🔭 astrophysics

From Hubble to HWO: Bridging the Frontier of White Dwarf Exoplanet Science

This white paper argues that preserving and prioritizing the Hubble Space Telescope's ultraviolet spectroscopic capabilities through at least 2035 is a high-return investment essential for analyzing the bulk composition of rocky exoplanets around white dwarfs, serving as both a standalone scientific endeavor and critical groundwork for future missions like the Habitable Worlds Observatory.

Original authors: Laura K. Rogers, Siyi Xu, Martin Barstow, Simon Blouin, Amy Bonsor, Andrew M. Buchan, Sarah L. Casewell, Tim Cunningham, John Debes, Patrick Dufour, Boris Gansicke, Joseph Guidry, Ted von Hippel, Mukr

Published 2026-05-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Laura K. Rogers, Siyi Xu, Martin Barstow, Simon Blouin, Amy Bonsor, Andrew M. Buchan, Sarah L. Casewell, Tim Cunningham, John Debes, Patrick Dufour, Boris Gansicke, Joseph Guidry, Ted von Hippel, Mukremin Kilic, Erika Le Bourdais, Carl Melis, Lou Baya Ould Rouis, Judith Provencal, Melinda Soares-Furtado, Andrew Swan, Isabella Trierweiler, Zachary Vanderbosch, Jamie Williams

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine our galaxy, the Milky Way, as a massive library of stars. Most of these stars (about 97%) are destined to end their lives as white dwarfs—the dense, glowing cores left behind after a star burns out. Think of a white dwarf as a cosmic "trash compactor" that has swallowed the remains of its own planetary system.

This white paper argues that we need to keep the Hubble Space Telescope (Hubble) running until at least 2035. Why? Because Hubble is the only tool we have right now that can read the "receipts" left behind by these dead stars to tell us what their planets were made of.

Here is the breakdown of the paper's main points using simple analogies:

1. The Cosmic "Receipt" (Polluted White Dwarfs)

When a white dwarf is young, its surface is pure hydrogen or helium, like a clean white sheet. But if it has planets or asteroids nearby, gravity sometimes pulls these rocky bodies in, tearing them apart. The debris falls onto the white dwarf like dust on a clean table.

Because white dwarfs have such strong gravity, heavy elements (like iron, silicon, or carbon) sink quickly below the surface. If we see these elements floating on the surface, it's proof that the star is currently "eating" a piece of a planet.

The Analogy: Imagine a chef (the white dwarf) who only cooks with salt and sugar (hydrogen/helium). If you taste a bite of their soup and detect a hint of garlic or pepper, you know for a fact that someone added those ingredients recently. Hubble is the only "taster" sensitive enough to detect these specific flavors.

2. Why We Need Hubble (The UV Flashlight)

To figure out exactly what these planets were made of, we need to look at them using Ultraviolet (UV) light.

The Problem: Many crucial ingredients for life (like Carbon, Nitrogen, and Phosphorus) and key building blocks of rocks (like Magnesium and Silicon) only show up clearly in UV light.
The Limitation: Earth's atmosphere blocks UV light, so we can't see it from the ground. We need a telescope in space.
The Hero: Currently, Hubble is the only telescope with the right "UV flashlight" (specifically its COS and STIS instruments) to see these elements clearly. The James Webb Space Telescope (JWST) is amazing, but it looks in infrared light, which misses these specific ingredients.

3. What We Are Learning (The Three Big Questions)

By keeping Hubble running, scientists want to answer three big questions about planets in our galaxy:

Are most planets like Earth?
So far, Hubble has shown that many dead stars are eating dry, rocky material, very similar to Earth or meteorites. This suggests that "dry, rocky worlds" are a common recipe in the universe. However, we only have a few samples so far. We need Hubble to look at many more to know if this is the rule or the exception.
Where is the water?
Some white dwarfs are eating material that is rich in water (oxygen). This proves that icy bodies can survive the violent death of a star and deliver water to the system. Hubble is needed to find out how common this "water delivery" is, which helps us understand where life might exist.
Do planets have cores and mantles?
Just like Earth has a heavy iron core and a rocky mantle, some of the planets being eaten by white dwarfs seem to be broken apart in a way that shows they had these layers. Hubble can detect the specific metals that tell us if a planet melted and separated into layers (differentiation), a process crucial for a planet to have a magnetic field and be habitable.

