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Hellishly hot planet orbits so close to its star that a year lasts a few days

Georgina Torbet
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A group of astronomers from the Physical Research Laboratory (PRL) in Ahmedabad, India has discovered a scorching new exoplanet where a year lasts just a few days, according to the Indian Space Research Organization.

The team, led by Prof. Abhijit Chakraborty of PRL, spotted the planet orbiting a star that is around 1.5 times the mass of our sun and is located 725 light-years away.

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“This discovery is made using PRL Advanced Radial-velocity Abu-sky Search (PARAS) optical fiber-fed spectrograph, the first of its kind in India, on the 1.2-meter Telescope of PRL at its Mt. Abu Observatory,” the Indian space agency ISRO writes. “These measurements were carried out between December 2020 and March 2021. Further follow-up measurements were also obtained from TCES spectrograph from Germany in April 2021, and also independent photometric observations from the PRL’s 43-centimeter telescope at Mt. Abu.”

The planet, called TOI 1789b or HD 82139b, is around 1.4 times the size of Jupiter, with around 70% of its mass. It also orbits its host star in just 3.2 days, meaning it is extremely close to the star — at a distance of just 0.05 times the distance between the Earth and the sun. That makes it a type of planet called a hot Jupiter.

As the planet is so close to the star, it experiences extremely high temperatures of up to 2000 K (3140 Fahrenheit or 1727 Celsius), which is hot enough to melt iron. The planet is also inflated, making it very low density, so it’s puffy and larger than Jupiter even though it has less mass.

Similar hot Jupiter planets have been discovered using other instruments like exoplanet NGTS-10b, discovered as part of the Next-Generation Transit Survey (NGTS), or the famous KELT-9b, the hottest exoplanet known with a surface temperature of up to 7,800 degrees Fahrenheit which was discovered using the Kilodegree Extremely Little Telescope.

Studying hot Jupiters can help astronomers understand how planetary systems form and evolve, as they are extreme outcomes and give rise to questions about how planets could have ended up so close to their stars. “The detection of such system enhances our understanding of various mechanisms responsible for inflation in hot Jupiters and the formation and evolution of planetary systems around evolving and aging stars,” the ISRO wrote.

The research is published in the journal Monthly Notices of the Royal Astronomical Society.

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