About this Article

Published September 17, 2025

The Hubble Cycle 33 proposal allocation peer review recently concluded. Overall, 282 reviewers from all over the globe reviewed 800 proposals. Of these, 151 proposals were accepted, a success rate of 19%, covering topics in solar system astronomy, exoplanets and exoplanetary systems, stellar physics, stellar populations and the interstellar medium, galaxy formation and evolution, circumgalactic and intergalactic media, supermassive black holes and active galaxies, and high-energy transients.

Large Programs

Seven of the accepted programs were Large programs (requesting 75 orbits or more), representing major investments of observatory time and resources. Here we summarize the exciting new science that will be enabled by these programs and the discovery space they will explore.

The HyperDeep Ultraviolet Field
GO 18004, 124 orbits, PI: Matthew Hayes (Stockholm University)

Hubble pioneered the field of observational galaxy evolution with the 1995 Hubble Deep Field and has continued to push boundaries over the past 30 years, only recently ceding the highest redshifts to JWST. This exciting Cycle 33 program marks the next step in the saga, using Hubble’s unique ultraviolet capabilities to probe moderate-redshift galaxies as means of constraining galaxy characteristics at high redshift.

There is general agreement that the epoch of reionization concluded at z~6, ~900 million years after the big bang. The exact mechanisms, however, remain unclear, particularly the role played by the ionizing emissivity of galaxies through the Lyman escape fraction, fesc(LyC). Multiple previous investigations have focused on observations at two redshift widows, at z~0.3, largely based on COS observations, and z~3, using ground-based data. Both use Ly-alpha emission to infer fesc(LyC), but the results are inconsistent, with the higher-redshift sample implying many more detections than are actually seen in existing deep-field UV data, the UV Ultradeep Field (UVUDF: GO 15234). In contrast, predictions based on low-redshift data are a much closer match.

Based on the characteristics of the current low-redshift sample, the proposal team hypothesizes that there are many LyC-leaking galaxies hiding just below the existing UVUDF limits and propose testing that hypothesis through a “hyper-deep” 124-orbit image of the UDF, using the wide-band F336W on Wide Field Camera 3. The new observations will extend ~2 magnitudes fainter than previous work and have the potential to identify 50 to 100 moderate-redshift LyC-emitters, as well as characterize the UV contribution from low-luminosity AGN at comparable redshifts.
 

Shoring Up the Cosmic Shoreline: Improving the Cumulative XUV Irradiance Knowledge of Rocky Worlds DDT Targets
GO 18030, 108 orbits over Cycles 33 to 35, 87 ksec Chandra, 60 ksec XMM, PI: Kevin France (University of Colorado Boulder)

The search for life on other planets underlies NASA’s key question: Are we alone? Hubble and JWST are playing their part in that multi-year, multi-mission project through the Rocky Worlds Director’s Discretionary Time program. Life requires an atmosphere. The ability of a planet to retain an atmosphere depends on two quantities: the mass of the planet and the corresponding gravity; and the energy input from the star, notably at ultraviolet wavelengths. Analyses, largely based on solar planets, suggest that there is a quasi-linear relation between those two parameters, mapped out as the cosmic shoreline, and separating planets that retain atmospheres (Earth, Venus) from those that are largely depleted (Mercury, Mars).

Rocky Worlds targets a handful terrestrial-like planets orbiting nearby low-mass red dwarfs; JWST targets the exoplanets for observations at 15-microns during their eclipse (when they go behind the star); Hubble is measuring the basic UV properties of the star. This Cycle 33 program builds on that foundation by targeting three Rocky Worlds exoplanet hosts for multi-year (2026 through 2029) observations in the UV with Hubble, and at X-ray wavelengths with Chandra and XMM. These observations will provide a more detailed characterization of the host star’s activity, particularly regarding flares and coronal mass ejections that could be particularly potent disruptors of exoplanet atmospheres.
 

