3.1

Signature Science Case #1: Finding habitable planet candidates

Indirect planet discovery techniques have shown that small, rocky planets are not rare. Results from NASA’s Kepler mission indicate that the fraction of Sun-like stars with roughly Earth-size planets in their habitable zones, is about 24% [2]. These zones span the range of distances from the stars where we think rocky planets can have liquid water – an essential material for all life on Earth – on their surfaces. This revelation drives us to proceed to the next step: build the capabilities to reveal the character of rocky exoplanets and measure the fraction that are truly Earth-like (a.k.a. exoEarths). We need to probe rocky planet atmospheres and determine their thermal/chemical states by measuring molecular abundances, including water vapor and other greenhouse gases.

Furthermore, measuring the frequency of habitable conditions requires observations of a large number of candidate exoplanets (dozens). To find and study that many candidates, an even larger number of stars must be observed (hundreds). A systematic survey of exoplanets most similar to the Earth – rocky planets in the habitable zones of Sun-like stars –guarantees that whatever we find will be of scientific value. In the absence of water vapor detections, we would know that global-scale surface oceans are rare on rocky worlds in the habitable zone. This null result would still transform our understanding of habitability as a planetary process, and further confirm the special nature of our home. On the other hand, if we find that Earth-like conditions are common on rocky exoplanets, then a stunning vista of hospitable new worlds will be unveiled.

Scientists have calculated that to study the habitability of rocky planets around Sun-like stars, we must collect and analyze light from the planets themselves, i.e. obtain direct images and spectra [3]. However, direct observations of rocky exoplanets around Sun-like stars demand the very high contrast possible only with a space telescope coupled to a high-performance starlight suppression system [3]. The LUVOIR Final Report describes the survey strategy developed to obtain the needed measurements while dealing with all of the expected complexities in an efficient manner, and the calculations to determine expected exoplanet yields. The expected exoplanet yields obtained in initial 2-year coronagraph surveys with LUVOIR-A and -B returned by our analysis appears in Figure 4. The LUVOIR-B observatory meets the requirement set by the LUVOIR STDT for a statistically significant exoEarth candidate survey (28 habitable planet candidates). With 54 habitable planet candidates, LUVOIR-A surpasses the requirement and provides ample margin against astrophysical and technological uncertainties. The same code with the same astrophysical input assumptions was used to calculate exoplanet yields for the Habitable Exoplanet Observatory (HabEx) mission concept.

Figure 4:

LUVOIR discovers dozens of habitable planet candidates and hundreds of other kinds of exoplanets. The chart shows exoplanet detection yields from an initial 2-year survey optimized for habitable planet candidates with LUVOIR-A (blue bars) and -B (green bars). The first column shows the expected yields of habitable planet candidates. Non-habitable planets are detected concurrently during the 2-year survey. Color photometry is obtained for all planets. Orbits and partial spectra capable of detecting water vapor and/or methane are obtained for all habitable planet candidates. Credit: C. Stark (STScI) / J. Friedlander (NASA GSFC)

Figure 5 shows that the expected yield of habitable planet candidates is a strong function of telescope diameter and changes with aperture geometry; these realities drove the design of the LUVOIR concepts. The inscribed diameter is the diameter of the largest circle completely contained within the telescope primary aperture; this is the parameter that has the single greatest impact on yields. The yields also depend on whether the aperture is unobscured (off-axis) or obscured (on-axis), which affects the choice of coronagraph type. In Figure 5, off-axis monolithic and segmented telescopes are shown with the green and red curves, respectively. Both sets of curves end at the largest size that appears feasible for that type of telescope. The range between the upper and lower curves spans different approaches to observing strategy. The off-axis telescopes are paired with vector vortex charge 6 (VC6) coronagraphs. The smooth joining of the green and red curves show that new coronagraph designs have eliminated any penalty for segmentation.

