Webb confirms the coldest planet ever found. It’s orbiting a white dwarf – Phys.org
2025-04-30T17:30:04Z
In 2020, astronomers detected WD 1856+534 b, a gas giant that orbits a star 81 light-years from Earth. This exoplanet, which is roughly six times the mass of Jupiter (making it a “super-Jupiter”), was the first transiting planet known to orbit a white dwarf (…
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In this illustration, WD 1856 b, a potential Jupiter-size planet, orbits its much smaller host star, a dim white dwarf. Credit: NASA, GSFC
In 2020, astronomers detected WD 1856+534 b, a gas giant that orbits a star 81 light-years from Earth. This exoplanet, which is roughly six times the mass of Jupiter (making it a “super-Jupiter”), was the first transiting planet known to orbit a white dwarf (WD) star. In a recent paper, an international team of astronomers describes their observations of this exoplanet using the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST). Their observation confirmed that WD 1856+534 b is the coldest exoplanet ever observed.
The research was led by Mary Anne Limbach, an Assistant Research Scientist with the Department of Astronomy at the University of Michigan, Ann Arbor. She was joined by researchers from the Kavli Institute for Astrophysics and Space Research at MIT, the Johns Hopkins University Applied Physics Lab (JHUAPL), the University of Victoria, the University of Texas at Austin, the Center for Interdisciplinary Research and Exploration in Astrophysics (CIERA), Center for Astrophysics, University of Southern Queensland, and the NSF NOIRLab and Gemini Observatory.
Their observations were part of the JWST Cycle 3 General Observation (GO) program, which aimed to use Webb’s sophisticated infrared optics and spectrometers to characterize the planet directly. This is in keeping with one of the JWST’s mission objectives, which is the characterization of exoplanets using the Direct Imaging Method. This consists of observing light reflected from an exoplanet’s surface or atmosphere and examining it with spectrometers to look for chemical signatures.
This can allow astronomers to determine the presence of potential biosignatures (oxygen, nitrogen, methane, water, etc.) and infer details about the planet’s formation and composition. Using advanced next-generation telescopes like the JWST, this method could lead to the first conclusive evidence of life beyond the solar system.
Emission spectra from these planets can also reveal details about the planet’s composition and migration history. However, as the authors note in their study posted to the arXiv preprint server, detecting light directly from an exoplanet remains challenging due to the overwhelmingly obscuring light from their host stars.
As a result, direct imaging has been largely confined to massive planets (e.g., gas giants) with wide orbits or extremely high atmospheric temperatures. Meanwhile, no terrestrial (or rocky) exoplanets have been observed orbiting closer to their stars. What’s more, no exoplanets with emission spectra cooler than 275 K (1.85°C; 35.33°F)—comparable to Earth—have been observed either. WD stars present a unique opportunity to detect and characterize colder planets. As the team noted:
“The low luminosity of WDs significantly reduces the contrast challenges that typically hinder direct detections around their main-sequence counterparts. As the evolutionary remnants of stars like the sun, WDs offer insight into the fate of planetary systems after stellar death. Understanding how planets interact with and survive post-main-sequence evolution provides crucial information on orbital stability, dynamical migration, and potential planetary engulfment.”
In addition, investigating WD planetary systems can shed light on whether planets can survive this late stage of stellar evolution and provide insight into whether habitable conditions can still exist around stellar remnants. Astronomers and astrobiologists are hoping to investigate these mysteries using Webb’s capabilities. For their study, Limbach and her colleagues confirmed the presence of WD 1856+534 b using the Infrared (IR) excess method with data from JWST Mid-Infrared Instrument (MIRI).
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This allowed them to constrain the mass of WD 1856+534 b and measure its atmospheric temperature. Their analysis revealed an average temperature of 186 K (-87°C; -125°F), making WD 1856+534 b the coldest exoplanet ever detected. They further confirmed that the exoplanet has a mass no greater than six times that of Jupiter, whereas previous observations produced an estimate of 13.8 Jupiter masses. Their results also constitute the first direct confirmation that planets can survive and migrate into close orbits near the habitable zones of WDs.
The team is looking forward to further observations of WD 1856 b by the JWST, which are scheduled for 2025. Hopefully, these observations will identify additional planets, which could reveal if WD 1856 b was perturbed into its current orbit. Furthermore, the results of previous observations made by Webb’s Near-Infrared Spectrometer (NIRSpec) in Cycle 1 will soon be released. These will provide an initial characterization of the planet’s atmosphere.
More information: Mary Anne Limbach et al, Thermal Emission and Confirmation of the Frigid White Dwarf Exoplanet WD 1856+534b, arXiv (2025). DOI: 10.48550/arxiv.2504.16982 Journal information: arXiv
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