The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope

Abstract: The near-term search for life beyond the solar system currently focuses on transiting planets orbiting small M dwarfs, and the challenges of detecting signs of life in their atmospheres. However, planets orbiting white dwarfs (WDs) would provide a unique opportunity to characterize rocky worlds. The discovery of the first transiting giant planet orbiting a white dwarf, WD 1856+534b, showed that planetary-mass objects can survive close-in orbits around WDs. The large radius ratio between WD planets and their host renders them exceptional targets for transmission spectroscopy. Here, we explore the molecular detectability and atmospheric characterization potential for a notional Earth-like planet, evolving in the habitable zone of WD 1856+534, with the James Webb Space Telescope (JWST). We establish that the atmospheric composition of such Earth-like planets orbiting WDs can be precisely retrieved with JWST. We demonstrate that robust > 5$\sigma$ detections of H$_2$O and CO$_2$ can be achieved in a 5 transit reconnaissance program, while the biosignatures O$_3$ + CH$_4$, and O$_3$ + N$_2$O can be detected to > 4$\sigma$ in as few as 25 transits. N$_2$ and O$_2$ can be detected to > 5$\sigma$ within 100 transits. Given the short transit duration of WD habitable zone planets (~ 2 minutes for WD 1856+534), conclusive molecular detections can be achieved in a small or medium JWST transmission spectroscopy program. Rocky planets in the WD habitable zone therefore represent a promising opportunity to characterize terrestrial planet atmospheres and explore the possibility of a second genesis on these worlds.

• The paper demonstrates that white dwarf-planet systems offer unique advantages for transmission spectroscopy with JWST due to high planet-to-host radius ratios, enabling the detection of atmospheric molecules.
• JWST can robustly detect H G ₂O and CO₂ on Earth-like planets around white dwarfs with just five transits, achieving over 4 sigma detection for biosignatures like CH G ₄ and O G ₃ with 25 transits.
• The study indicates that moderate JWST programs can detect key biosignature combinations, positioning rocky exoplanets around white dwarfs as prime targets in the search for extraterrestrial life.

Detecting Molecules in Earth-like Exoplanets Orbiting White Dwarfs with JWST

This paper investigates the potential of the James Webb Space Telescope (JWST) to detect and characterize molecules in the atmospheres of Earth-like exoplanets orbiting white dwarfs (WDs). The study highlights the unique advantages these systems present, particularly in the context of rocky planets experiencing transits. Utilizing the recent discovery of the giant planet candidate WD 1856+534b, this research explores the feasibility of using JWST to conduct atmospheric characterization for a hypothetical Earth-like planet in the habitable zone of this system.

The authors point out that the high planet-to-host radius ratio inherent in transiting WD systems offers exceptional opportunities for transmission spectroscopy, a technique sensitive to atmospheric molecules. This advantage stems from the enhanced contrast between the planet's atmosphere and its host star, which is more pronounced in WD-planet systems compared to main sequence star planets.

Key Findings:

1. Robust Molecular Detection: JWST can achieve high-confidence detections—exceeding 5 sigma—for atmospheric H$_2$O and CO$_2$ on Earth-like planets in WD systems with only five transits. Notably, biosignature gases such as CH$_4$ and O$_3$ can reach detection levels greater than 4 sigma with 25 transits.
2. Abundance Constraints: The paper outlines how trace gas abundances can be constrained to within a factor of two with 25 transits, while massive data collection programs (100 transits) can yield precisions of approximately 10%.
3. Detectable Molecules: In addition to H$_2$O and CO$_2$, moderate JWST programs can potentially detect biosignature combinations such as CH$_4$ paired with O$_3$ and N$_2$O—underscoring the value of prioritizing rocky exoplanets around WDs for biosignature studies.

Implications and Further Research:

The paper’s findings suggest that WD-planet systems are particularly promising targets in the search for extraterrestrial life, due to favorable observational conditions and longer timescales over which such planets remain in their host's habitable zones. A critical aspect driving future research is the development of techniques that manage the challenges posed by short transit durations typical of WD systems. With an eye toward maximizing the efficiency of transits, strategies that advance instrumentation capabilities for capturing shorter-duration transits will be advantageous.

Furthermore, the occurrence rate of small, rocky planets in the habitable zones of WDs remains an unknown variable. This paper implicitly encourages continued surveys and monitoring to establish a concrete statistical base regarding the distribution and frequency of such planets, which, when discovered, may offer unparalleled insight into atmospheric characterization opportunities using tools like JWST.

In summary, by extending transmission spectroscopy to rocky exoplanets orbiting white dwarfs, the paper paves the way for understanding their atmospheric composition and potential habitability, offering a promising frontier in the study of exoplanetary systems.