Figure 5:

Telescope size, aperture geometry, and coronagraph type all affect the expected detection yields of exoEarth candidates. On the x-axis, the inscribed diameter is the diameter of the largest circle completely contained within the telescope aperture. The green, red, and blue curves show yields for different combinations of telescope aperture geometry and coronagraph type, more fully explained in the main text. The yellow curve shows the yields for a single starshade paired with a 4-m telescope. Credit: Stark et al. (2019)

On-axis segmented telescopes are shown with blue curves in Figure 5. They are paired with apodized pupil Lyot coronagraphs (APLC), which perform better with obscured apertures than VC6. The yield gap between red and blue curves shows that there remains a penalty for obscuration. The two black points show the expected yields for LUVOIR-B (6.7-m inscribed diameter, off-axis segmented paired with VC6) and LUVOIR-A (13.5-m inscribed diameter, on-axis segmented paired with an APLC). The yellow curve shows the expected yield from a single starshade paired with a 4-m telescope and without refueling of the shade’s propulsion system. This curve demonstrates that, unlike coronagraph-only systems, the yield from starshade-only systems does not continue increasing with telescope diameter.

3.2

Signature Science Case #2: Searching for biosignatures and confirming habitability

LUVOIR will not only achieve the exoEarth census but will probe the atmospheres of those planets for biosignature gases, possibly revealing the first signs of life outside our home planet. The spectrum of the Earth contains features arising from gases of biological origin, like O₂ and methane, in addition to features from water vapor and other greenhouse gases (Figure 6). The Earth is the only planet we know of teeming with surface life that strongly affects the planet’s atmosphere. Thus, there are good reasons to focus serious searches for life outside the solar system on exoplanets that are as much like the Earth as possible. By targeting exoplanets with sizes and orbits similar to Earth’s and host stars similar to the Sun, we hope to increase our chances of finding – and recognizing – extraterrestrial life.

Figure 6:

Earth’s atmospheric composition has changed significantly over its inhabited history (left), as has its spectral features (right). For instance, the canonical biosignatures of modern Earth (e.g. O₂, O₃) cannot be observed in the Archean eon. A wide wavelength range enables higher fidelity spectral characterization by providing observations of many atmospheric species, and often multiple absorption bands of a given species. This breaks degeneracies between overlapping bands (e.g., overlapping CH₄. and H₂O absorption features in the near-infrared) and enables the search for non-traditional biosignatures. Credit: G. Arney, S. Domagal-Goldman, T. B. Griswold (NASA GSFC)

However, it is important to understand that no one molecule (or pair of molecules) is automatically a biosignature in every context. Many studies over the last decade have identified pathways to atmospheres containing abundant oxygen without life, especially for planets around M dwarf stars [4]. The key is to measure the abundances of many molecules in the whole atmosphere, and then use those abundances to estimate the production and destruction rates predicted by physics and chemistry. To calculate the production and destruction rates, a good deal of knowledge about the atmosphere’s context will be needed, including the planet’s orbit, mass, and the characteristics of the host star. Scientists will try to explain the character of an atmosphere with two sets of models: one where the physics and chemistry of the atmosphere are purely driven by geological and astrophysical processes; and a second where biological sources are also considered. Those planets for which only the second set of models can explain the data are those for which evidence of life may be robustly claimed.

Therefore, confident life-detection has at least three fundamental requirements: 1) assess a wide range of atmospheric molecules, 2) measure or constrain their abundances with some precision, and 3) understand the planet’s context within its system. The first requirement demands direct spectra with broad wavelength coverage from the near-ultraviolet (NUV) to the near-infrared (NIR). The LUVOIR telescopes and starlight suppression systems can span these wavelengths of light and access the important constituents of planetary atmospheres, including water (H₂O), carbon dioxide (CO₂), molecular oxygen (O₂), ozone (O₃), and methane (CH₄). Important indicators of false positive biosignatures (e.g., CO) can be observed as well. The second requirement demands good quality spectra to measure abundances, which drives the required telescope and instrument sensitivity. The third requirement demands collection of a range of supporting information, including planet masses and far-UV spectra of stellar activity-driven emission from the host stars. LUVOIR’s great sensitivity and flexibility will enable the variety of observations needed to properly understand a planet as a complete system.

