It’s no secret that the James Webb Space Telescope (JWST) is the most advanced telescope in history. As such, demand to use it is high—incredibly high. Last week the Space Telescope Science Institute (STScI) in Maryland announced which programs it had picked to win some of the telescope’s precious time in its third year of science observations. Beginning this July and officially called Cycle 3, JWST’s latest solicitation received a record-breaking 1,931 proposals—the most ever for any space telescope in history. Such immense interest is “not surprising,” says Christine Chen, an associate astronomer at STScI. “Basically, out of the box, everything has worked extremely well.” With that popularity, there are winners and many more losers; only 253 proposals were selected.* But among the winners there is a wealth of exciting science, including surveys to look for the universe’s first galaxies (JWST’s primary forte), studies of possibly life-harboring exoplanets and, for the first time, an attempt to leverage JWST’s power to find exomoons—natural satellites orbiting worlds beyond the solar system.
David Kipping of Columbia University has been banging the exomoon drum for many years. Having unsuccessfully proposed exomoon searches for every previous JWST cycle, he had tempered his expectations about his latest hopeful attempt—which was why, when he glanced at his smartphone while teaching a class on February 29, he was so surprised to see an e-mail notification from STScI. “It just said, ‘We are pleased to inform you...,’” he says. “I was pinching myself. I didn’t think it was real—I thought it was a cruel joke. I had to stop class!” That e-mail confirmed that Kipping’s wildest dreams had come true; JWST, he says, is by far the most capable current observatory for finding—and then confirming—candidate exomoons. An exomoon program he had proposed, led by his graduate student Ben Cassese, also at Columbia, had been selected. “I took a screenshot,” says Cassese. “I was like, ‘no takebacks.’”
Their program will look at a Jupiter-sized planet orbiting the star Kepler-167 about 1,100 light-years from Earth. The planet transits its star, crossing the star’s face to cast a worldly shadow toward Earth about once every 1,000 days. The next transit will unfold on October 25, when Cassese and Kipping will be looking with JWST’s far-seeing infrared eye in search of telltale evidence for one or more accompanying exomoons down to the size of Jupiter’s own Ganymede or Callisto. The team had submitted three exomoon proposals, but this was its favorite, Kipping says. “We didn’t have anything that was as good as this,” he says, owing to the size of the planet, its distance from the star and the fact that it transits. If the researchers find something, we will have proof that exoplanets can host moons, opening up a new realm of scientific inquiry. If they don’t, that null result will help set important constraints on moon formation—and perhaps hint that the lunar-laden worlds of our system are a cosmic rarity rather than commonplace.
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Emily Pass of the Center for Astrophysics | Harvard & Smithsonian also had an exomoon proposal selected—the only other JWST exomoon search approved by STScI. Hers will examine two Earth-sized worlds around a red dwarf star, TOI-700, about 100 light-years away. The goal is to look for “moons that are true analogues to the Earth-moon system,” she says. Her study will also be an important data point for learning the likelihood of moons forming among a red dwarf star’s worlds. Early analysis of the famous TRAPPIST-1 red-dwarf system, which contains seven Earth-sized planets, has already dampened that expectation. By analyzing existing JWST data of one of the planets, TRAPPIST-1h, “we could rule out a moon orbiting that planet that was larger than Earth’s moon,” Pass says.
TRAPPIST-1 is, of course, a system of immense scientific interest every JWST cycle. Ongoing studies of data gathered during previous cycles are working out if some of the planets in the star’s notional habitable zone actually possess an atmosphere—and, thus, the possibility of liquid water on their surface. From there, future work might then use JWST to scour any of TRAPPIST-1’s promising planets for signs of life. Natalie Allen of Johns Hopkins University will lead a program in Cycle 3 to study TRAPPIST-1 e, one of the habitable-zone planets, for signs of an atmosphere. Past efforts have been stymied by the planets’ tempestuous host star, which continually belches out powerful flares that all too easily disrupt observations. So Allen and her colleagues will watch the moment 1 e transits the star at the same time as the innermost planet, 1 b, which is known to have no atmosphere. “It’s a bare rock,” she says. By simultaneously observing those dual, overlapping transits, subtracting the star’s activity and discerning the existence of an atmosphere on 1 e should become easier. The boost in sensitivity, in fact, might even allow JWST to sniff out some of that world’s chemistry. “We should be able to get the spectroscopic composition of the atmosphere,” Allen says: a key step toward identifying any prospects of life that involves parsing a planet’s light into a rainbowlike spectrum of multiple wavelengths, or colors.
Much of JWST’s focus in Cycle 3 will, as expected, be on studies of galaxies. Of the approximately 5,500 hours of observing time awarded in Cycle 3, about a third went to galaxies, with exoplanets close behind and other fields of astronomy, such as studies of objects in our solar system, taking the rest of the time. Already JWST has upended our knowledge of the earliest galaxies in the universe; it has broken the record again and again for the most distant one known—a record that at present stands at just 320 million years after the big bang 13.8 billion years ago.
