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Thermal dissociation
Planets orbiting close to their stars are exposed to large amounts of ultraviolet light. This can cause
their atmospheres to evaporate, in a process known as thermal dissociation. However, until now this
process was very difficult to study, because being so close to a very bright star, signals from an
orbiting exoplanet can be difficult to distinguish from stellar activity; according to researchers, the
discovery of the brown dwarf WD0032-317B may help us understand precisely this process. This is
because the star around which it orbits is a white dwarf: WD0032-317. With a temperature of
37,000 degrees Kelvin, WD0032-317 is very hot, but has a relatively low mass: only 40% of the mass
of the Sun. That makes this white dwarf, which is actually a star in the final stage of its life, fainter
and therefore its companion easier to detect. This has made it possible to find WD0032-317B, its
companion brown dwarf, which is more than 80 times the mass of Jupiter.

New discovery
According to our knowledge of the Universe, if a space object revolves around a star, it has to be a
planet and a relatively cold body: Like the planets revolving around the Sun, which can reach a
temperature above 5000 degrees Celcius.
However, a new finding challenges precisely this understanding of the Universe. It is the discovery of
a brown dwarf, a substellar object that is not quite a planet, but not quite a star either.
The brown dwarf found by an international team led by astrophysicist Na’ama Hallakoun, from the
Weizmann Institute of Science in Israel, is called WD0032-317B and its orbit is so close to that of its
hot star that its temperature exceeds 8,000 Kelvin (7,727 degrees Celsius, or 13,940 Fahrenheit).
This star would then be the hottest object of its kind ever recorded, surpassing even the
temperature of the Sun, which is around 5,778 degrees Kelvin, or 5,200 degrees Celsius.

In conclusion
The most striking thing about this finding is that the brown dwarf is a kind of smoking star: its hyperheated companion, the white dwarf, slowly evaporates it. The finding of WD0032-317B is not only very important because it is the hottest stellar body, but also an excellent candidate for studying how extremely hot stars can evaporate their lower-mass companions. The astrophysicists’ paper has been accepted for publication in Nature Astronomy and is currently available on the arXiv preprint server. (For more information on pre-accepted articles).

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