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A Hellish Planet in Outer Space; Its Heat Shreds Molecules Apart Into Atoms

Astronomers have discovered a ultra hot Jupiter with extreme conditions. Credit: Haven Giguere, Nikku Madhusudhan

Ultra-hot Jupiter TOI-2109b is 20 times closer to its star than Mercury is to the Sun, and both iron and steel will easily melt on its daytime side.

The TESS space telescope discovered the ultra-hot Jupiter TOI-2109b, which turned out to be a unique object. This is the second record-breaking hot exoplanet of this type, which rapidly changes its orbit under the influence of tidal forces from its star and will subsequently fall into it.

What is a hot Jupiter?

Hot Jupiters are short-period gas giants that are rare enough to be found in only about 0.5 percent of Sun-like stars. Despite this, such exoplanets have played a huge role in creating the current picture of the diversity of exoplanet atmospheres, which, in turn, significantly influenced theories of formation, evolution, and dynamics of planets. This was achieved due to the fact that hot Jupiters are convenient to detect and study due to their proximity to their stars and their large size using a variety of observation techniques.

What about their ‘ultra’ relatives?

In recent years, a separate group of such planets has attracted special attention of scientists – ultra-hot Jupiters, the temperature of the daytime side of which exceeds 2500 Kelvin. Such planets include, for example, KELT-9b, which holds the record for temperature – its outer layers are heated to temperatures comparable to the photospheres of some dwarf stars.

The Hellish Ultra-hot Jupiter TOI-2109b

A group of astronomers led by Ian Wong of the Massachusetts Institute of Technology announced the discovery of a new ultra-hot Jupiter TOI-2109b, which is located in an F-star system at a distance of 855 light-years from the Sun. Initially, the object was discovered by the TESS space telescope using the transit method, later the discovery was confirmed using a number of ground-based telescopes.

The host star has a mass of 1.44 solar masses and a radius of 1.69 that of the sun. An exoplanet with a mass of 5.02 Jupiter masses and a radius of 1.34 Jupiter’s radius orbits around it with a period of 16 hours.


The temperature of the outer layers of TOI-2109b on the dayside is estimated at 3631 Kelvin, making this exoplanet the second hottest known object of this type. It is possible that the planet’s night hemisphere remains extremely cold. In this case, a kind of chemical circulation acts on it: at extremely high temperatures, compounds decompose into individual atoms on the dayside, and on the cold night side, they again form bonds with each other.

What else makes this hot Jupiter unique?

TOI-2109b turned out to be a unique object also because the planet’s extremely short orbit and the intense gravitational interaction between it and the star make the system an ideal target for studying the decay of the orbit under the influence of tidal forces, which will eventually lead to the exoplanet falling onto the star along a spiral trajectory. It is expected that over the next few years, telescopes will be able to detect the change in the planet’s orbit, as well as investigate the influence of powerful streams of radiation from the star on the atmosphere of the hot Jupiter.

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