A team of scientists led by astronomer Jan Šubjak from the Stellar Department of the Astronomical Institute of the Czech Academy of Sciences, and the Harvard University Center for Astrophysics and the Smithsonian Institution, recently announced a significant discovery in the field of exoplanet research. Using data from the TESS space mission and ground-based observations with the HARPS spectrograph in Chile, they confirmed the existence of a mini-Neptune exoplanet named TOI-2458 b. This discovery provides new insights into possible scenarios for the formation of planetary systems.

The discovery of the mini-Neptune TOI-2458 b

TOI-2458 b was first identified as a transiting planet – its passage in front of its host star causes a periodic dimming of the star's brightness. These changes in the star's brightness led scientists to conduct further, more detailed observations. The combination of data from the TESS satellite and spectroscopic measurements allowed them to determine the planet's size, mass, and other properties.

The planet TOI-2458 b has a radius approximately 2.8 times that of Earth and a mass of about 13.3 times that of Earth. It orbits very close to its star – one orbit takes only 3.74 days. This means the planet is in a very hot environment, where conditions are not conducive to the existence of life.

The host star and its unusual properties

The star around which TOI-2458 b orbits also attracted the attention of scientists. It is an F-type star, which is slightly more massive than our Sun and has 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 approximately 5.7 billion years.

Another surprising discovery was the star's rapid rotation, which takes only 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 been observed in only a few stars, such as τ Boo, which is known for its interaction with a "hot Jupiter" exoplanet (note: within our solar system, we have planets like Jupiter or Neptune, but when referring to a type of exoplanet, it is recommended to use terms like "hot Jupiter," "mini-Neptune," etc.).

"Our observations suggest that this system is very dynamically interesting," says Dr. Šubjak. "The rapid rotation and magnetic activity of the star may be a result of interactions with planetary objects in the past."

The discovery of another planet in the TOI-2458 system

During further investigation of the TOI-2458 planetary system, scientists identified the presence of a second planet orbiting at a greater distance from the star. This planet has an orbital period of approximately 16.5 days, which means it is located further from the star than TOI-2458 b. Dynamical models suggest that its mass is between 10 and 25 times the mass of Earth, but further observations will be needed to determine its properties more precisely.

The history of the system's formation and the role of a lost hot Jupiter

One of the most significant findings of this study is the hypothesis that the TOI-2458 planetary system may have once included a hot Jupiter. Unlike hypotheses that assume the migration of these planets from the outer regions of the protoplanetary disk, this hot Jupiter may have formed directly in place, very close to the star.

Over time, however, the giant planet was engulfed by the star due to tidal interactions. This process may explain why the TOI-2458 star now exhibits unusually rapid rotation. "The engulfment of a massive planet could have imparted a significant amount of angular momentum to the star," explains Dr. Šubjak.

Impact on the Current System Dynamics

If this hypothesis about a lost hot Jupiter is correct, its presence in the past would have significantly influenced the orbital dynamics 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 currently orbits around the poles of its star. This inclination could be the result of secular resonances that acted on the planet in the early stages of the system, before the hot Jupiter spiraled into its star.

Implications for Exoplanet Research and Planetary System Formation

If this hypothesis is confirmed, it could provide a new perspective on how stellar systems interact with the planets orbiting them. Future observations could provide further evidence for this theory, while also revealing more details about the dynamic 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 presents new challenges for our planet formation models," adds Dr. Šubjak.

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Astronomical Institute of the Czech Academy of Sciences / gnews - RoZ

Illustration of the TOI-2458 system, as imagined by artificial intelligence. Image created using DALL·E, OpenAI