On December 25, 2021, an Ariane 5 rocket launched from Europe’s spaceport in French Guiana at precisely 7:20 a.m. The payload was the largest and most advanced orbiting optical observatory ever developed: NASA’s James Webb Space Telescope. Twenty-seven minutes and seven seconds after launch, the telescope separated from the second stage of the launcher. A jubilant Jean-Luc Voyer, director of operations for the launch, announced the separation, adding “Go Webb!”1 After a two-month journey, Webb arrived at its final destination 1.5 million kilometers from Earth. It is now in orbit at the second Lagrange point, a stable gravitational position created through the interaction of the sun and Earth.2
The successful launch was the culmination of many years’ work and planning by thousands of scientists and engineers working with NASA, the European Space Agency, and the Canadian Space Agency.3 Teams of technicians at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, are in the final stages of testing and calibrating the telescope and its instruments.4 By all accounts, the first calibration images (Figure 1) delivered by Webb surpass expectations.
Figure 1.
Images of the Large Magellanic Cloud as taken by the Spitzer Space Telescope (left) and the James Webb Space Telescope (right).
(Images: NASA/JPL-Caltech, NASA/ESA/CSA/STScI)
In the months and years ahead, Webb will observe light from the first galaxies that formed after the universe began nearly 14 billion years ago;5 study objects in our own solar system; observe the life cycle of stars in distant galaxies; and investigate the properties of planets orbiting other stars, so-called exoplanets. In all likelihood, Webb’s observations will transform our understanding of astronomical phenomena.
Webb observes light in the infrared spectrum from 800 to 24,000 nanometers (nm).6 This is well beyond the wavelength capabilities of the Hubble Space Telescope (100 to 2,500nm). Webb is in fact the successor to both Hubble and the Spitzer Space Telescope (3,000 to 160,000nm),7 and carries a mirror with a collecting area five times larger than that of Hubble and 45 times larger than that of Spitzer.
The first data from the new telescope are due to begin arriving in the next month.
The First Year
Some of Webb’s observational programs will be carried out by the scientists who built the instruments, developed the software, and set the scientific and design requirements of the observatory.8 These scientists have been allocated approximately 4,000 hours during the first year of operation. Roughly 460 additional hours are reserved for programs established by the STScI director, Ken Sembach. These programs are designed to demonstrate Webb’s scientific and observational capabilities.9
The majority of time available in the first year, around 6,000 hours, has been left to the astronomical community. NASA received approximately 1,200 proposals for first-year research and, after a peer review process, selected 266.10 Approximately one-third of the time available will be spent studying the properties of galaxies, one-quarter will be devoted to characterizing exoplanets, and one-sixth will focus on the stellar life cycle: the births, lives, and deaths of stars. In the remaining allocated time, Webb will study other kinds of objects, including those in our own solar system and gas accreting onto supermassive blackholes in the most distant galaxies found so far.
What do astronomers expect to learn from Webb’s observations? Based on everything we have learned from Earth-based telescopes and Hubble, we can make some educated guesses. In its first year alone, Webb is poised to answer deep questions about the nature of planets, the formation of galaxies, and the formation of their stars following the first moments after the Big Bang.
A Cornucopia of Planets
Our solar system contains a variety of planets. The inner planets—Mercury, Venus, Earth, and Mars—are all rocky, terrestrial worlds. Three receive enough sunlight that they could support liquid water. Mercury is barren and Mars has only a tenuous atmosphere. Earth is ideal for supporting animal and plant life, while Venus is enshrouded in a thick atmosphere of heat-trapping gasses. The outer planets include the two gas giants, Jupiter and Saturn, and, in Neptune and Uranus, two smaller ice giants. It is only in the last three decades that we have been able to investigate how common these types of planets are and how they are configured within solar systems.11
In March 2022, NASA announced that more than 5,000 planets beyond our solar system had now been catalogued.12 The first exoplanet orbiting a solar-type star was discovered in 1995. It was found orbiting the star 51 Pegasi in the constellation Pegasus based on periodic changes in 51 Pegasi’s radial velocity. This motion is manifested as a Doppler shift in the wavelength of light from the star’s motion.13 Radial velocities change in response to gravitational tugs on the planet from its parent star, which cause the star to wobble toward and away from Earth with a periodicity equal to the period of the orbit. The scale of the change in velocity amplitude depends on the planet’s mass and distance to its parent star. Further analysis showed that 51 Pegasi is orbited by a gas giant like Jupiter, but with an orbital period of only four days. The distance between the planet and the star is so small—about one-eighth that of Mercury and our Sun—that the estimated temperature of the planet is more than 1,000 kelvin (K), compared to Earth’s 288K. This was the first of a new kind of so-called hot Jupiter planet, for which there is no analog in our solar system.14
Most exoplanets are discovered by observing planetary transits. When an exoplanet passes between Earth and the parent star, the exoplanet blocks a small fraction of the star’s light. The sensitive telescopes now used to monitor large fields of stars are able to detect the telltale dips in light from a star when a planetary transit occurs. These transit events are repeated as the exoplanets orbit their parent stars, where the periodicity of the transit is equal to the period of the orbit.15 The amount of starlight blocked can be determined from the ratio of the cross-sectional area of the exoplanet to that of the parent star. This method is also used to measure an exoplanet’s size.16
In 2009, NASA launched the Kepler space telescope to monitor a single field of more than half a million stars in our galaxy.17 Kepler continued operating until 2018, by which time it had detected more than half of all the known exoplanets.18 Webb will be part of NASA’s ongoing mission to identify and catalog these objects.
