KELT-9b is the hottest exoplanet ever found


The bizarre gas giant is hotter than many stars and may have a comet-like tail, writes Ariella Heffernan-Marks.


An illustration of KELT-9b in orbit around its host star KELT-9.
An illustration of KELT-9b in orbit around its host star KELT-9.
NASA/JPL-Caltech

Scientists have discovered a planet that is hotter than many stars.

In May 2016, astronomers at the Winer Observatory in Arizona were observing the sky using the Kilodegree Extremely Little Teloscope (KELT), which allows them to look at many stars at once, at a low resolution. They noticed that a star named KELT-9 would dim slightly every 1.5 days.

An animation of KELT-9b in orbit around its host star KELT-9.
An animation of KELT-9b in orbit around its host star KELT-9.
NASA/JPL-Caltech

Further investigation led to the discovery of KELT-9b: an exoplanet that completes a circuit of KELT-9 every 1.5 days. It stood out from previous discoveries. According to Scott Gaudi, astronomy professor at the Ohio State University in Columbus who has led a study on this topic, it is the “hottest gas giant planet that has ever been discovered”.

The temperature on KELT-9b is 4600 Kelvin. To put this in perspective, it is only 1200 K cooler than the surface of our Sun, and three times as hot as volcano lava. The heat is due to extreme radiation that is projected onto the planet from its very bright host star KELT-9. Due to the nature of the planet’s orbit, only one face receives the ultraviolet blast, while the other side remains in darkness. The star-facing side of the planet is constantly exposed to radiation, making it exceptionally hot but also putting it at risk of evaporating away.

Ariella Heffernan-Marks in a Melbourne-based science writer.
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