4. The Teamwork: Hubble, JWST, and the Future

The paper argues that Hubble shouldn't work alone; it needs to be part of a team.

Hubble + JWST: Think of JWST as a camera that takes beautiful pictures of the dust around the star, showing us the minerals (the shape of the rocks). Hubble acts as a chemical analyzer, telling us the ingredients (the elements) inside those rocks. When you combine the two, you get a complete picture of the planet's geology.
Hubble + HWO (The Future): NASA is building a new, super-powerful telescope called the Habitable Worlds Observatory (HWO) for the future. Hubble is needed now to find the best targets for HWO to study later. Without Hubble doing the groundwork, the new telescope might waste time looking at the wrong stars.

5. The Urgent Timeline

The paper warns that we are in a race against time.

New Discoveries: New ground-based surveys are finding thousands of these "polluted" white dwarfs every year. We have a massive backlog of targets to study.
The Gap: If Hubble shuts down before 2035, we will have a "blind spot." We will have the new telescope (HWO) and the infrared telescope (JWST), but we will lack the specific UV tool needed to read the chemical composition of these planets.
The Request: The authors are asking NASA to prioritize Hubble's UV capabilities and keep it running through 2035. This ensures we can finish our current studies, prepare for the future, and fully understand the building blocks of planets across the galaxy.

In short: Hubble is the only tool we have that can read the chemical "receipts" of dead stars to tell us what their planets were made of. To understand if Earth-like, water-rich, life-supporting planets are common in the universe, we must keep Hubble's UV "flashlight" shining until at least 2035.

Technical Summary: From Hubble to HWO: Bridging the Frontier of White Dwarf Exoplanet Science

Problem Statement
Despite the discovery of over 6,000 exoplanets, a fundamental limitation persists in understanding their interior structure and bulk elemental/mineralogical composition. While mass, radius, and orbital properties are measurable, these data points only allow for educated guesses regarding geological evolution, volatile inventories, and habitability potential. Polluted white dwarfs (WDs)—stars accreting tidally disrupted planetary bodies—offer a direct solution by exposing the bulk composition of rocky exoplanetary material. However, determining this composition requires measuring a full suite of diagnostic elements, including volatile species (C, N, O, P, S) and key rock-forming elements (Fe, Si, Mg, Al, Ni). Most of these elements, particularly the volatiles essential for habitability studies, possess spectral signatures only in the ultraviolet (UV) regime ( $\lambda < 3000$ Å). Currently, the Hubble Space Telescope (HST), specifically through its Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS), is the sole facility capable of performing high-resolution ( $R > 15,000$ ) UV spectroscopy required for this science. Existing data collection has been piecemeal and exploratory; a systematic, statistically powerful survey is needed to move from individual case studies to population-level insights.

Methodology and Instrumentation
The paper advocates for a continued, extended HST mission through at least 2035 to conduct systematic UV spectroscopic surveys of polluted white dwarfs. The methodology relies on:

Target Selection: Leveraging large-scale multi-object spectroscopic surveys (e.g., DESI, SDSS-V, 4MOST, WEAVE) and future UV surveys (UVEX, CASTOR) to identify heavily polluted WDs. UVEX is expected to detect over 20,000 bright WDs suitable for follow-up.
Spectroscopic Capabilities: Utilizing HST's COS and STIS instruments.
- COS: The primary tool for high UV sensitivity, employing gratings like G130M (for hot WDs $>15,000$ K) and G185M (for cooler WDs).
- STIS: Used for bright WDs where COS saturates, utilizing modes such as E140M, E230M, and G230L.
- Resolution: High resolution ( $R > 15,000$ ) is strictly required to resolve lines and disentangle photospheric metal lines from interstellar medium (ISM) contributions.
Observational Strategy: The paper calls for targeted follow-up of identified systems, rapid response (Target of Opportunity) for transient tidal disruption events, and strict calibration stability to ensure reproducibility across multi-epoch visits spanning months to years. It also advocates for relaxing the standard 90% program completeness threshold for statistical surveys to ensure unbiased samples.

