Astronomers have discovered a massive, ultra-hot Jupiter that comes dangerously close to his star. It is more massive than Jupiter and circles his star faster than Mercury circles the sun. There are only three possible results, and none of them are good for the planet.
The discovery is published in new research, which were published in the Astrophysical Journal entitled “Orbital decay of the Ultra-Hot-Jupiter Toi-21109b: Tidal restrictions and transit-timing analysis”. The senior author is Jaime Alvarado-Montes, a research scholarship holder at Macquarie University in Sydney, Australia.
The exoplanet revolves an F-type star in about 870 light years away. It is enormous, about five times as massive as Jupiter, circles his star in just 16 hours. That means it is very close to his star.
“Just to bring it into the context of the mass of Mercury is almost 6,000 times smaller than Jupiter, but it still takes 88 days to circle our sun. For a huge gas giant like TOI-2109B, in order to fully corberate in 16 hours-we say that this is a planet that is super close close for his star,” said, “said Lead authors-alvarado-montes in a press release.
This figure shows the distribution of USP-J-like planets with orbits of less than one day of the earth. The colors represent their equilibrium temperatures. Photo credits: Alvarado-Montes et al. 2025. APJ
The researchers found that the orphanage of Toi-219b was slowly and yet perceptibly expired. This is based on 14 years of combined transit timing data from the Tess and Cheops space telescopes as well as on floor-based data from many telescopes, including the Global Telescope Network of the Las Cumbres. These time measurements in combination with theoretical models showed that the orbit of the planet is going through subtle changes and that in the next three years its circulation will shrink by at least 10 seconds. This means that the planet has started a spiral of death in its star, and it is an almost ideal candidate for further observations and studies.
“Since the orbital period of this planet is shorter than the star rotation period, TOI-219B can be an optimal candidate for examining the decay of orbital,” the authors write. This is due to the flood rotation. Turning Tidal increases the tidal arch to the star, and since the planet is “in front”, the boosting is drawn forward. This transfers the angle impulse from the orbit of the planet to the rotation of the star. This turns the star faster and the planet is pulled further inwards.
Now that the planet's orbit is shrinking, there are only three possible results. The planet could cross the Roche border, the gravity of the star overwhelms the gravity of the star and is torn apart. Or the planet could immerse yourself directly in its star in a fiery finish. However, the third option is the most scientifically fascinating: the planet could have freed its voluminous atmosphere and only leave a rocky core.
This figure shows how complex tidal interactions are and shows some of the primary physical and orbital parameters that describe how the system develops. Photo credits: Alvarado-Montes et al. 2025. APJ
If that happens, it could explain the existence of some rocky exoplanets. You could be the remaining cores of once massive gas giants.
“This planet and its interesting situation could help us to find out some mysterious astronomical phenomena that we really have really not explained many evidence to,” says Dr. Alvarado-Montes. “It could tell us the story of many other solar systems.”
“Toi-2109b is one of the best candidates for orbital decline among well-known USP-Js, who is standing out due to his remarkably short orbital period and the characteristics of his guest star,” the researchers explain. There are 12 stars that organize Ultra Short Period Jupiter (USP-J). The star, toi-2109, has the highest orbital rate of all and is the most solid. “All of these functions can have profound effects on the fate of TOI-219B,” they write.
However, there are some things that could confuse further studies.
A rare process that could come into play here is called Core-Envelope tie coupling and plays in stars. In this process, the core and the shell revolve around different rates, and instead of turning evenly like a solid body, they are essentially separated from each other. This can happen if material attacks the star from the planet. “This could play an important role in the observable signatures of the orbital loss,” the researchers explain.
The efficiency of the star flood is also unclear. “In the event of an orbital decline, the rate to which the period is reduced is determined by the efficiency of the star tide processes that drive the exchange of the angle impulse with the planetary orbit,” the authors write. However, TOI-2109 is an F-type star, and in this type of star, these processes may not be sufficient to lead to a larger decrease in orbital. It can depend on the age of the star and uncertainties at the age of the star means that these processes are not well understood.
The researchers examined two cases, one with a young host star and one with an old host star. They found that the old guest star would induce a very fast orbital decline. The opposite applied to the young host star. “Conversely, the planetary companion against the decay of orbital is much more stable if the Host star toi-2109 is younger,” the authors explain.
Unfortunately, some of the data for TOI-2198B are unclear. There are great uncertainties in the times with medium transmission, and although the researchers are confident that the planet's orbit falsifies, this makes predictions challenging. “Future photometric and spectroscopic data will therefore be necessary to further restrict such a scenario,” they write.
The illustration of this artist shows Wasp-19b, another ultra-hot Jupiter in a very short orbit. The orbital period is less than 19 hours and the theory says that it should expire like TOI-219B. Photo credits: from ESA/Hubble & NASA, CC from 4.0.
The researchers intend to continue monitoring the star and planet for several years. If you see it looking until death, you will learn more about these tragic scenarios. Although these types of planets are very rare, you can keep important lessons with a performance rate of only ~ 0.5%.
“From temporary hot Jupiders who only exist for a short time, to the name of Jupiter, which paradoxically have water in their atmospheres, USP-Js represent the perhaps most extreme planetary systems and thus the knowledge that we gain through the study of these systems,” write the authors.
It is possible that some of the rocky exoplanets we find are only the remains of these once Giante planets that came too close to their stars.
