How can some planets be hotter than stars? We've started to unravel the mystery.

This article was originally published at The Conversation. The publication contributed the article to’s Expert Voices: Op-Ed & Insights.

Quentin Changeat, Postdoctoral Research Fellow in Astronomy, UCL

Billy Edwards, Project Scientist of the Twinkle Space Mission, Research Fellow of Astronomy, UCL

Until the early 2000s, the only known planets were located in our own neighbourhood, the Solar System. They broadly form two categories: the small rocky planets in the inner Solar System and the cold gaseous planets located in the outer part. With the discovery of exoplanets, planets orbiting stars other than the Sun, additional classes of planets were discovered and a new picture started to emerge. Our Solar System is by no means typical.

For example, data from the Kepler mission has shown that large, gaseous exoplanets can orbit very close to their star – rather than far away from it, as is the case in our Solar System, causing them to reach temperatures exceeding 1,000 Kelvin (727 degrees Celsius). These have been dubbed “hot” or “ultra-hot” Jupiters. And while most other exoplanets are smaller, between the size of Neptune and Earth, we don’t know much about their composition.

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But how can hot, gaseous planets form and exist so close to their star? What kind of extreme physical processes happen here? Answers to those questions have large implications in our understanding of exoplanets and solar system planets. In our recent study, published in The Astrophysical Journal Letters, we have added another piece to the puzzle of planet formation and evolution.

Kelt-9 b

In essence, hot Jupiters are a window into extreme physical and chemical processes. They offer an incredible opportunity to study physics in environmental conditions that are near impossible to reproduce on Earth. Studying them enhances our understanding of chemical and thermal processes, atmospheric dynamics and cloud formation. Understanding their origins can