What To Know
- A resonant planetary system is a system in which the planets have orbital periods which are in simple ratio to each other.
- In other words, the orbits of the planets are synchronized in such a way that the relative positions of the planets repeat themselves regularly.
- The TTV method measures transit time variations caused by gravitational interactions between planets, while the radial velocity method estimates variations in a star’s speed induced by the gravity of an orbiting planet.
Largely absent from the Solar System, but omnipresent in our galaxy, sub-Neptune exoplanets continue to fascinate planetologists. A recent study conducted by researchers from the University of Geneva (UNIGE) and the University of Bern (UNIBE) now reveals that these planets, smaller than Neptune but larger than Earth, can be divided into two distinct categories: “inflated” and “non-inflated” sub-Neptunes. This discovery sheds light on the mysteries of their density and formation, particularly in relation to their orbital resonance within their planetary systems.
Resonant systems
A resonant planetary system is a system in which the planets have orbital periods which are in simple ratio to each other. In other words, the orbits of the planets are synchronized in such a way that the relative positions of the planets repeat themselves regularly. This synchronization is known as orbital resonance. A recently discovered system at 100 light years of the Earth around the star HD 110067 perfectly illustrates this phenomenon. The latter includes six sub-Neptunes which follow a fascinating harmonic resonance. For example, the innermost planet completes an orbit in 9.1 Earth days, while the planet the outermost one completes one in 54.7 days. The resonance ratios between these planets are such that for every orbit of the outer planet, the inner planet completes six. These precise resonances have existed for about four billion years, a period comparable to the age of our own solar system. Researchers have recently observed that these resonant sub-Neptunes are generally less dense than those that do not follow orbital resonances. This difference in density raises intriguing questions about their formation and evolution.
The six planets orbit their central star HD 110067 in a harmonic rhythm, with the planets aligning every few orbits. Credits: Thibaut Roger, NCCR Planets
The inflated and non-inflated
Planetologists have used two main methods to measure the density of sub-Neptunes: temporal variation of transit (TTV) and the radial velocity. The TTV method measures transit time variations caused by gravitational interactions between planets, while the radial velocity method estimates variations in a star’s speed induced by the gravity of an orbiting planet. The results showed that the TTV method tends to detect less dense sub-Neptunes, while the radial velocity method is biased toward planets with higher density. By analyzing these data, the researchers then confirmed the existence of two distinct families of sub-Neptunes. “Swollen” sub-Neptunes are often in resonance with other planets in their systemwhile the “uninflated” ones are not. This resonance appears to play a crucial role in their density and evolution. The research team proposes that all planetary systems begin their existence by converging into a resonant chain. However, only about 5% of these systems manage to maintain this resonance over time. When the resonant chain breaks, it can cause planets to collide and merge, creating denser worlds. Resonant systems, on the other hand, retain their inflated sub-Neptunes because they escape these catastrophic events. This discovery sheds new light on the formation and evolution of sub-Neptune exoplanets and helps explain why our own Solar System does not contain such planets. The link between orbital resonance and sub-Neptune density thus opens up exciting perspectives for the study of exoplanets and the dynamics of planetary systems. Details of the study are published in the journal Astronomy & Astrophysics.
