They are known as "Hot Jupiters," massive gas giants that orbit very close to their stars. As a result, they have very short orbital periods (less than a day in some cases) and experience extreme temperatures of up to 1,650 °C (3,000 °F). In fact, these planets are superheated to the point that minerals become vaporized and form clouds in their atmospheres. While this class of planet is rare, accounting for about 500 of the more than 5,900 exoplanets confirmed to date, the existence of these planets has raised questions about our planetary formation models.
Even rarer than individual Hot Jupiter systems are binary systems where two Hot Jupiters exist, one orbiting each star. Thanks to new research from a team of Yale astronomers, scientists may have solved the mystery of these planets. As they state in their paper, they may have determined the origin story of double Hot Jupiters in binary systems. According to their findings, the long-term evolution of binary star systems can lead to Hot Jupiters forming naturally around each star. These findings could provide insight into this rare class of planet and could help inform future models of planet formation.
The research team responsible was led by Malena Rice, a planetary astrophysicist and Assistant Professor in the Yale Department of Astronomy. She was joined by Yurou Liu and Tiger Lu, an undergraduate student of physics and computer science at Yale University and an Astrophysics Ph.D Candidate at Yale University. The paper that describes their findings, "The Formation of Double Hot Jupiter Systems through von Zeipel–Lidov–Kozai Migration" (on which Liu was the lead author), appeared on June 10th in the Astrophysical Journal.
Astronomers have been pondering the existence of Hot Jupiters since the first was discovered in 1995, which was 51 Pegasi b (Dimidium). This was the first exoplanet ever confirmed, which orbits a Sun-like star roughly 50 light-years from Earth. Traditionally, scientists theorized that gas giants form in the outer reaches of a star system, where they are most likely to be found in stable orbits (similar to the Solar System). However, the fact that many gas giants have been observed orbiting closely to their stars has raised questions about planet formation and whether or not they migrate.
The discovery of binary systems where each star has a Hot Jupiter raises additional questions about the dynamics that shape planetary systems. For their study, Rice and her colleagues ran numerical simulations of two stars and two planets in a binary system using the Grace computing cluster at the Yale Center for Research Computing, the NASA Exoplanet Archive, and data from the European Space Agency's Gaia mission. Their simulations incorporated a mechanism formally known as von Zeipel-Lidov-Kozai (ZLK) migration.
Per this process, over long timescales, planets with unusual orbits will be influenced by the gravity of a distant secondary object (the stellar companion). Rice and her team observed that this mechanism can produce Hot Jupiters around twin suns. "The ZLK mechanism is a dance of sorts. In a binary system, the extra star can shape and warp planets' orbits, causing the planets to migrate inward," said Rice. "We show how planets in binary systems can undergo a mirrored migration process, so that both stars end up with hot Jupiters."
"Our proposed mechanism works best when the stars are at a moderate separation," said Lu. "They need to be far enough apart that giant planets are still expected to form around each star, but close enough together for the two stars to influence each other during the system lifetime." The research team also theorizes that future exoplanet surveys could find more examples of double Hot Jupiter systems. As they indicate, there are dozens of confirmed Hot Jupiters in star systems that have a stellar companion where another giant planet could be found. As Lui said:
"With the right code and enough computing power, we can explore how planets evolve over billions of years — movements that no human could watch in a lifetime, but that still could leave imprints for us to observe. We would expect giant planets to form far away from their host stars. This makes hot Jupiters both accessible and mysterious — and a worthwhile subject to study."