A team of scientists led by astronomer Ján Šubjak from the Stellar Division of the Astronomical Institute of the CAS and the Center for Astrophysics at Harvard University and the Smithsonian Institution recently announced a major discovery in the field of exoplanet research. Using data from the TESS space mission and ground-based observations with the HARPS spectrograph in Chile, the existence of a mini-neptun exoplanet, named TOI-2458 b, has been confirmed.
The discovery of the mini-neptun TOI-2458 b
TOI-2458 b has been identified for the first time as a transiting planet - as it moves in front of its parent star, it causes a periodic decrease in its brightness. Changes in the star's brightness have led scientists to make further detailed observations. The combination of data from the TESS satellite and spectroscopic measurements made it possible to determine the size, mass and other properties of the planet.
The planet TOI-2458 b has a radius about 2.8 times that of Earth and a mass about 13.3 times that of Earth. It orbits in a very close orbit - one revolution around the star takes only 3.74 days. This means that the planet is in a very hot environment where conditions are not conducive to life.
The parent star and its unusual properties
The star around which TOI-2458 b orbits has also caught the attention of scientists. It is an F-type star that has a slightly higher mass than our Sun and a surface temperature of around 6000 K. Analyses show that the star is in an advanced stage of its evolution and is beginning to leave the main sequence, suggesting an age of around 5.7 billion years.
Another surprising discovery was the star's rapid rotation, which lasts just 9 days, along with a very short magnetic activity cycle of 54 days. This combination is unusual for F-type stars. Similar short cycles have only been observed in a few stars, such as τ Boo, which is known for its interaction with a hot Jupiter-type exoplanet (note: according to the Institute of Czech Language of the CAS we have a planet Jupiter or Neptune in the Solar System, but if it is a type of exoplanet, it is recommended to say "hot Jupiter", "mini Neptune", etc.).
"Our observations suggest that this system is dynamically very interesting," says Dr. Šubjak. "The star's rapid rotation and magnetic activity may be the result of past interactions with planetary objects."
Discovery of another planet in the TOI-2458 system
Further investigation of the TOI-2458 planetary system identified the presence of a second planet orbiting at a greater distance from the star. This planet has an orbital period of about 16.5 days, which means it is farther from the star than TOI-2458 b. Dynamical models suggest its mass is between 10 and 25 times that of Earth, but further observations will be needed to determine its properties more precisely.
The history of the formation of the system and the role of the lost hot Jupiter
One of the most important findings of this study is the hypothesis that the TOI-2458 planetary system may have included a hot Jupiter in the past. Contrary to hypotheses suggesting the migration of these planets from the outer parts of the protoplanetary disk, this hot Jupiter may have formed in situ at a very close distance from the star.
Over time, however, the giant planet was swallowed by the star due to tidal interactions. This process could explain why TOI-2458 is now rotating unusually fast. "The ingestion of a massive planet could have given the star a significant amount of rotational torque," explains Dr. Šubjak.
Impact on current system dynamics
If this hypothesis of a lost hot Jupiter is valid, its presence in the past would have significantly affected the dynamics of the orbits of the remaining planets. Scientists believe that gravitational interactions may have led to the unusual orbital inclination of the mini-Neptune TOI-2458 b, which now orbits the poles of its star. This tilt could be the result of secular resonances that acted on the planet in the early stages of the system, before the hot Jupiter spiralled into its star.
Implications for exoplanet research and planetary system formation
If confirmed, this hypothesis could provide new insights into how star systems interact with planets orbiting in their vicinity. Future observations could provide further evidence for this theory while revealing more details about the dynamical processes in exoplanetary systems. Similar studies could also reveal how common such scenarios are in our Galaxy and what conditions must be met for the formation and long-term stability of such systems. "The TOI-2458 system shows us that the Universe is full of surprises and brings new challenges to our models of planet formation," adds Dr. Šubjak.
Institute of Astronomy of the CAS/ gnews - RoZ
Illustration of the TOI-2458 system in an artificial intelligence vision. Image created with DALL-E, OpenAI
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