NASA's James Webb Space Telescope

Science Themes

Image showing the birth of stars and protoplanetary systems.

JWST will provide cutting edge observations that will delve into the mysteries of the first objects to form in the early universe, the assembly of galaxies, the birth of stars and planetary systems, planetary systems and the origins of life, and much more.

The End of the Dark Ages: First Light and Reionization

Theory and observation have given us a simple picture of the early universe. The Big Bang produced (in decreasing order of present mass-energy density):

  • dark energy (the cosmic acceleration force)
  • dark matter
  • hydrogen
  • helium
  • cosmic microwave and neutrino background radiation
  • trace quantities of lithium, beryllium, and boron

As the universe expanded and cooled, some hydrogen molecules were formed, and these in turn enabled the formation of the first individual stars. The first stars formed in those regions that were the most dense.

According to theory and the Wilkinson Microwave Anisotropy Probe (WMAP), the universe has expanded by a factor of 20 since that time, the mean density was 8000 times greater than it is now, and the age was about 180 million years. Also according to theory, these first stars were 30 to 1000 times as massive as the Sun and millions of times as bright and burned for only a few million years before meeting a violent end. Each one would produce either a (superluminous) pair-instability supernova or collapse directly to a black hole. The supernovae would enrich the surrounding gas with the chemical elements produced in their interiors, and future generations of stars would all contain these heavier elements (“metals”). The black holes would start to swallow gas and other stars to become mini-quasars, growing and merging to become the huge black holes now found at the centers of nearly all galaxies. The distinction is important because only the supernovae return heavy elements to the gas. The supernovae and the mini-quasars, if beamed, should be observable by the JWST. Both might also be sources of gamma ray bursts and gravity wave bursts that could be discovered by other observatories and then observed in followup by JWST. In addition to the supernovae of the first light stars, JWST will also be able to detect the first galaxies and star clusters.

The JWST First Light theme science goal is to find and understand these predicted first light objects. To find them, the JWST must provide exceptional imaging capabilities in the near IR band. To verify that the high-z galaxies are indeed made of primordial stars and do not contain older stellar populations, mid-infrared observations are required. An observational approach to identify these objects has been described in the JWST SWG First Light white paper.

Four images and simulations of the deep field

Distant galaxies as seen by Hubble and JWST. The two images at the top show the Hubble Ultra Deep Field obtained with WFC3/IR in three filters. The two images at the bottom are simulations of what the deep field may look like with JWST/NIRCam. JWST images will be both sharper and extend to fainter limits compared to Hubble.

Assembly of Galaxies

Galaxies are the visible building blocks of the universe. Theory and observation also give us a preferred picture of the assembly of galaxies. It seems that small objects formed first, and then were drawn together to form larger ones. This process is still occurring today, as the Milky Way merges with some of its dwarf companions, and as the Andromeda Nebula heads toward the Milky Way for a future collision. Galaxies have been observed back to times within one billion years after the Big Bang.

Despite all the work done to date, many questions are still open. We do not really know how galaxies are formed, what controls their shapes, what makes them form stars, how the chemical elements are generated and redistributed through the galaxies, whether the central black holes exert great influence over the galaxies, or what are the global effects of violent events as small and large parts join together in collisions.

The JWST Assembly of Galaxies theme goal is to observe galaxies back to their earliest precursors (z > 10) so that we can understand their growth and their morphological and metallicity evolution. The JWST must provide imaging and spectroscopy over the 0.6 to 27 µm band to meet this objective.


Birth of Stars and Protoplanetary Systems

While stars are a classic topic of astronomy, only in recent times have we begun to understand them with detailed observations and computer simulations. A hundred years ago we did not know that they are powered by nuclear fusion, and 50 years ago we did not know that stars are continually being formed. We still do not know the details of how they are formed from clouds of gas and dust, or why most stars form in groups, or how planets form with them. We also do not know the details of how they evolve and liberate the “metals” back into space for recycling into new generations of stars and planets. In many cases these old stars have major effects on the formation of new ones.

Observations show that most stars are formed in multiple star systems and that many have planets. However, there is little agreement about how this occurs, and the discovery of large numbers of massive planets in very close orbits around their stars was very surprising. We also know that planets are common around late-type stars (cooler and less massive than the Sun), and that debris disks might signal their presence.

The JWST Birth of Stars and Protoplanetary Systems theme goal is to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars, to the genesis of planetary systems. JWST is uniquely primed to solve these mysteries given the combination of its high resolution observing modes, imaging, spectroscopy, and coronographic capabilities, and superb near and mid-IR sensitivity.

Planetary Systems and the Origins of Life

Understanding the origin of the Earth and its ability to support life is a key objective for all of astronomy and is central to the JWST science program. Key parts of the story include understanding the formation of small objects and how they combine to form large ones, learning how they reach their present orbits, learning how the large planets affect the others in systems like ours, and learning about the chemical and physical history of the small and large objects that formed the Earth and delivered the necessary chemical precursors for life. The cool objects and dust in the outer Solar System are evidence of conditions in the early Solar System, and are directly comparable to cool objects and dust observed around other stars.

The JWST Planetary Systems and Origins of Life theme goal is to determine the physical and chemical properties of planetary systems, and investigate the potential for the origins of life in those systems. JWST must provide near and mid IR imaging and spectroscopy to observe exoplanets.

JWST's planetary exploration theme also includes a rich solar system science case that includes imaging and spectroscopic characterization of Mars and the outer planets, Kuiper belt objects, dwarf planets, icy moons, and comets.