“Death by star” is a fate awaiting most planets in star systems. That includes our Sun, Venus, and Mercury a few billion years from now. And, astronomers now see that same fate awaiting Kepler-1658b. It’s a hot Jupiter exoplanet orbiting an evolved F-type yellow-white dwarf star about 2600 light-years away from Earth.
A number of things conspire on a fiery fate for this world: closeness to its star, a slowly decaying orbit, and tidal physics. Exoplanet Kepler-1658b lies 0.054 AU from its primary—way closer than the distance between the Sun and Mercury in our solar system. It’s also whipping around in a 3.8-day orbit. That rapid, close-up orbit is decaying, thanks to a couple of factors. The first is a simple one: tides. Essentially, the gravitational interaction between Kepler-1658b and its aging host star is changing the planet’s orbit. That’s bringing it ever closer to the star. The second factor is the age of the star itself. It’s just old enough now to start expanding to become a subgiant. That’s an interesting bit of astrophysics. That, coupled with the tidal physics acting to decay the planet’s orbit, brings this overheated planet ever closer to its eventual “death by star”.
“We’ve previously detected evidence for exoplanets inspiraling toward their stars, but we have never before seen such a planet around an evolved star,” said Shreyas Vissapragada, a 51 Pegasi b Fellow at the Center for Astrophysics | Harvard & Smithsonian and lead author of a new study describing the results. “Theory predicts that evolved stars are very effective at sapping energy from their planets’ orbits, and now we can test those theories with observations.”
Detecting the Decline of an Exoplanet Orbit
The change in the orbital period between Kepler-1658b and its stellar parent is just 131 milliseconds per year. That means in about 2.5 billion years, Kepler-1658b will get swallowed up by its star, never to be seen again.
So, how did Vissaparagada and colleagues know that this is happening? Confirming the orbit of this exoplanet was a long, painstaking process that required many careful observations over several years. First, astronomers had to confirm that Kepler-1658b is an actual exoplanet and not just a perturbation in the data. This hot Jupiter planet was actually the first suspected exoplanet discovered by the Kepler mission. But, it took nearly ten years to confirm the finding with follow-up observations.
Continued observations turned up something odd about the planet—its dayside temperature is about 3450 K, which is hotter and brighter than astronomers expected. That may be explained by the planet’s interactions with the parent star. The tidal pull may be generating more heat inside the planet (much as the interaction between Jupiter, Io, and Europa do with Io’s interior). To confirm that, astronomers will need to do more observations of the planet and its star.
Learning from Kepler-1658’s Impending Demise
The next step after discovery required even more observations to understand the “inspiraling process” sending this exoplanet into its star. Orbital changes of 133 milliseconds per year are not really detectable in just a few observations. It took a village of telescopes and observatories, starting with Kepler, followed by ground-based work at Palomar Observatory’s Hale Telescope. Then, the Transiting Exoplanet Survey Telescopes (TESS) got its chance to take a look at the system. All of these captured data when the exoplanet transited across the face of its star (as seen from our point of view). The net result? Over the past 13 years, the time between transits has very slightly decreased. The root cause? Energy dissipation thanks to tidal interactions.
Okay, so now we know that tidal friction heats Kepler-1658b while its orbit decays into a spiral of doom. It will happen over the next few billion years. That’s a long death march. So, what’s next for astronomers to learn from this? “Now that we have evidence of inspiraling of a planet around an evolved star, we can really start to refine our models of tidal physics,” Vissapragada said. “The Kepler-1658 system can serve as a celestial laboratory in this way for years to come, and with any luck, there will soon be many more of these labs.”
The physics of this situation obviously can be applied to many other stars and their planets, including our own. Watching Kepler-1658b interact with its star in a gravitational battle to the death should give insight into what will happen in 5 billion years or so when Earth and a puffed-out red giant Sun interact.