When planets orbit their star closely, the star's gravity creates strong tidal forces that can slow the planet down and narrow its orbit even further. The planet thus gets on a spiral death course, which ends with the fall into the star. Now astronomers have identified a hot gas giant that embarked on such a death course. Hot Jupiter Kepler-1638b is gradually approaching its star and will be engulfed by it in the distant future. This gas giant is the first exoplanet around an aging, already expanding star where such a spirally narrowing orbit has been detected.
As our Sun nears the end of its life cycle in billions of years, it will gradually expand and transform into a red giant. Its radius increases so much that it will gradually tear apart and swallow up all the inner planets of the solar system. But even before that, getting too close to the star can become dangerous. The star's gravitational influence creates tidal forces in nearby planets, slowing them down and gradually narrowing their orbits. The earth's gravity exerts a similar effect on the moon: its speed and orbit have also shortened over billions of years of earth's history due to tidal interactions. A few years ago, astronomers also discovered an exoplanet in such a spiraling orbit.
Exoplanet around a swelling subgiant
Now a team led by Shreyas Vissapragada from the Center for Astrophysics | Harvard & Smithsonian in the US identified another exoplanetary “death row inmate”. This is a very prominent exoplanet, because the hot gas giant Kepler-1638b was the first exoplanet in 2009 that the Kepler space telescope had detected based on its transit. However, it took another ten years before further observations could confirm the existence of this planet. Kepler-1638b lies about 5,000 light-years from us and is about the size of Jupiter but five times heavier. The extrasolar gas giant orbits an aging star that has almost completely depleted its core of hydrogen. As a result, nuclear fusion has shifted to a shell surrounding the core. As a result of this shell burning, the star swells and becomes what is known as a subgiant.
The gas giant Kepler-1638b orbits its star at a distance of just 0.05 astronomical units — about one-eighth the distance from Mercury to the Sun. The planet therefore only needs about 3.8 days for one orbit around its star. It is one of fewer than a dozen known exoplanets orbiting a swelling subgiant. Already a few years ago there were first indications that this would affect the orbit of Kepler-1638b. To investigate this, Vissapragada and his colleagues have now collected and analyzed further observation data of this exoplanet. In addition to transit data from the Kepler space telescope, they used data from the Hale Telescope in southern California and NASA's Transiting Exoplanet Survey Telescope (TESS), which has been in operation since 2018. Together, these data made it possible to measure and compare the planet's orbital periods over 13 years.
131 milliseconds slower per year
The analyzes confirmed that the orbit of Kepler-1638b has already changed slightly under the influence of stellar tidal forces. According to the measurements, the orbital period of the hot gas giant is reduced by 131 milliseconds per year. "We have previously observed exoplanets spiraling toward their stars, but never before have we detected this in an exoplanet around an aging star at the end of its life cycle," says Vissapragada. "The theory is that such subgiants are particularly effective at extracting energy from the orbits of the planets orbiting them." Thus, the contracting stellar core and expanding outer layers ensure that such advanced stars exert stronger tidal forces on their planets. "Now we can test this theory based on the observations," says Vissapragada.
The star's strong tidal forces may also explain why the planet Kepler-1638b appears hotter and brighter than it should: tugging gravitational pulls stretch and compress its interior, heating it even further. Astronomers compare this to the effect of Jupiter on its moon Io. Its interior is heated up so much by the tidal forces that Io is the most volcanically active celestial body in the entire solar system. Similarly, Kepler-1638b could also be heated by its star's tidal forces. "Now that we have the first evidence for a planet's spiral orbit around a mature star, we can use it to refine our models of tidal physics," says Vissapragada. "The Kepler-1638 system can thus serve as a heavenly laboratory for us in the years to come." The astronomers also plan to keep an eye on the hot gas giant and its star.
Source: Shreyas Vissapragada (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA) et al., The Astrophysical Journal Letters, doi: 10.3847/2041-8213/aca47e