On a collision course with its star, the closest hot Jupiter on record at only 870 light years away, TOI-2109b, has astronomers keenly observing the exoplanet’s orbital decay to see how it will compare to three proposed scenarios for the cosmic finale’s resolution.
According to a new study, the exoplanet’s orbit is a tight one—much closer to its star than that of Mercury—and only lasts 16 hours, giving it an extremely high surface temperature. Compared to the planet Jupiter, TOI-2109b contains five times its mass and is close to twice its size.
Studying a Hot Jupiter
TOI-2109b was not discovered until 2021, although it had been present, albeit unnoticed, in data dating back over a decade. The team based their findings on observations collected between 2010 and 2024 by NASA’s TESS mission and the European Space Agency’s CHEOPS satellite, supplemented with material from ground-based telescopes. Located relatively close by (within the Milky Way galaxy’s Hercules constellation), astronomers are able to get a relatively clear view of its cosmic happenings.
Only about one planet in 500 falls into the “hot Jupiter” category of gas giants, which travel extremely close to their stars, and TOI-2109b is the closest one ever observed.
“Just to put it into context – Mercury’s mass is almost 6,000 times smaller than Jupiter, but it still takes 88 days to orbit our Sun. For a huge gas giant such as TOI-2109b to fully orbit in 16 hours, it tells us that this is a planet located super-close to its star,” says lead author Dr Jaime A. Alvarado-Montes, a Macquarie Research Fellow.
As the researchers analyzed the timing data, they identified that the planet’s orbit was shrinking, with both theoretical models and observations suggesting a 10-second decrease over the next three years. This orbital reduction indicated that the planet is likely on a collision course with its parent star.
“Using all of the data available for this planet, we were able to predict a small change in its orbit,” Dr Alvarado-Montes says. “Then we verified it with our theory and with our planet evolution models, and our predictions matched the observations. That’s quite exciting.”
Exoplanet Collision Course
Based on these projections, the team has presented three hypotheses for how the exoplanet’s end will unfold. The first possibility is destruction by tidal forces. Under this scenario, those forces would reach a threshold called the “Roche limit” where tidal forces overcome the exoplanet’s gravity to tear it apart. First, the planet would elongate into a donut-like shape as its gravity failed to hold it together, before shredding completely.
The second possibility is simpler—but also destructive—where the planet ends in a fiery flash as it impacts its parent star.
“The star will absorb it and kill it, of course, in the process – completely burn it, and the planet will disappear,” says Dr Alvarado-Montes.
The most intriguing scenario is a third possibility, where intense stellar radiation may strip away the planet’s abundant gases, leaving only its rocky core through a process astronomers call “photoevaporation.”
“As the planet gets even closer to the star, all of the gas molecules could start being dissociated, and the planet gets smaller and smaller,” Dr Alvarado-Montes explains. “And if the planet shrinks quickly enough, then when the planet reaches the position where its Roche limit would have been, it’s not going to be five Jupiter masses anymore, but it will be small enough that the Roche limit moves closer to the star, so it could escape destruction.”
Potentially Rethinking Planetary Evolution
The most unusual of the hypotheses, the stripping of TOI-2109b’s gases, has significant ramifications for how astronomers understand planetary evolution. If that scenario proves true, it may indicate that some rocky planets across the universe were once gas giants prior to losing their atmospheres.
If photoevaporation does occur, the planet will transform from a hot Jupiter into a “super-Earth,” a large rocky planet that may be the size of Neptune or Uranus. The action would also stabilize TOI-2109b’s orbit, prolonging its existence for another few million years.
“This planet and its interesting situation could help us figure out some mysterious astronomical phenomena that so far we really don’t have much evidence to explain,” Dr Alvarado-Montes says. “It could tell us the story of many other solar systems.”
The event is a natural laboratory for astronomers to study planetary migration and evolution. Currently, astronomers plan to monitor the planet’s death spiral for the next three to five years, seeking evidence to support their predictions.
“This planet and its interesting situation could help us figure out some mysterious astronomical phenomena that so far we really don’t have much evidence to explain,” Dr Alvarado-Montes says. “It could tell us the story of many other solar systems.”
The new paper, “Orbital Decay of the Ultra-Hot Jupiter TOI-2109b: Tidal Constraints and Transit-Timing Analysis,” appeared on July 15, 2025, in The Astrophysical Journal.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.