3.3

Signature Science Case #3: The search for habitable worlds in the solar system

The search for habitable environments also takes place closer to home. Scientists have discovered that several moons of the outer solar system have liquid water beneath their icy surfaces. These sub-surface oceans must be heated from below, which may also provide the energy needed for life. Plumes emanating from openings in the icy shells, such as those observed from Europa and Enceladus [5, 6], may allow glimpses into the deep oceans. LUVOIR has an important role to play in determining the currently unknown strength and frequency of plume activity by providing observations over long time baselines. Such information will be a valuable support for future spacecraft visiting these other ocean worlds to search for signs of life with focused investigations. The FUV multi-object spectroscopy capability of LUMOS is particularly well suited to spectroscopic imaging of auroral emission from Europa’s plumes in multiple atomic lines (e.g., the neutral hydrogen Lyman α line at 121.5 nm and the neutral oxygen line at 130.4 nm). Furthermore, the large apertures of the LUVOIR telescopes may permit emission from individual jets to be spatially resolved (Figure 7). This key feature will aid disentangling plume strength vs. plume morphology.

Figure 7:

LUVOIR can monitor individual plumes from solar system ocean moons. The left panel shows an aurora on Europa observed with HST [5]. This far-UV hydrogen emission comes from dissociation of water vapor in plumes escaping through the moon’s ice shell. The center and right panels show simulations of how this emission might look observed with LUMOS on LUVOIR-B and -A. The moon’s surface is bright due to reflected solar hydrogen emission, which was below the background in the HST image. Credit: G. Ballester (LPL) / R. Juanola-Parramon (NASA GSFC)

3.4

Signature Science Case #4: Comparative atmospheres

The capabilities that will allow LUVOIR to investigate dozens of potentially habitable worlds will also allow comprehensive study of a huge range of exotic exoplanets, which by and large will be easier to observe than Earth-like exoplanets. Many of these comparative planetology studies cannot be done by studying the planets of the solar system alone; for example, investigations of the effects of host star mass, stellar metallicity, system age, and a broad range of planet masses and orbits. For this diverse set of planets, the primary objectives are to measure the abundances of the primary atmospheric constituents; characterize hazes and clouds; and assess atmospheric loss rates. These objectives will be achieved with a combination of direct and transit spectroscopy, both obtained with LUVOIR. The initial 2-year survey for habitable planet candidates will uncover large numbers of other types of exoplanets (Figure 4), which will range from hot rocky planets to cold giant planets.

3.5

Signature Science Case #5: The formation of planetary systems

LUVOIR will provide unprecedented measurements to help understand the processes that give rise to the extraordinary diversity of planetary systems found over the last decades. LUVOIR will examine the formation and evolution of planetary systems over a wide range of stellar ages by executing unprecedented spectroscopic studies of the most abundant materials in protoplanetary disks (Figure 8); revealing the morphology of young planetary systems; and providing the data for dynamical studies of mature planetary systems to uncover clues to their formation eons ago.

Figure 8:

LUVOIR can powerfully and efficiently measure the hidden materials of planet formation. Left: The Orion Star Forming Region with the LUMOS multi-object spectroscopic field of view overlaid three times (green squares). The blow-up panels show several planet-forming disks that fall in each field, with LUMOS spectroscopic apertures overlaid (green rectangles). UV spectra of all the disks in each field can be obtained simultaneously. Right: Simulated partial LUMOS spectrum of a protoplanetary disk showing absorption from two key molecular species (H₂ and H₂O). Credit: NASA / ESA / HST / K. France (CU – Boulder) / J. Tumlinson (STScI)

