What happens to hot Jupiters when their star turns red?

This artist’s impression shows an ultra-hot exoplanet as it is about to transit in front of its host star. Using the ESPRESSO instrument of ESO’s Very Large Telescope, astronomers have found the heaviest element yet in an exoplanet's atmosphere, barium, in the two ultra-hot Jupiters WASP-76 b and WASP-121 b. Credit: ESO

Extrasolar planet research has yielded some remarkable findings, many of which have confounded astronomers' assumptions and challenged our understanding of the many shapes that planetary systems may take. The finding of Jupiter-sized planets that orbit close to their sun (hot Jupiters), for example, challenged what astronomers thought about gas giants. Previously, it was widely assumed that gas giants originate beyond the frost line, which is the barrier beyond which volatile elements (such as water) freeze solid and remain so for the remainder of their lives.

This will occur when our sun exits its main sequence phase and enters its red giant branch (RGB) phase. This begs the issue of what happens to hot Jupiters when their parent stars erupt into red giants. A team of researchers led by the Compact Object Mergers: Population Astrophysics and Statistics (COMPAS) consortium simulated how red giants will expand to engulf hot Jupiters using advanced 3D simulations. Their findings could help astronomers solve another mystery: why do some binary systems have one rapidly rotating star with strange chemical compositions?

Mike Lau, a Ph.D. student at Monash University's School of Physics and Astronomy, and other members of the COMPAS consortium, a collaborative effort to study the evolution of binary systems, led the research. They were joined by researchers from The ARC Center of Excellence for Gravitational Wave Discovery (OzGrav), Princeton University's Flatiron Institute's Center for Computational Astrophysics, and the Harvard & Smithsonian Center for Astrophysics. Their paper, Hot Jupiter engulfment by a red giant in 3D hydrodynamics, has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

As Lau noted to Universe Today via email, astrophysicists are interested in hot Jupiter engulfment because they believe it may explain some of the "strange" stars identified in our galaxy, such as swiftly revolving and chemically enriched massive stars. The recent surge in exoplanet discoveries has allowed for the testing of numerous hypotheses, including the potential that when stars expand to become red giants, planets that used to orbit at a safe distance would spiral into the star's center, churning up stellar material in the process. Lau stated:

This is, therefore, one way of explaining observed rapidly rotating giant stars. Also, any planetary material that comes off during the in-spiral could alter stars' surface chemical makeup. This may help us understand why a small fraction of stars is observed to be abnormally rich in lithium. Finally, we may be able to directly detect this process by looking for stars that have swollen up and brightened from eating a planet, though we will have to be very lucky to catch them in the act.

The capacity to directly view engulfments and their effects on stars will be available owing to next-generation space observatories such as the James Webb Space Telescope and ground-based telescopes with 30-meter (98-foot) main mirrors. This comprises the Extremely Large Telescope (ELT) and the Giant Magellan Telescope (GMT), both of which are being built in Chile's Atacama desert, and the Thirty Meter Telescope (TMT), which is now being built on Mauna Kea in Hawaii. These observatories will be able to identify exoplanets orbiting near to their stars using a mix of adaptive optics, coronographs, and spectrometers.

Meanwhile, Lau and his colleagues ran a series of 3D hydrodynamic simulations of the engulfment process. He put it like way:

We used a method called smoothed particle hydrodynamics. This represents the giant star and hot Jupiter as collections of particles that follow the fluid's motion, like a ball pit but with millions of balls. This technique has also been used to visualize fluids in video games and animations. A key result from our simulation is that the hot Jupiter may lose most of its material due to friction as it spirals into the star.

Lau and his colleagues expect that future breakthroughs in computers will enable higher-resolution simulations. If validated, these findings might explain quickly revolving stars with unusual chemical compositions in binary systems. They also provide a taste of what future surveys will reveal when they explore these systems and their exoplanets and get spectra straight from them.

Journal Infomation: Mike Y. M. Lau et al, Hot Jupiter engulfment by a red giant in 3D hydrodynamics, arXiv (2022). DOI: 10.48550/arxiv.2210.15848