Scientists identify the coldest exoplanet known to date

The planet with the somewhat complicated name WD 1856+534 b was discovered a few years ago. A new investigation has revealed that it is the coldest of the exoplanets known to date.

An international team led by astronomer Mary Anne Limbach observed the planet with the Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST). Although it orbits its star at a very small distance, its surface has an average temperature of minus 87 degrees Celsius (186 Kelvin).

Until now, Epsilon Indi Ab was considered the record holder. However, it is significantly warmer there: the average temperature on this exoplanet is 2 degrees Celsius.

The planet is almost as big as Jupiter

WD 1856+534 b was discovered in 2020 with the help of the Transiting Exoplanet Survey Satellite (TESS) and Spitzer space telescopes. The planet is almost as large as Jupiter, but has around six times the mass. Earlier estimates were based on 13.8 Jupiter masses.

The planet orbits a white dwarf about 80 light years away from Earth, i.e. a star at the end of its life. At first, the discoverers were not sure whether WD 1856+534 b was not itself a star, a brown dwarf.

"We were all a little surprised – and excited – when we realized that it was actually a planet, and a really cold one at that," Limbach told US news site Space.com. The astronomer at the University of Michigan is the lead author of the study, which is available as a preprint on the Arxiv document server.

JWST enables direct observation of the planet and thus better insights into the celestial body. The team was able to verify that it is a planet. It orbits its star at a short distance – it only needs 1.4 days for one orbit.

An orbit in the forbidden zone

It is therefore in the so-called forbidden zone. This is the area that the star occupied when it was in the red giant stage before it shrank into a white dwarf. Everything in this forbidden zone is being swallowed up by the expanding star. This suggests that the planet only assumed its current orbit later.

"This is compelling evidence that planets can not only survive the brutal death of their star, but can also move into orbits where we wouldn't necessarily have expected them before," said Limbach.

The team plans to observe the system again with JWST in July. They hope to discover more planets and gain insights into how WD 1856+534 b came to be in its orbit.

(wpl)