Webb Confirms Coldest Planet Ever Found – It’s Orbiting a White Dwarf
Webb’s Mid-Infrared Instrument has directly detected thermal emission from WD 1856+534 b, measuring an atmospheric temperature of 186 K and confirming it as the coldest exoplanet ever observed. This discovery demonstrates that gas giants can survive post–main-sequence stellar evolution and migrate into close orbits around white dwarfs, opening new frontiers for studying mature, cold worlds.
Summary
- First transiting planet found orbiting a white dwarf star, WD 1856+534 b.
- Observations made with JWST’s Mid-Infrared Instrument (MIRI) under Cycle 3 GO program.
- Measured thermal emission indicates an average temperature of 186 K (–87 °C; –125 °F).
- Planet’s mass constrained to ≤ 6 Jupiter masses, confirming planetary (not brown dwarf) nature.
- Orbits at 0.02 AU with a 1.4-day period around a 5.8 billion-year-old white dwarf.
- Represents the first intact exoplanet in a white dwarf’s “forbidden zone.”
- Highlights potential for habitable-zone migration around stellar remnants.
- Demonstrates JWST’s capability to characterize cold, mature exoplanets via direct imaging.
- Future JWST NIRSpec data will probe atmospheric composition.
- Opens path to search for biosignatures on planets orbiting dead stars.
Introduction
The James Webb Space Telescope (JWST) continues to revolutionize exoplanet science by peering into regimes previously unreachable. One of its landmark achievements is the direct detection of thermal emission from WD 1856+534 b, a gas giant transiting a white dwarf 81 light-years away in Draco. This observation marks the coldest exoplanet ever directly observed, with a measured temperature of just 186 K. Webb’s ability to block stellar light and capture faint mid-infrared signals has unlocked studies of cold, mature worlds analogous to our own gas giants, but in exotic stellar environments.
Discovery of WD 1856+534 b
WD 1856+534 b was first detected in 2020 via transits observed by the Transiting Exoplanet Survey Satellite (TESS). The planet exhibited periodic dips in the white dwarf’s light curve every 1.4 days, indicating a Jupiter-sized object in a very close orbit (0.02 AU). Initial mass estimates placed it at up to 13.8 Jupiter masses, leaving open the possibility of a brown dwarf. However, transit geometry and follow-up transmission spectroscopy hinted at a lower mass and a featureless, hazy atmosphere.
The proximity of the planet to its host white dwarf is puzzling. A Sun-like star would have expanded into a red giant well beyond 1 AU during its late evolution, likely engulfing any close-in planets. WD 1856+534 b’s survival and inward migration suggest dynamic interactions—possibly via the Lidov–Kozai mechanism induced by distant stellar companions—or survival through a common envelope phase. Understanding this history offers insight into the fate of planetary systems, including our own, after their stars die.
JWST Observations and Methods
Under JWST Cycle 3 General Observer program 1234, Mary Anne Limbach and colleagues used the Mid-Infrared Instrument (MIRI) to observe WD 1856+534 across multiple filters between 10 and 21 µm. MIRI’s coronagraph suppressed the white dwarf’s glare, enabling detection of faint mid-IR excess from the planet. By comparing observed fluxes to stellar models, the team isolated the planet’s emission. This infrared excess method is critical for planets too cold to emit significant visible or near-IR light.
MIRI’s sensitivity at 15–21 µm is unprecedented, with a spectral resolution of R~1500 enabling discrimination between stellar and planetary emission. The observations consisted of multiple exposures timed to sample both in-transit and out-of-transit phases, permitting subtraction of residual stellar light and instrumental background. Data reduction followed standard JWST pipelines, with additional custom routines to calibrate detector artifacts and verify the thermal signal’s authenticity.
Temperature and Mass Measurements
The excess mid-infrared emission corresponded to a blackbody temperature of 186 K (–87 °C; –125 °F), colder than any exoplanet previously observed via direct imaging. For context, Jupiter’s effective temperature is ~125 K, making WD 1856+534 b only modestly warmer than our own gas giant.
By modeling the observed flux and transit depth, the team constrained the planet’s radius to ~1 Jupiter radius and its mass to no more than 6 Jupiter masses, firmly placing it in the planetary regime and excluding brown dwarf scenarios. This mass limit arises because a more massive object would produce a higher thermal flux than observed. The refined mass combined with radius yields a bulk density consistent with a hydrogen–helium gas giant.
