundance of lunar satellites is a bizarre deviation from cosmic norms. Time is of the essence, considering JWST is the only telescope now or for the foreseeable future that can look for exomoons. “JWST could last ten years, maybe longer,” Kipping notes. “If we never look for exomoons with it, we would really regret it. That would be such a shame.”
Levenson knows there will be some disappointment from the programs that were not selected. “There are lots of great ideas that we are not going to be able to observe during this cycle,” she says. For those that missed out, the deadline to try again and apply for Cycle 3 is October. “We have to keep trying,” Kipping says. “JWST is not going to be there forever.” For those lucky few that did make the cut, there are scientific riches to be had. “There’s this whole range of science that JWST is just great for,” says Levenson. “We’re definitely not done yet.”