Super-Earths and Planetary Formation
One of the most surprising discoveries that has emerged from the search for exoplanets is that most of these objects are larger than Earth but smaller than Neptune, whose diameter is about four times that of our own planet. Exoplanets in this category are dubbed super-Earths, or sometimes mini-Neptunes, depending on their composition.19 Although super-Earths appear to be four times as common as the larger gas giants such as Jupiter and Saturn,20 they have no counterpart in our solar system. For this reason, we have almost no information on their properties. Modeling super-Earth formation is challenging and requires carefully tuning their gas-accretion rate in protoplanetary disks; the gaseous envelope must grow, but not so quickly that the gas dominates the total mass of the planet and it becomes a gas giant.21
In its first year, Webb will spend thousands of hours studying the diverse properties of exoplanets, in particular the composition of their atmospheres.22 During a planetary transit, the light from a parent star passes briefly through the atmosphere of an exoplanet, where molecules such as CO2, H2O, CH4, and NH4 absorb light from the parent star. A measurable change in the star’s spectrum can then be observed. By measuring the strength of the absorption, astronomers can determine gas conditions, such as pressure and temperature, and the abundance of elements such as carbon, oxygen, and nitrogen.23
One of Webb’s largest exoplanet research programs will examine eleven super-Earths, each with a mass between one and three times the size of our planet.24 The goal is to measure changes in atmospheric composition as a function of planetary size. Several of the exoplanets in this study orbit the same star, which will help offset any potential issues arising from differences among solar systems.25 This program, led by Natasha Batalha of NASA Ames Research Center, and Johanna Teske of the Carnegie Institution for Sciences in Washington, will also study how atmospheric conditions change as planets age, a process shaped by interactions with gas escaping from planetary interiors.
Rocky Worlds and Habitability
Located 40 light years from Earth, TRAPPIST-1 is a relatively cold star in the constellation of Aquarius.26 It has a surface temperature of around 2,500K, compared to 5,800K for our sun, and only about a tenth of the mass. In 2016, a team led by Michaël Gillon from the University of Liège discovered that TRAPPIST-1 had at least three exoplanets.27 Subsequent observations from both the Spitzer Space Telescope and ground-based telescopes identified a further four exoplanets.
The ratio of a planet’s mass to its volume yields a constraint on its density. This can be compared to those of
- gas giants, mainly made up from hydrogen and helium with an average density of 0.7g/cm3;
- rocky worlds, with a core and mantle predominantly composed of ferrosilite and enstatite (Fe, Mg)SiO3 and an average density of 6g/cm3; and
- mixtures, including rocky worlds and ice giants with oceans and atmospheres.28
In the case of TRAPPIST-1, all seven exoplanets have densities consistent with rocky worlds—like Venus, Earth, and Mars—and at least three of them orbit at habitable distances from their star, where conditions are neither too hot nor too cold for liquid water to be present on their surface.
Laura Kreidberg of the Max Planck Institute for Astronomy is leading a first-year Webb program that will use mid-infrared imaging at wavelengths of 15,000nm to assess the atmosphere of TRAPPIST-1c, the second-closest exoplanet to the star.29 The objective is to determine if TRAPPIST-1c has a hot, thick atmosphere dominated by CO2, like that of Venus. This will be the first time the atmosphere of a rocky exoplanet is characterized.
Several other programs will attempt to characterize the atmospheres of TRAPPIST-1’s other exoplanets using spectroscopy.30 Simulations have shown that molecular features, such as the presence of CO2, will be easily identifiable in the the Webb data.31 All seven exoplanets were formed from the same protoplanetary nebula and have the same elemental composition. Any variations in their properties can be attributed to slight differences in their formation or interior outgassing.
