An article published in “The Astrophysical Journal Letters” reports a study on the exoplanet Kepler-1658b which predicts the decay of its orbit until it’s destroyed by its star. A team of researchers used data gathered from observations conducted over several years, from those by the Kepler Space Telescope that discovered Kepler-1658b to those by the TESS Space Telescope to examine its orbit. The conclusion is that this gas giant is slowly approaching its star and will be destroyed in the future.
The history of the exoplanet Kepler-1658b is somewhat curious since its traces were among the first found in NASA’s Kepler space telescope mission but it took almost a decade to confirm its existence. This hot or perhaps ultra-hot Jupiter very close to its star continues to be of interest to astronomers within the study of planetary migrations. Kepler-1658b appears to be such a case that will lead to its destruction.
For planets very close to their stars, destruction may be a common fate. If that’s proven, it would be due to tidal forces generated by the interaction between a planet and its star. The force of gravity they exert on each other can distort their shape leading to the release of energy. The dynamics can be very different depending on the combination of distance, size, and rotation speed of the interacting bodies and are still under study. We know for example that the Moon is moving away from the Earth due to those dynamics but a planet already close to its star could continue to approach.
Changes in a planet’s orbit are very slow, making it really hard for us to detect. In the case of the exoplanet Kepler-1658b, the fact that its first traces were discovered in 2009 is an advantage because it means that the authors of this study had data collected over about 13 years.
After the Kepler space telescope, the Hale telescope in California observed the exoplanet Kepler-1658b up to the most recent observations conducted with the TESS space telescope, Kepler’s successor. These instruments captured a series of transits of this gas giant in front of its star from the point of view of the Earth or its orbit. This made it possible to accurately measure its year’s length, less than four Earth days. It turned out that its year has shortened by about 131 milliseconds per year, meaning that its orbit is shortened as it gets closer to its star.
The star Kepler-1658 is more massive than the Sun and has reached a point in its life where it’s beginning to expand into the so-called subgiant phase. According to the models, such evolved stars have an internal structure that should allow the tidal energy subtracted from their planets to be more easily dissipated than stars still in a stable phase such as the Sun. For this reason, an evolved star should accelerate the orbital decay of a planet close to it allowing to detect it without having to wait centuries. Astrophysicist Shreyas Vissapragada, the lead author of this study, explained that now he and his colleagues will be able to test this theory with observations.
Another result of the study of the exoplanet Kepler-1658b concerns its brightness and heat emissions, which seemed anomalous. According to the researchers, the tidal interactions with its star could generate energy within the planet and explain its characteristics. In practice, that’s a phenomenon similar to what happens to the moons of gas giants in which enough heat is generated to have underground oceans of liquid water or volcanoes, such as Jupiter’s moon Io.
Kepler-1658b is probably only the first in a series of exoplanets whose orbital decay will be observed, a kind of celestial laboratory to improve models of tidal interactions. The TESS space telescope is increasing the number of exoplanets orbiting evolved stars and this could help this type of research.