3.6

Signature Science Case #6: Small bodies in the solar system

There are many things still to be discovered and understood about the bodies within the solar system itself. Sensitive, high resolution imaging and spectroscopy of solar system asteroids, dwarf planets, comets, and Trans-Neptunian Objects (TNOs) that will not be visited by spacecraft in the foreseeable future can provide vital information on the processes that formed the solar system ages ago. In particular, the Kuiper Belt is the only available remnant of the solar system’s primordial planetesimal population accessible for direct study. The characteristics of this population, including both size and orbital distributions, are directly relevant to understanding process of planetesimal formation and the migration of the giant planets. In particular, the size distribution of the smallest bodies provides critical constraints on theories of the formation of the first planetary building blocks.

HST remains the most sensitive optical telescope ever built, with the ability to detect TNOs with diameters in the range of ~35 km at 40 AU. However, this limit may leave 99% of the inner TNO population invisible to observers. Deep imaging programs with LUVOIR can detect smaller TNOs than any other current or planned observatory: ~2 km bodies at 40 AU with LUVOIR-A and ~4 km with LUVOIR-B. These size limits are two orders of magnitude smaller than the limit for the Large Synoptic Survey Telescope (LSST) wide field survey and an order of magnitude smaller than the limits for proposed or potential deep drills with JWST and future ground-based 30-m-class telescopes (Extremely Large Telescopes, or ELTs).

3.7

Signature Science Case #7: Connecting the smallest scales across cosmic time

The Universe began smoothly, but is now richly structured on all scales. The prevailing cold dark matter (CDM) paradigm for structure formation describes how the first seeds of structure evolved into the dynamic shape of the “cosmic web” of filaments and voids traced out by galaxies. The most current CDM models accurately predict the numbers and properties of “dark matter halos” where galaxies and clusters of galaxies form. However, fundamental questions remain unsolved: What is the dark matter? Do dark matter-based models of structure formation correctly predict small (sub-galactic) size scales? Does dark matter decay? CDM models were built to explain large-scale structure. Whether they can explain smaller structures is a key test that will reveal whether we have captured the physics of interactions between normal matter, dark matter, and ionizing light.

Dwarf galaxies are highly sensitive to the nature and distribution of dark matter – such as the particle mass and any self-interactions – because almost all their mass is dark matter. The space density and internal structure of dwarf galaxies are closely connected to the fundamental properties of the dark matter particle, the history of reionization, and the granular limits of the galaxy formation process. Each type of dark matter property manifests itself on astronomical scales through a gravitational signature, in the statistical description of dark matter structure. The measure of this signature is the matter power spectrum today and at earlier times. LUVOIR will measure the matter power spectrum by mapping the regions around spiral galaxies out to ~15 Mpc to find low-mass dwarf galaxies, far more sensitively than JWST and the ELTs.

3.8

Signature Science Case #8: Constraining dark matter using high precision astrometry

A complementary constraint on the nature of dark matter comes from using the orbits of stars within dwarf spheroidal galaxies (dSphs) to study their density profiles. These galaxies are dark matter-dominated, and their central density profiles are a sensitive probe of dark matter properties. Those density profiles can be measured using the 3D motions of individual stars within the galaxies. However, for the smallest dSphs that are most dominated by dark matter, current and planned observatories cannot precisely measure transverse velocities for sufficient numbers of stars to accurately determine the galaxies’ central density profiles. LUVOIR can provide the necessary velocity precision for enough stars, thanks to its angular resolution, sensitivity, and a pixel geometry calibration system within the HDI instrument. LUVOIR will measure the proper motions of stars within dSphs with ~10 km/s accuracy out to 4 Mpc with LUVOIR-A or 2 Mpc with LUVOIR-B. Combined with radial velocities from the ELTs, these proper motion measurements will provide galaxy density profiles and dark matter maps in cases where it is not currently possible due to the difficulty of obtaining transverse velocities.