Key Contributions and Scientific Outcomes
The paper outlines four primary scientific domains enabled by this extended program:

Bulk Elemental Composition and Volatile Inventories:
- Current data suggests most polluted WDs accrete dry, rocky material analogous to bulk Earth and chondritic meteorites, with volatile depletion mirroring Solar System formation.
- However, statistical limitations prevent definitive conclusions. An extended program would quantify the distribution of exoplanetary compositions, determining the prevalence of Earth-like rocky material and water-rich bodies.
- Crucially, UV spectroscopy is required to detect C, N, O, P, and S. Only three polluted WDs currently have the full suite of these detections. Expanding this sample is vital for understanding volatile retention and delivery pathways.
Core-Mantle Differentiation:
- Differentiation (segregation into iron-rich cores and iron-poor mantles) controls planetary habitability factors like magnetic dynamos and tectonics.
- UV spectroscopy is essential for measuring siderophilic (iron-loving) and lithophilic (rock-loving) elements to distinguish between core-rich and mantle-dominated accretion.
- Current evidence suggests differentiation is common outside the Solar System. A larger sample will provide statistically robust insights into the fragmentation and fate of differentiated bodies, with specific abundances (Si, Ni, Cr) constraining parent body mass and oxygen fugacity.
Synergy with the Habitable Worlds Observatory (HWO):
- The HWO, a future NASA flagship, will possess a high-resolution UV spectrograph with greater collecting area.
- Continued HST observations are critical to curate a list of well-characterized, promising targets for HWO. Without this groundwork, HWO risks spending its early years on discovery rather than probing underlying physics.
Synergy with the James Webb Space Telescope (JWST):
- JWST characterizes the mineralogy of circumstellar dust disks around WDs via infrared spectroscopy (e.g., 10 $\mu$ m silicate features).
- HST provides the bulk elemental composition of the same material via UV photospheric abundance measurements.
- Combining these datasets creates a self-consistent "geochemical fingerprint," linking mineralogy (e.g., olivine, pyroxene ratios) to bulk composition. This integrated approach is necessary to evaluate planetary properties such as volcanism, crustal recycling, and long-term nutrient supply.
- Furthermore, simultaneous time-resolved observations from HST (UV) and JWST (IR) are required to study the physical mechanisms connecting circumstellar gas, dust, and photospheric accretion, including viscous timescales and transient tidal disruption events.

Results and Claims
The paper asserts that the scientific returns from HST UV spectra over the past two decades have been transformative but limited by sample size. The proposed extended mission aims to transition the field from isolated case studies to volume-complete, statistically significant surveys.

Statistical Power: The authors claim that only an extended HST mission can establish the true distribution of exoplanetary bulk compositions and definitively answer how common Earth-like compositions and water-rich bodies are.
Necessity of UV: The paper emphasizes that without HST's UV capabilities, key diagnostic elements (C, N, O, P, S) and the full suite of siderophilic/lithophilic elements cannot be measured with the required precision.
Timeline: The authors argue that preserving HST's UV capabilities through at least 2035 is crucial to maximize the scientific return from HST, JWST, and the future HWO, particularly to capitalize on the influx of targets from UVEX (launching 2030) and CASTOR.

Significance
The paper concludes that continued UV spectroscopic capabilities with HST represent a high-return investment for white dwarf and exoplanet science. It posits that losing HST before the HWO era would create a critical gap in the ability to characterize exoplanet material composition. By bridging the gap between current capabilities and future flagship missions, an extended HST program is presented as essential for answering fundamental questions regarding the bulk composition, mineralogy, and habitability potential of rocky exoplanets across the Galaxy.