Half Planet-Half Star Found To Be Hotter Than Our Sun

Deep in the void of space, an object hotter than our sun has been discovered that is blurring the lines between what defines a star and a planet.

Orbiting a star around 1,400 light-years away, astronomers have found a brown dwarf that is much hotter than any other measured, even exceeding the temperature of our own sun.

According to a paper on the preprint server arXiv, due to be published in the journal Nature Astronomy, this object—named WD0032-317B—has a temperature of around 8,000 Kelvin, equivalent to 7,730 C. In comparison, our sun's surface is "only" around 5,500 C.

Brown dwarfs are a kind of half star, half planet, sometimes called "failed stars." They are huge balls of gas between 13 times and 80 times the mass of Jupiter that are not massive enough to sustain the full-blown nuclear fusion process required to ignite into a star, like our sun.

They do have some form of fusion, however, as heavy isotopes of hydrogen called deuterium require less temperature and pressure to fuse.

This places them in a gray area between an enormous Jupiter-like gas planet, and a star, and usually burn at a much lower temperature than most other stars.

Usually, brown dwarfs are nowhere near the temperature of WD0032-317B, which was named by a team including astrophysicist Na'ama Hallakoun of the Weizmann Institute of Science in Israel, making it the hottest of its kind ever found. Brown dwarfs are usually between 480 and 1,930 C, due to not having the same level of energy generation as a standard star.

WD0032-317B was found to have a mass of between 75 and 88 Jupiters, and orbits its star in only 2.3 hours.

The reason for its unusually scorching temperatures is that this brown dwarf orbits its parent star at a very close distance, bombarding its surface with ultraviolet light. This can lead to a process called thermal dissociation occurring, which is when the atmosphere of the orbiting planet evaporates, with the molecules being torn apart.

"To date, only 12 white dwarf + brown dwarf systems are known. This makes WD0032−317 the 13th known such system (assuming the companion is a brown dwarf), with the hottest irradiated companion," the authors wrote in the paper.

One other example of a planet orbiting so close to its star that it begins thermal dissociation is KELT-9b, a gas giant exoplanet that orbits a blue supergiant star, and reaches surface temperatures of around 4,430 C on the star-facing side.

The authors state that they hope that this discovery can help them figure out the conditions on Jupiter-like gas giants, like KELT-9b, orbiting close to extremely hot, massive stars, as observation of these planets can be difficult due to their proximity to the star.

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