3.9

Signature Science Case #9: Tracing ionizing light across cosmic time

The major open questions of cosmic structure formation go far beyond the nature of dark matter. The opacity and temperature of the intergalactic medium (IGM) influences where and how gas can enter dark matter halos to form stars, making the history of “reionization” a major unsolved problem. The problem is complex because dwarf galaxies are the predominant sources of ionizing photons and are also most greatly impacted by being ionized and heated. LUVOIR will push beyond JWST and the ELTs to constrain the number and distribution of the faintest galaxies at high redshift – those that feel the effects of reionization most directly. Figure 9 shows the redshift to which telescopes of different sizes can detect dwarf galaxies. With deep imaging of 12 HDI fields in the I, J, and H bands, obtained in parallel with exoplanet observations, LUVOIR-A can measure photometric redshifts for galaxies as faint as AB≈33 mag, corresponding to the mass of the Milky Way’s Fornax dwarf spheroidal galaxy at redshift z = 7. This unprecedented depth – two magnitudes deeper than JWST – will definitively constrain the faint end slope of the galaxy luminosity function, which in turn constrains the critical influence of reionization on early galaxy formation. However, this specific observing program requires unfeasibly long times with LUVOIR-B.

Figure 9:

LUVOIR can detect smaller galaxies than any other current or planned observatory. The curves show sensitivity to detecting galaxies of a particular total stellar mass (y-axes) as a function of redshift (and lookback time) for different telescopes: a 2.4-m space telescope, a 4-m space telescope, the LUVOIR concepts, and the 39-m ground-based ELT. The limits for the space-based telescopes are for a fiducial 500 ksec (139 hour) observation that returns a SNR=5 detection of a 200-parsec diameter source. The limits for the ELT are for a 10-night (80 hour) integration and a 100-night (800 hour) integration. Results for JWST are similar to those for the ELT 100-night exposure. Credit: M. Postman (STScI)

3.10

Signature Science Case #10: The cycles of galactic matter

Galaxies are built from gas that surrounds them and fills up their disks. Gas flows from the space between the galaxies (the intergalactic medium; IGM), into the region around a galaxy (CGM), then into the galaxy itself where it can become the fuel for new stars. Eventually, massive stars, supernovae, and AGN drive material back out. Much of this cycling of matter is currently unobserved, since the gas flows are hot and tenuous. LUVOIR’s uniquely powerful multi-object UV spectroscopy will reach an entirely new discovery space in the exploration of galactic gas flows. LUMOS has an array of 840 x 420 individually configurable microshutters, each of which covers 0.07” x 0.14”. This angular resolution corresponds to < 100 pc out to z = 0.1 (400 Mpc) and < 1 kpc at all redshifts. The ability to probe the powerful UV diagnostics of diffuse gas, hot stars, and dust at such small spatial scales over wide fields will permit the very complex interactions of these processes to be studied as a function of local conditions.

3.11

Signature Science Case #11: The multiscale assembly of galaxies

Since the original Hubble Deep Field in 1996, tracing the growth of galaxies has pushed all the way back to about 400 million years after the Big Bang (z ~ 11). JWST will press further back and farther down the galaxy mass function with rest-frame optical light observed in the near- and mid-IR. Because of its large size and optimization for optical wavelengths, LUVOIR with HDI will push up to 3 mag fainter. Reaching AB=33–34 mag at the diffraction limit, LUVOIR will exquisitely image the detailed assembly of galaxies from their smallest parts. Telescope aperture is the critical factor enabling this capability, since it sets the spatial resolution and the photometric depth that can be achieved in reasonable times. Figure 9 shows that LUVOIR can detect analogs of the Milky Way’s smallest satellite galaxies at the time they were forming stars (z > 4). The LUVOIR-A deep fields discussed in Section 3.9 will capture the high-redshift building blocks of larger galaxies, thus achieving another science goal with the same dataset.