There is a remote possibility that signs of life, such as the presence of molecular oxygen and methane,32 may be detected in the atmosphere of these exoplanets. The detection of life on a planet beyond our solar system would be an incredible achievement.
The Big Bang and the First Galaxies
By the latter part of the twentieth century, scientists had succeeded in measuring the cosmic microwave background (CMB) left over from the Big Bang.33 And by the end of the century, astronomers had determined that the expansion of the universe is accelerating.34 These are enormously consequential discoveries. They suggest that the universe has evolved dramatically over its 14-billion-year history. “The most important thing accomplished by the ultimate discovery of the [cosmic] radiation background,” Steven Weinberg remarked, “was to force us all to take seriously the idea that there was an early universe.”35
The density of baryonic matter prior to the formation of the first galaxies can be determined from the CMB.36 Directly following the Big Bang, the universe was too hot for neutral atoms to form. All matter existed in a plasma of free electrons, protons, and ionized helium nuclei, interacting through the exchange of photons. After 370,000 years, the adiabatic expansion of the universe caused the gas to cool to 3,000K. Free electrons could then bind to nuclei, forming neutral hydrogen and helium. Photons remaining in plasma no longer had sufficient energy to re-ionize atoms and streamed freely in all directions. Cosmologists refer to this moment in cosmic history as recombination.37 Observations have shown that the CMB is smooth and uniform in all directions. There are small fluctuations in the CMB, but only on the order of one part per 100,000.38 These correspond to inhomogeneities in the density of matter and radiation at the time of recombination, where higher densities were slightly colder than their surroundings. After finding evidence for dark matter, cosmologists developed a model in which gravitational attraction in the denser fluctuations allows them to overcome the expansion from the Big Bang. These fluctuations merge to form clumps, termed dark-matter halos, that grow over time, merging with other halos.39 It is at the center of these structures, where gas cools and condenses, that galaxies are formed. The theory has many holes, and cosmologists have great difficulty in using it to explain how baryonic matter can coalesce and form stars. Webb will likely not see these first galaxies as they form, but it will accurately measure their numbers and their properties.
The existence of the CMB means the entire universe was once filled with neutral hydrogen and helium. But in the modern universe, all observations of intergalactic gas—the intergalactic medium (IGM)—show that it is almost entirely ionized, with only a small fraction of neutral gas.40 Measurements of the IGM illuminated by quasars suggest that this state of ionization has existed for at least 12 or 13 billion years. As the clock winds back even earlier, the neutral fraction of the IGM is significantly greater.41 The period when the IGM became ionized again following recombination is known as reionization.
The cause of this reionization remains one of the most significant unsolved problems in cosmology. Ionizing the gas in the IGM requires sources of ultraviolet emission amounting to more than 13.6eV, the binding energy of an electron in the ground state of hydrogen. Astronomers expect that stars in the first galaxies produced this ultraviolet radiation, but observations of galaxies both nearby and 10 to 12 billion light-years distant show that most of this radiation is absorbed before it can leak from galaxies into the IGM.42 The timescale is also an issue. To match our observations, the galaxies must reionize the IGM in roughly the first 700 million years. Searches for galaxies with sufficient ultraviolet radiation have come up short.43 To make inferences about reionization, astronomers are currently extrapolating from existing trends. The first billion years of the universe, commonly referred to as the dark ages, remain the least understood.44 Studying galaxies in the reionization era is a priority for Webb.
The Epoch of Galaxy Formation
In 1994, Robert Williams, then director of STScI, dedicated more than 100 hours of his discretionary time with Hubble to study a tiny patch of sky—smaller than a fingernail at arm’s length—in the constellation Ursa Major. Hubble was able to detect the light from approximately 3,000 galaxies within the region chosen by Williams and his colleagues.45 The resulting composite image, known as the Hubble Deep Field, was a revelation.46
Most of the galaxies in the Hubble Deep Field are up to 12 billion light years away, meaning that we are seeing them as they looked 12 billion years ago. Due to the expansion of the universe, there is a correlation between distance and a galaxy’s redshift. In the Hubble Deep Field, we see galaxies to redshifts of approximately 5. The galaxies with the highest redshifts are smaller and ablaze with star formation, forming new stars many times faster than the Milky Way.
