The phenomenon is known, but this is the first time that astronomers have observed it directly. Around the dying star Kepler-1658, the orbit of one of its planets is gradually declining. So that in a few billion years, it will collide with its star.
Located 2,629 light-years away, the star Kepler-1658 was discovered in 2009 by the télescope spatial Kepler, and it is home to a single exoplanet, Kepler-1658b. The discovery of the latter was not confirmed until 10 years later, in 2019. It is a “hot Jupiter” type planet, with a mass 5.9 times greater than Jupiter, a similar size, and above all, an orbit very, very close to its host star. Indeed, it completes a complete revolution of Kepler-1658 in just 3.85 days and is eight times closer to its star than Mercury is to the Sun!
But this is another characteristic that has interested a group of scientists, as evidenced by a study published in the journal The Astrophysical Journal Letters. The planet Kepler-1658b is slowly but surely approaching its star! To the point where it will be, within a few billion years, swallowed up by it! This is not the first time that such an observation has been made on an exoplanet, we can notably cite the case of Wasp-12b. But this time, the host star is a little special: it would already be half dead.
The phenomenon of orbital decline
This approach of Kepler-1658b to its star is called “orbital decline”. The phenomenon was also observed between Neptune and its moon Io, or between Mars and its moon Phobos. It can have several causes, but in the study it is mainly about tidal effects. When two bodies orbit around a common center of gravity, as in these planetary systems, they exert a gravitational force on each other. The force exerted will however be more intense at the level of the parts closest to the center of gravity, and less at the level of the more distant places.
But the tidal forces, alone, would not be enough to modify the orbit of Kepler-1658b, otherwise the same phenomenon would apply to all similar systems! It is then the direction of rotation of the two stars which defines the consequences of the phenomenon. In the case of the Earth-Moon system, they impose a synchronous rotation: the Moon is locked by the effects of the tide, and performs at the same time a turn around the Earth and a turn on itself. So much so that it always shows us the same face!
In the case of Kepler-1658b, its movement is retrograde: the planet rotates in the opposite direction to that of its star. And this is what causes the rapprochement observed by the researchers! Or at least in part. Because the star Kepler-1658 also turns out to be unusual! With 1.45 solar masses but a diameter almost three times greater than that of the Sun, the star is currently in the “subgiant” stage: after having consumed all its fuel, the star’s core contracts , and the nuclear fusion reactions start in a slightly more external layer. The result is a swelling of the star, hence the diameter of three times that of the Soleil !
The same fate awaits our planet, in 5 billion years
By scrutinizing the star Kepler-1658 and its planet thanks to the transit method for nearly 13 years, researchers have observed a decrease in its orbit time of 131 milliseconds per year, a first around this type of star! “We have already detected evidence of exoplanets being sucked into their stars, but we have never seen such a planet around an evolved star”said Shreyas Vissapragada in a communiquéfirst author of the study and an astrophysicist at Harvard-Smithsonian Center for Astrophysics. “The theory predicts that evolved stars are very efficient at extracting energy from the orbits of their planets, and now we can test these theories with observations. »
The opportunity to learn more about this star and the phenomenon. Because it is the same fate that awaits our Planet, when the Sun will pass to the stage of sub-giant, then red giant. But do not panic, this phenomenon will not occur before 5 billion years, the Sun being only halfway through its life! “Now that we have evidence of a planet being sucked around an evolved star, we can really start refining our tidal physics models,” enthused S. Vissapragada. “The Kepler-1658 system can serve as a celestial laboratory in this way for years to come, and hopefully there will soon be many more such laboratories. »