Why puffy exoplanets often dance with perfect rhythm

Information Regarding Sub-Neptune Planets and Their Densities:

1. Sub-Neptune planets, which are worlds of a size between Earth and Neptune, constitute a significant number of the planets orbiting sun-like stars in the Milky Way, accounting for between 30% to 50% of such stellar systems.

2. These planets are often categorized as either ‘puffy’ or ‘non-puffy,’ but it has been debated whether these differences are due to separate planetary populations or measurement methodology. Recently, a study from the University of Geneva (UNIGE) and the University of Bern (UNIBE) suggests that there are indeed two different populations of sub-Neptunes.

3. Intriguingly, sub-Neptunes that are in orbiting resonance with their planetary companions (i.e., synchronous orbits) seem more likely to be puffy than those not in resonance. Examples of such resonant systems are rare, but one exemplary case is the HD 11067 system, which hosts six sub-Neptunes moving harmoniously around a single central star.

4. The hypothesis posed by the research team attributes this lightness to the methods and processes involved in the sub-Neptunes’ formation, explaining that they may have originated during periods in which planetary systems move through a resonant chain and a small percentage of these are stable enough to maintain this equilibrium over the long term.

5. Upon discovering a resonant planetary system, scientists are puzzled as to why the sub-Neptunes within it are lighter despite years of orbiting its parent star.

6. Sub-Neptune density can be estimated using two primary measurement methods – the Transit Timing Variation (TTV) and radial velocity methods. The TTV technique tends to identify exoplanets with lower densities when a planet passes in front of its host star from Earth’s perspective and measuring variations in transit time is key. In contrast, the radial velocity technique identifies planets by calculating the gravitational tug they create on the host star.

7. Recent examination of the two methods employed indicates that the TTV approach selectively identifies less-dense exoplanets, a possible reason for the bias seen toward higher densities within populations characterized by the radial velocity technique. Consequently, it is evident that fewer puffy sub-Neptunes are detected using the radial velocity technique since it requires more time and risks the observation being interrupted before calculating the planet’s mass, subsequently increasing the detection bias of densier planets.

8. Finally, the study found that not only did the TTV method primarily yield less dense exoplanets but also that these planets presented a lower density in resonant planetary systems than their counterparts observed within non-resonant systems — a finding consistent regardless of whether it was the TTV method or radial velocity technique that estimated their mass.

9. Having resolved the question of sub-Neptunes’ distinct populations and their connections with orbital resonance, future discoveries and research will equip us with a deeper understanding of the life histories of our galaxy’s most commonly occurring planet type. This inquiry may also eventually help illuminate why our solar system doesn’t host a sub-Neptune of its own.

10. This research has been published in the Astronomy & Astrophysics journal.

.st1{display:none}See more