The Metallicity Distribution Functions of M31 Ultra-Faint Dwarf Galaxies
GO 18043, 99 orbits, PI: Sal Wanying Fu (Princeton University)

The story of a galaxy’s evolution is written in its star formation history. Simulations of galaxy formation tell us that many galaxies formed stars early and then stopped forming stars (or “quenched”) when the universe re-ionized. In reality, some galaxies keep forming stars after reionization — this points to other processes at work, such as inflows and outflows, or enrichment from exploding supernovae. A galaxy’s metallicity distribution function can tell us which other processes shaped their evolution and how.

This program will measure the first well-populated stellar metallicity distributions in seven ultra-faint dwarf galaxies that orbit the Andromeda galaxy (M31). All seven have star formation histories that are extended, meaning that they kept forming stars long after reionization, and diverse, meaning that each had different processes at play. The newly revealed metallicity distribution functions will help us better understand these other processes that drive galaxy evolution. The metallicity distribution functions can only be measured using Hubble’s unique ultraviolet capabilities, making Hubble the only observatory that can enable this science.

These galaxies are especially interesting as the ultra-faint dwarf galaxies in the Milky Way appear to have quenched at reionization, so these Andromeda galaxies are the closest ultra-faint dwarfs we can study with extended star formation histories.

Reaping the Whirlwind: Accretion & Outflow in the Inner Milky Way
GO 18050, 75 orbits, PI: Hannah Bish (Space Telescope Science Institute)

Large-scale gas accretion powers star formation in spiral galaxies, but we currently have little information on how gas is flowing into the inner regions of our own Milky Way. This Cycle 33 program aims to address that lack through COS far-UV spectroscopy along multiple pencil-beams toward the inner Galaxy. Galactic stars serve as background light sources for this work, with atomic and molecular gas along the line of sight showing up as absorption features in the stellar spectra. However, interpreting the spectra can be complicated since gas is found rotating with the disk as well as in inflows and outflows.

To break that degeneracy, this program couples observations of 20 Galactic stars with eight nearby (angular separation) quasars. The smorgasbord of sightlines probes multiple star-forming regions at Galactic radii between 2 and 7 kpc, with the stars mapping out the nearby gas and the quasars providing information spanning the full Galactic disk. The anticipation is that the observations will yield the first measurements of halo gas rotation in the inner Milky Way and provide insight into connections between inflows, outflows, and the impact of star-forming regions.
 

Hubble Census of Nearby Satellites
GO 18061, 149 orbits, PI: Michael Jones (University of Arizona)

Our Milky Way lives in the Local Group of galaxies. This comprises two massive spiral galaxies — the Milky Way and Andromeda — and a plethora or smaller satellite galaxies that orbit these two massive hosts. But how did the Local Group form, and how typical is it compared to other galaxy systems in the universe?

To answer these questions, we need to study and characterize other nearby systems of similar mass. There are 22 such systems nearby, with a total of 260 satellites overall. Of these, 111 satellites have been previously observed and characterized with Hubble, but this only tells part of the story. To really make progress, we need to understand the whole galaxy sample.

This exciting proposal will study the remaining 149 satellites in the sample. Combined with the 111 galaxies that have already been studied, this will give us an unprecedented understanding of other nearby systems superficially like our own and help us to investigate just how similar they truly are. In particular, this study will investigate the relative number of satellites with different brightness (the satellite luminosity function) in each system and reveal star formation histories for every satellite in the sample, both of which will provide new insights into galaxy formation and evolution.

These galaxy studies require Hubble’s unique ultraviolet capabilities, and only by combining new and archival Hubble data can this program achieve its science goals — highlighting the continuing need for new Hubble observations, the richness of Hubble’s archives, and the power of combining both.
 

Anatomy of a fall: Dissecting the environment-driven transformation of late-type Virgo cluster galaxies with HST UV-optical imaging of star clusters, associations, and HII regions
GO 18103, 145 orbits, PI: David Thilker (Johns Hopkins University)

Galaxies in present-day clusters have formed within substantially higher-density environments than the overwhelming majority of galaxies in the local universe. The Virgo cluster is the nearest such system and therefore offers a striking opportunity to investigate how global galaxy properties might differ with respect to average systems in the Local Group and looser environments. This major Cycle 33 program will obtain multi-wavelength imaging using Wide Field Camera 3 of 40 late-type galaxies in the cluster.