In the last few decades, astronomers have targeted other deep fields with more sensitive instruments, surveying larger areas in greater depth and incorporating information from light at other wavelengths—from x-rays to radio waves. The peak period of star formation in galaxies is now thought to have occurred nearly 10 billion years ago around a redshift of 2.47 Our own solar system was formed about 4.5 billion years ago, at the tail end of this period.48
Upgrades in the instrumentation to Hubble have enabled astronomers to detect light from distant galaxies at redshifts of between 7 and 8,49 with a tenuous detection of at least one galaxy at a redshift of 11.50 At this point, Hubble has reached the limit of its capabilities. The wavelength of light from distant galaxies is redshifted by a factor of (1 + z). As a result, Hubble can observe the light from galaxies only until it is redshifted further into the infrared, beyond the sensitivity of its instruments.
Webb will allow astronomers to investigate an even earlier era, when the universe was younger than 700 million years old. At that time, the first galaxies must already have formed from clouds of gas containing primal elements left by the Big Bang—hydrogen, some helium, and trace amounts of other light elements such as lithium. The gas clouds could coalesce in dark-matter halos, but they would still have needed to cool below 10,000K in order to obtain the densities necessary for stellar fusion. Star formation in modern galaxies relies on gas cooling mechanisms involving atomic and molecular transitions of oxygen and carbon.51 These heavy elements are fused in stars and would not have been available during the formation of the first galaxies.52 Yet all the galaxies identified to date—out to redshifts of between 8 and 9—show evidence for emission from carbon and oxygen.53 The data and observations are paradoxical: stars need heavy elements to form; heavy elements come from stars. Webb may provide the breakthrough that resolves these questions.
The First Galaxies and Reionization
In its first year, astronomers expect Webb to characterize galaxies and their properties during the epoch of reionization.54 Webb is capable of detecting light from galaxies with redshifts greater than 10, and possibly as high as 15,55 a mere 300 million years after the Big Bang. This spans the full range of redshift when reionization is expected to have occurred (z = 6 – 11).
Dan Coe, at STScI, leads a program that will use Webb’s spectroscopy capabilities to confirm the redshift and study the nature of a galaxy at z ~ 11,56 currently the best candidate for the most distant known object.57 Webb will also look for nebular emission from heavy elements such as carbon and oxygen. The intensity of emission from these elements will reflect their abundance and the nature of the ionizing stars. This area will be a major focus for Webb in its first year.58
Other first-year programs will use imaging in the near-infrared to address two fundamental questions about galaxies during reionization. A program led by Steven Finkelstein at the University of Texas at Austin, Nor Pirzkal at STScI, and me will investigate whether there are sufficient galaxies to produce the ultraviolet light needed to reionize the universe. Our team will use more than 100 hours of Webb imaging to target one of the Hubble Ultra-Deep legacy fields in the constellation of Fornax.59 The combination of Hubble and Webb ultra-deep imaging will measure the number of galaxies at redshifts of 10, and possibly to 14, dating from the beginning of reionization. Webb data will reveal how fast galaxies are forming stars and will be used to test models for how efficiently baryons in these objects can cool and form stars. This is an important consideration in estimating how rapidly these galaxies can reionize their surroundings.
A second question concerns how reionization occurred. One of the conclusions drawn from studies using other telescopes is that reionization did not happen everywhere at once. Forming more stars, large galaxies are the first to ionize regions of space.60 The nature of dark matter means that these galaxies will reside in structures that are highly clustered. Astronomers can use the size and clustering of galaxies during reionization to understand how effectively the ionizing ultraviolet radiation from galaxies escapes as a function of dark-matter halo size.
A program led by Jeyhan Kartaltepe of the Rochester Institute of Technology and Caitlin Casey of the University of Texas at Austin will use more than 200 hours to image an area of sky larger than the full moon in the constellation Sextans.61 The program will detect more than a half-million galaxies, including many thousands experiencing reionization. The goal is to identify rare, luminous galaxies—the ones forming stars most rapidly—and to measure their clustering. This will also be our first chance to see if reionization is mostly a result of luminous galaxies, which cluster more, or fainter galaxies, which cluster less.
The first year of Webb is only the beginning for astronomers exploring the early universe. Indeed, its data may generate more questions than answers. Even Webb, powerful as it is, may not be enough—the first stars may still remain beyond its detection capabilities.62 Nevertheless, Webb will be a time machine into the reionization era. The galaxies it observes will have been enriched by, at most, only a few generations of stars. By reconstructing the panchromatic light from these galaxies, astronomers can infer their formation histories and model the first galaxies: how they formed, and how they reionized the intergalactic gas around them.
I am sure surprises await.
