How did hot Jupiters end up orbiting so close to their stars, thus earning their moniker? This is what a recent study published in *The Astronomical Journal* hopes to address as a team of researchers from The University of Tokyo investigated the orbital evolution of hot Jupiters ended, specifically regarding where their orbits started before orbiting so close to their stars. This study has the potential to help scientists better understand the formation and evolution of exoplanets and what this could mean for finding life beyond Earth.
For the study, the researchers used a series of mathematical equations on more than 500 hot Jupiters to ascertain the orbital origin of a long-known process known as disk migration and high-eccentricity migration (HEM). Disk migration is when a planet’s orbit changes while the planet is still within the protoplanetary (young) disk orbiting its star, while HEM is when a planet’s orbit is elongated and eventually forms into a circular orbit. The team specifically explored the timescales of planetary orbit changes from highly-eccentric to circular versus the system’s age and what this could mean for both processes.
In the end, the researchers found that while the vast majority of the planets examined in this study were found to have orbital times for planets to go from highly-eccentric to circular were less than the system’s age, the team found that approximately 30 hot Jupiters didn’t meet this criteria, meaning the timescales for going from highly-eccentric to circular was greater than the system’s age.
The study notes in its conclusions, “In this paper, we identified close-in Jupiters that likely arrived via disk migration by leveraging the idea that when the circularization timescale of a planet is longer than system age, HEM would not be able to complete in time. To do this, we empirically calibrated the tidal quality factor using the eccentricity distribution of 578 Jovian mass planets with measured masses and radii.”
Going forward, the researchers note a larger sample size is required, along with studying the obliquity (tilt) of protoplanetary disks and how this plays a role in disk migration. The team emphasizes the importance of studying archival data from NASA’s now-retired Kepler telescope and NASA’s active Transiting Exoplanet Survey Satellite (TESS) mission.
Hot Jupiters are one of the most captivating objects in the universe since they do not mirror anything in our solar system, since the gas giants like Jupiter orbit much farther out. As fate would have it, the first confirmed exoplanet in 1995 was a hot Jupiter, thus immediately challenging our understanding of planetary system formation and evolution. Since then, scientists have confirmed the existence of approximately 500-600 hot Jupiters, which is a tenth of the total confirmed exoplanets. Additionally, scientists have also gained greater insight into their formation and evolution, specifically regarding whether their orbits began so close to their host stars or originally started much farther out.
What gives them the moniker “hot Jupiter” is owed to their extremely close orbits, ranging between 1 to 10 days, with some orbiting shorter than one day. In the early years of exoplanet discovery, the ratio of hot Jupiters to other types of exoplanets was very lopsided, primarily due to the infancy of discovery methods. While this ratio has substantially decreased in recent years, the origin of hot Jupiters has remained a mystery, with scientists debating these scorching planets resulting from disk migration or from highly eccentric orbits. While hot Jupiters, and any potential moons, have temperatures far too extreme for life as we know it to exist, they could provide key insight into exoplanet formation and evolution.
What new insight into hot Jupiters will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!