The WFC3 data include broadband imaging from the near-UV to the I-band, plus H-alpha, and build on previous surveys at millimeter wavelengths by ALMA and optical imaging spectroscopy with MUSE on the VLT. Combined, those observations will provide a complete census of HII regions, molecular clouds, ionized and molecular gas, large-scale population structures, and the overall metallicity distribution, probing the star-formation history and structure.

The primary goals are to disentangle the star-formation history in the disk by age-dating the stellar constituents; quantify the efficiency of feedback into the ISM from stars younger than a few hundred million years, probing gas stripping models; and study how gas removal processes during infall affect the physical conditions and impact star formation. The results will be compared against recent similar studies of fields galaxies, notably from the PHANGS survey. 
 

Beyond hydrogen and helium: an exhaustive survey of escaping metals in ultra-hot Jupiters around bright, hot stars
GO 18108, 80 orbits, PI: Leonardo Dos Santos (Space Telescope Science Institute)

Hot Jupiters were the first type of exoplanet to be detected around solar-type stars and the first, via Hubble observations in 2001, to offer spectroscopic insight into their atmospheric composition. Since then, transit spectroscopy has yielded crucial insights into exoplanet properties. In particular, UV observations with Hubble have shown that the close-in, short-period hot Jupiters are subject to significant stellar interactions, with extensive plumes of gas driven by atmospheric escape. Extensive theoretical efforts have been devoted to understanding stellar-driven escape processes, but most assume that the exoplanet atmosphere is pure hydrogen and helium. This innovative program aims to test that hypothesis and probe whether heavier elements (metallicity) may have an impact.

The program targets five hot Jupiter companions orbiting relatively massive A-type stars. These highly irradiated planets are likely to be experiencing significant atmospheric escape, and are the most likely to provide detectable signals, via UV spectroscopy, of exospheric metals in transit spectra. These new observations will complement data from previous programs and provide a comprehensive survey of metals in exospheric outflows.

Medium Programs

A further 12 programs were Medium (requesting 35 to 74 orbits), also significant investments of observatory time and resources. They are as follows:

Completing Panchromatic Spectra for High Priority HWO Target Stars
GO 18001, 37 orbits, PI: Sarah Peacock (University of Maryland, Baltimore County)

Proper Motions of the M31 Satellites: The missing pieces of the 6D puzzle
GO 18029, 48 orbits, PI: Paul Bennet (Space Telescope Science Institute)

Spatially resolving the conditions for ionizing radiation escape in galaxies
GO 18034, 40 orbits, PI: Alberto Saldana-Lopez (Stockholm University)

The Size Scale and Baryonic Content of Low Redshift Intergalactic Absorbers
GO 18053, 73 orbits, PI: Bart Wakker (Eureka Scientific Inc.)

Dynamically Mapping the Satellite Galaxies in the Outer Halo of the Milky Way
GO 18068, 64 orbits, PI: Nitya Kallivayalil (University of Virginia)

The High Redshift Lyman Continuum Survey
GO 18080, 53 orbits, Claudia Scarlata (University of Minnesota Twin Cities)

A New Benchmark Set of Massive Stars Spectra to Distinguish Wolf-Rayet Stars and Very Massive Stars
GO 18088, 55 orbits, PI: Danielle Berg (University of Texas at Austin)

HST Superluminous Supernova UV Legacy Program
GO 18105, 56 orbits, PI: Robert Quimby (San Diego State University)

Baryonic Escape, Abundances, and Thermodynamics in Cosmic Ecosystems (BEATriCE)
GO 18107, 70 orbits, PI: Sean Johnson (University of Michigan)

Improving BH Scaling Relations in High-Mass AGNs: Insights from HST Imaging and Dynamical Modeling
GO 18116, 44 orbits, PI: Shu Wang (Seoul National University)

A Far-Ultraviolet Spectroscopic Survey of AGN Dwarf Galaxies
GO 18134, 59 orbits, PI: Sylvain Veilleux (University of Maryland)

Tracing Accretion in the Planetary Regime: A Comprehensive UV/Optical Survey of the Late Stages of Planet Formation
GO 18139, 60 orbits, PI: Lillian Jiang (UC Santa Barbara)
 

View the Complete List of Cycle 33 Approved Programs