Supplementing the LUVOIR deep fields with additional optical images at shorter wavelengths achieves yet another goal: resolving individual star-forming regions within moderate-redshift (z ~ 1–2) galaxies. These images will have a spatial resolution that Hubble only achieves for the very limited set of gravitationally lensed galaxies (Figure 10). Finally, LUVOIR can reveal the story of star formation in individual galaxies by detecting the main-sequence turnoff in color-magnitude diagrams of resolved stellar populations, providing their ages and metallicities. Previous observing programs could only achieve this for very nearby galaxies, limiting studies to dwarf galaxies and a couple of giant spirals; JWST will be similarly limited. LUVOIR will enable these studies out to the distances needed to reach the nearest giant elliptical galaxies.

Figure 10:

LUVOIR can routinely reveal the internal details of galaxies. Simulations of a low mass (10⁹ M_⊙) galaxy at z=2 imaged with a 2.4-m space telescope (top left), a 39-m ground-based telescope (top right), and LUVOIR (bottom panels). The space-based simulations are a combination of B, I, and J band optical images. The ground-based simulation uses longer wavelengths (Y, J, and H bands), where extreme adaptive optics systems may provide the best spatial resolution. These simulations all assume the same total exposure time (500 ksec, corresponding to 17 nights for the ground-based simulation). Credits: M. Postman, G. Snyder (STScI)

3.12

Signature Science Case #12: Stars as engines of galactic feedback

Stars themselves are the most numerous sources of feedback on galaxies, returning energetic radiation to their environments over their lives and substantial amounts of kinetic energy and heavy elements when they die. With its high spatial resolution and uniquely powerful UV spectroscopy, LUVOIR will make fundamental contributions to our understanding of the stellar initial mass function (IMF) in two key areas: very massive stars (VMSs) and stellar multiplicity. VMSs (stars with M_* > 150 M_⊙) can heavily influence their surrounding environment, providing 25–50% of the ionizing flux from the host cluster. Diagnostics for VMSs lie in the UV, through emission and absorption lines in the 120–200 nm range. Characterizing VMSs and other massive stars will require resolving young and massive star clusters out to ~100 Mpc and obtaining multiplexed UV spectra to measure stellar and wind lines.

Most stars, including massive stars, are in binary and multiple systems. Knowledge of stellar multiplicity and system properties (number of companions, distribution of short vs. long periods, distributions of mass ratios and eccentricities, and their dependence on the stellar mass) provides tight constraints on models of star formation, the IMF, and star cluster evolution. Establishing the impact of local and global conditions on the multiplicity of B type and lower-mass stars will require a census of both short-period and long-period binaries in massive star clusters and in galaxies other than the Milky Way and the Magellanic Clouds. Short-period spectroscopic binaries will be studied with adaptive optics-assisted spectrographs on the ELTs. However, studies of long-period binaries require high angular resolution, a very stable point-spread function, and photometry across entire ~arcmin fields-of-view, as well as UV multi-object spectroscopy to characterize the stars in the system. LUVOIR will obtain such observations for binary star systems in nearby galaxies.

3.13

Magnetic fields everywhere: the science of POLLUX

POLLUX is a high-resolution, point-source UV spectropolarimeter studied and designed by a consortium of European institutions, with leadership and support from the Centre National d’Etudes Spatiales (CNES). The instrument was designed to occupy the fourth instrument bay on LUVOIR-A, and represents a potential future international contribution to the LUVOIR mission. As the POLLUX study is separate from the overall NASA-led LUVOIR study, POLLUX’s science goals supplement the main LUVOIR science goals and are discussed along with the instrument’s technical details in Chapter 13 of the Final Report. The most innovative characteristic of POLLUX is its spectropolarimetric capability, which enables studies of a broad range of important astrophysical environments that are out of reach for standard high-resolution spectroscopy. Spectropolarimetry enables the detection and characterization of magnetic fields and local environments of astrophysical objects. POLLUX will address a variety of science cases related to LUVOIR’s science portfolio, thereby complementing the other instruments.