Вадим Дудченко
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With an extremely short orbital period of 0.67 days (16 hours), the newly-discovered ultrahot Jupiter — named TOI-2109b — has the shortest orbital period among all known extrasolar gas giants.

The ultrahot Jupiter TOI-2109b is relatively close to its star, TOI-2109, at a distance of only about 2.4 million km (1.5 million miles) out. Image credit: NASA / ESA / G. Bacon.

TOI-2109b was initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission.

Its planetary nature was then confirmed through radial-velocity measurements using several ground-based telescopes.

The planet orbits TOI-2109, an F-type star located 861 light-years away in the southern portion of the constellation Hercules.

Also known as TIC 392476080, the star is roughly 50% larger in size and mass compared to the Sun.

TOI-2109b is about 5 times Jupiter’s mass and 1.35 times Jupiter’s radius.

Due to its extremely tight orbit and proximity to its star, the planet’s day side is estimated to be at around 3,500 K — about as hot as a small star. This makes the planet the second hottest detected so far.

“Meanwhile, the planet’s night side brightness is below the sensitivity of the TESS data, which raises questions about what is really happening there,” said Dr. Avi Shporer, an astronomer in the Kavli Institute for Astrophysics and Space Research at MIT.

“Is the temperature there very cold, or does the planet somehow take heat on the day side and transfer it to the night side? We’re at the beginning of trying to answer this question for these ultrahot Jupiters.”

Judging from its properties, Dr. Shporer and colleagues believe that TOI-2109b is in the process of ‘orbital decay’ — it is spiraling into its star at a rate of 10 to 750 milliseconds per year, faster than any hot Jupiter yet observed.

“In one or two years, if we are lucky, we may be able to detect how the planet moves closer to its star,” said Dr. Ian Wong, an astronomer at NASA’s Goddard Space Flight Center.

“In our lifetime we will not see the planet fall into its star. But give it another 10 million years, and this planet might not be there.”

The study was published in the Astronomical Journal.


Ian Wong et al. 2021. TOI-2109: An Ultrahot Gas Giant on a 16 hr Orbit. AJ 162, 256; doi: 10.3847/1538-3881/ac26bd


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