Coldest exoplanet ever found is confirmed by James Webb
A gas giant exoplanet with a mass equivalent to about 13.8 times that of Jupiter was announced in 2020. Orbiting a star 81 light-years from Earth, which is relatively close in astronomical terms, it is the first known exoplanet to orbit a white dwarf.
Called WD 1856+534 b, it was detected in transit, that is, passing in front of its star from our point of view, which allows it to be observed indirectly. The discovery was surprising because, when a star dies, it collapses and becomes a white dwarf, a process that destroys nearby planets.
Now, in a recent paper, not yet peer-reviewed, an international team of astronomers has confirmed another record: WD 1856+534 b is the coldest exoplanet ever observed . The discovery was made using the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST).
The current observations are part of the JWST General Observations Program – Cycle 3, which corresponds to the third year of planned scientific operations of the telescope, starting in 2025. This cycle includes study proposals submitted by researchers and selected for scientific merit.
These measurements represent one of the main purposes of the JWST mission, which is to use the direct imaging method to characterize exoplanets . Instead of detecting them in transit (as in the discovery of WD 1856+534 b), in this technique, the telescope captures the light coming directly from the planet.
How to capture direct light from exoplanets?
Performed by spectroscopic analysis, the direct imaging technique of exoplanets allows astronomers to identify biosignatures such as oxygen, methane and water on these distant worlds, revealing details of their composition and formation. The expectation is that we are close to evidence of extraterrestrial life.
To this end, emission spectra provide data on planetary composition and migration history. However, capturing light directly from exoplanets remains a difficult task because of the blinding brightness of their host stars, which limits direct observations.
Proof of this is that, to date, no rocky planet has been observed directly close to its star , nor have any exoplanets with temperatures below 1.85 °C, comparable to that of Earth. Therefore, direct imaging has been limited to gas giants with wide orbits or superhot atmospheres.
The discovery of an exoplanet orbiting a white dwarf (WD) now presents a unique opportunity to detect cooler worlds. This is because the low luminosity of these stars reduces the so-called “contrast challenges”, which make it difficult to observe exoplanets around Sun-like stars.
Since they are “dead” and no longer produce energy through nuclear fusion, white dwarfs also offer insights into what happens to planets after the death of their star. Therefore, studying them reveals information about the stability or orbital migration, and possible survival of the planets.
Studying surviving planets orbiting dead stars
Studying planets around white dwarfs could help answer one of the big questions in astrobiology: can these worlds survive the death of their stars? If so, scientists can assess whether these systems would still maintain the minimum conditions for habitability.
In the current study, astronomers led by Mary Anne Limbach confirmed the existence of the exoplanet WD 1856+534 b, with the help of the James Webb. To do this, they used data from the MIRI instrument and analyzed the excess infrared light emitted by the planet.
Using this technique, it was possible to constrain the exoplanet's mass and measure its atmospheric temperature. The data revealed an average of 186 kelvins, equivalent to –87 °C, making WD 1856+534 b the coldest exoplanet ever detected.
Using more recent data obtained by JWST, the authors refined what was known since the original discovery of WD 1856+534 b in 2020. The mass, for example, is smaller than previously thought and is no more than six times the mass of Jupiter.
But the new study's major contribution is to provide the first direct evidence that planets can survive the death of their stars, and even migrate to orbits close to the habitable zones of white dwarfs. James Webb promises new observations as early as 2025.
The study is hosted on the arXiv preprint platform .
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