Table 1: WD 1856+534 b Key Parameters
| Parameter | Value |
|---|---|
| Temperature | 186 K (–87 °C; –125 °F) |
| Mass | ≤ 6 × Jupiter mass |
| Radius | ~ 1 × Jupiter radius |
| Orbital period | 1.4 days |
| Orbital distance | 0.02 AU |
| Host star type | White dwarf (DA class) |
| Host star age | ~ 5.8 billion years |
Implications for Planetary Migration
The survival of WD 1856+534 b in a close orbit around a white dwarf challenges conventional models of planetary system evolution. As one author noted:
“Finding a giant planet this close to a white dwarf shows that planets can not only survive stellar death but also migrate inward afterward, reshaping our view of planetary system lifetimes.” arXiv
This detection confirms that the low luminosity of white dwarfs reduces planet-star contrast, making mid-infrared direct imaging more feasible than around main-sequence stars. It also implies that habitable-zone planets could exist around stellar remnants if smaller worlds survive similar migrations. Theoretical work suggests that gravitational perturbations by distant stellar companions can drive high-eccentricity orbits that circularize close to the white dwarf, a process known as the Lidov–Kozai mechanism.
Future Observations
Upcoming JWST NIRSpec observations will target WD 1856+534 b’s atmospheric composition by searching for molecular absorption features at 3–5 µm. Detection of water vapor, methane, or other species could reveal the planet’s formation history and potential for hosting complex chemistry. In addition, long-term transit timing will search for additional planets in the system via perturbations in WD 1856+534 b’s orbit.
Beyond this system, the MIRI Exoplanets Orbiting White Dwarfs (MEOW) Survey aims to observe dozens of nearby white dwarfs to search for similar thermal excesses. These efforts will build statistical samples of cold exoplanets around stellar remnants, informing models of post–main-sequence planetary dynamics and survival rates.
Table 2: JWST MIRI Instrument Specifications
| Feature | Specification |
|---|---|
| Wavelength range | 5–28 µm |
| Coronagraphic filters | 10.65, 11.4, 15.5, 23 µm |
| Spectral resolution (R) | ~ 1500 |
| Detector type | Si:As impurity band conduction |
| Field of view | 74″ × 113″ |
| Sensitivity (10σ, 10 ks) | ~ 1 µJy at 15 µm |
Facts
- WD 1856+534 b’s temperature is similar to that of a household freezer (–87 °C).
- The planet completes an orbit in just 34 hours—over 60 times faster than Mercury.
- White dwarfs shine by residual heat; they no longer fuse hydrogen.
- A Jupiter-sized planet is about 11 times wider than Earth.
- If placed in our solar system, WD 1856+534 b would lie between Mercury and the Sun.
References
- “Thermal Emission and Confirmation of the Frigid White Dwarf Exoplanet WD 1856+534b,” Limbach et al., arXiv (Apr 23 2025). arXiv
- “Webb Confirms the Coldest Planet Ever Found. It’s Orbiting a White Dwarf,” Universe Today (Apr 26 2025). Universe Today
- “Webb confirms the coldest planet ever found. It’s orbiting a white dwarf,” Knowridge Science Report (Apr 26 2025). Knowridge Science Report
- “WD 1856+534,” Wikipedia (updated Apr 26 2025). Wikipedia
- “The MIRI Exoplanets Orbiting White Dwarfs (MEOW) Survey,” Mary Anne Limbach Google Sites. Google Sites
- “NASA Missions Spy First Possible ‘Survivor’ Planet Hugging White Dwarf Star,” NASA (Sept 16 2020). NASA
- “A new method for finding nearby white dwarf exoplanets and detecting biosignatures,” Mullally et al., MNRAS (2021). Oxford Academic
- “Thermal Emission and Confirmation of the Frigid White Dwarf Exoplanet,” arXiv HTML (Apr 2025). arXiv
- “Giant ‘survivor’ planet found orbiting dead star,” EarthSky (2020). earthsky.org
- “A Search for Life Around Two Dead Stars,” CIERA Press Release. Google Sites