Jeyhan Kartaltepe of the Rochester Institute of Technology will lead a survey in Cycle 3 to significantly bolster the number of known high-redshift galaxies—those in the earliest universe, with their light most stretched to the red end of the electromagnetic spectrum by the expanding cosmos. The researchers will do so by employing a novel technique. In the past JWST has found such galaxies via simple, quick photometry by looking for them by their comparative brightness at reddish wavelengths; more time-intensive spectroscopy can then be used to follow up on the best candidates to determine their true distance and age. Rather than continue this tedious two-step process, Kartaltepe will instead rely on a potentially more efficient approach called slitless spectroscopy, essentially allowing the telescope’s spectroscopic capabilities to measure every object in an image at once. This, she says, could net about 3,000 high-redshift galaxies from the first 750 million years of cosmic history—an immense increase. “Cycle 1 and 2 were really about discovery,” she says. “Now the idea is, what does it all mean?”
Rohan Naidu of the Massachusetts Institute of Technology will be tackling a similar topic in Cycle 3 by studying some of the particularly large and red galaxies that have surprisingly popped up in JWST’s observations of the first billion or so years post–big bang. Called “little red dots,” they appear much brighter and more massive than theorists have expected galaxies at this epoch to be. “The position that I and my collaborators have argued for is that there is that there’s hidden phases of black hole growth that we didn’t really see until now,” Naidu says. His program will seek to settle the debate about little red dots once and for all. Not all of Naidu’s proposals to use the telescope were as successful, though, and many other astronomers missed out entirely. “It’s just so brutal,” he says. “There’s so many excellent proposals that don’t go through.”
Grant Tremblay, an astronomer at the Center for Astrophysics | Harvard & Smithsonian, was part of JWST’s Telescope Allocation Committee (TAC), a group of hundreds of scientists tasked by STScI to pick winners and losers from the torrential flood of proposals. The TAC ranked each proposal on a one-to-five scale based on scoring for scientific merit and various other factors. But while some proposals clearly rise to the top for selection, others in a murkier middle ground can be picked seemingly by chance. Tremblay wonders if there might be a better way to allocate JWST’s truly precious time. “What is the difference between a 2.8 ranked proposal and a 2.9?” he says. “I would love to try a random number generator on that vast middle ground.”
In our own solar system, Mike Brown of the California Institute of Technology will use JWST to study Saturn’s icy moon Enceladus. He will be looking to work out the origin of plumes of water that erupt from the moon’s poles. It’s thought these plumes may originate from an ocean beneath the moon’s icy surface, in which case they could carry molecular evidence of habitability and even life within that dark abyss that could be collected by spacecraft. But they may alternatively originate from within the icy crust itself, quelling exciting visions of such remote submarine sampling. Brown will watch with JWST to see if the plumes are constant. If they are, that would point to an icy crust explanation. If they instead erupt in bursts, they more likely originate from the ocean, timed to the periodic bending and cracking of the overlying ice sheet. “It should be pretty dead obvious,” Brown says. “We need to know the answer to this question before we send anything.”
Michael Zhang of the University of Chicago will use JWST to study a peculiar type of planet, one orbiting not a regular star but a pulsar, a rapidly spinning neutron star left behind by an explosive supernova. For the first time ever, he and his colleagues will attempt to study the atmosphere of such a planet, in this case a gas giant orbiting the pulsar PSR J2322-2650, some 750 light-years from Earth. The world orbits so near its host, Zhang says, that its atmosphere “might be escaping” and “accreting onto the pulsar.” That may explain how pulsars such as this one—a so-called millisecond pulsar that spins thousands of times per minute—are able to rotate so fast. “One possible answer is accreting material from a planet,” he says.
This is not the only cosmic mystery JWST might solve in Cycle 3. Elena Gallo of the University of Michigan will use the telescope to study a galaxy in the distant universe that may contain a direct-collapse black hole. These are hypothesized astrophysical products of Einstein’s general theory of relativity, in which clouds of gas in the universe’s first 250 million years collapsed not to form stars but rather straight into black holes; such objects may serve as “seeds” for the subsequent formation of supermassive black holes, the circa billion-solar-mass titans found at the centers of large galaxies (including our own Milky Way). The galaxy that Gallo will observe appears to harbor a black hole as massive as the galaxy itself. “A black hole that [becomes] so massive so fast is a telltale [sign] of a direct collapse,” she says.
Not all JWST Cycle 3 proposals will actually use the telescope itself. Danna Qasim of the Southwest Research Institute in Texas will perform a laboratory experiment as part of Cycle 3 to try and solve the sulfur-depletion problem. We know that sulfur is one of the six elements crucial to life, but its cosmic origins are somewhat mysterious. “Our own solar system came from a dense interstellar cloud,” Qasim says. “We don’t know where 99 percent of the sulfur is in these clouds—in the gas or in ice or in minerals.” Without knowing such specifics, scientists can’t accurately trace the pathway this life-giving element takes to reach embryonic worlds in nascent planetary systems. So Qasim will shoot electrons into an ice analogue we might expect to see in such clouds to learn what sulfur-bearing molecules are formed. That might give us a handle on the problem—and highlights that for the most sought-after telescope in history, even space is not the limit.
*Editor’s Note (3/8/24): This sentence was edited after posting to correct the number of proposals that were selected.