A Giant Planet Candidate Transiting a White Dwarf

Abstract: Astronomers have discovered thousands of planets outside the solar system, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star, but more distant planets can survive this phase and remain in orbit around the white dwarf. Some white dwarfs show evidence for rocky material floating in their atmospheres, in warm debris disks, or orbiting very closely, which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted. Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey. So far, the detection of intact planets in close orbits around white dwarfs has remained elusive. Here, we report the discovery of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red-giant phase and shrinks due to friction. In this case, though, the low mass and relatively long orbital period of the planet candidate make common-envelope evolution less likely. Instead, the WD 1856+534 system seems to demonstrate that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs.

• The paper identifies a giant planet candidate transiting a white dwarf, constraining its mass below 14 Jupiter masses to support a planetary rather than brown dwarf nature.
• It employs TESS, Spitzer, and ground-based data to capture a 56% dimming transit, emphasizing challenges in spectroscopic analysis due to the white dwarf's featureless spectrum.
• The study proposes alternative dynamical scenarios for planetary migration and survivability post-stellar evolution, prompting future theoretical and observational research.

Overview of "A Giant Planet Candidate Transiting a White Dwarf"

The paper, "A Giant Planet Candidate Transiting a White Dwarf," explores an astronomical observation that challenges our understanding of planetary systems associated with degenerate stars. The detection of a Jupiter-sized object orbiting the white dwarf WD 1856+534 presents a unique case that compounds existing theories on the survivability of planetary systems through stellar evolution phases. This essay provides an intricate summary of the findings and their implications in both observational astronomy and theoretical astrophysics.

The research reports the discovery of a planetary candidate of comparable size to Jupiter, with an estimated upper mass limit of 14 Jupiter masses. This mass constraint and the non-detection of thermal emission suggest the transiting body is not a brown dwarf but rather a giant planet. The white dwarf WD 1856 resides approximately 25 parsecs away and is part of a triple star system with two distant M-dwarfs.

Key Observational Findings

• Transit Detection: The transit event was first identified through the TESS mission data, revealing periodic dimming every 1.4 days. The short duration of the transit suggests a small host star, confirming the event primarily originates from the white dwarf, not background stars.
• High-Precision Photometry and Lack of Spectroscopic Features: Observations from ground-based telescopes and NASA's Spitzer Space Telescope further characterized the transit, indicating a substantial dimming of 56%. The absence of significant infrared emission aids in constraining the companion’s maximum mass.
• White Dwarf Parameters: WD 1856 has a well-documented cooling age of ~5.9 billion years, indicating a long evolutionary history that allows ample time for dynamical interactions in its planetary system.
• Non-detection of a Significant Doppler Shift: Due to the featureless DC-type spectrum of the white dwarf, radial velocity measurements were infeasible, necessitating alternative methods to determine planetary characteristics purely from the photometric transit and spectral energy distribution.

Theoretical Implications

The presence of a giant planet closely orbiting a white dwarf raises implications about planetary survivability and dynamical stability following stellar evolution:

• Challenging the Common Envelope Hypothesis: The paper challenges the notion that planets close to stellar remnants emerge from common-envelope evolution, given the low-mass and moderate orbital period of WD 1856 b.
• Proposing Alternative Dynamics: Alternative hypotheses such as dynamical instabilities and perturbations leading to the current orbit are explored. The study speculates that the planet could have migrated through complex gravitational interactions post-main-sequence.
• Potential for Historical Analysis of Planetary Dynamics: The finding encourages further investigation into the dynamics of planetary systems around white dwarfs. It emphasizes the necessity for computational models to simulate planetary scattering and orbital changes resulting from host star evolution.

Speculation on Future Research Directions

The elucidation of WD 1856 b as a planet will likely drive future research in several directions:

• Spectroscopic Studies: Advanced facilities like JWST could provide transmission spectra to definitively classify the object and assess atmospheric composition.
• Modeling Stellar and Post-main-sequence Planetary Dynamics: Research will focus on simulating multi-planet systems to constrain better how similar planets might survive stellar engulfment and subsequent gravitational interactions.
• Extending Exoplanet Surveys: Systematic searches around white dwarfs might reveal more giant planets or smaller rocky worlds, advancing the understanding of planetary survivability and evolution across cosmic timelines.

In conclusion, the research scrutinizes a peculiar planetary configuration that serves not only as a testament to the resilience of planetary bodies through stellar life cycles but also as a stepping stone for future astronomical pursuits in understanding the final chapters of planetary system evolution. The study invites further exploration into the dynamic histories of planets orbiting white dwarfs, potentially reshaping comprehension of planetary migration and survivability post-stellar demise.