Why inflated exoplanets often dance with a perfect rhythm

Planets beneath Neptune that dance in time with the rest of their planetary systems have lower densities than planets that don’t, planetary scientist David Brown has discovered.

Though conspicuously absent from the solar system, the most common planets in the Milky Way are known as “sub-Neptunes,” or worlds with sizes between Earth and the ice giant Neptune. It’s estimated that between 30% and 50% of Sun-like stars are orbited by at least one sub-Neptune — but despite the ubiquity of these worlds, scientists studying exoplanets, or exoplanets, have traditionally had difficulty measuring the density of sub-Neptunes.

Depending on the techniques used to make these measurements, sub-Neptunes appear to split into two distinct categories: “puffy” and “non-puffy.” The question, however, is whether there are actually two distinct populations of sub-Neptunes or whether these differences are a result of which method was used to measure density. On that front, new research from the University of Geneva (UNIGE) and the University of Bern (UNIBE) suggests that there are indeed two physically distinct families of sub-Neptunes. And puffy sub-Neptunes are more likely to be in resonance with their planetary siblings.

Waltzing with planetary partners

Planets are said to be in resonance when, for example, one planet completes one orbit in the same time that another planet completes two orbits.

An extraordinarily resonant planetary system recently discovered is HD 110067, located 100 light-years from Earth. The six sub-Neptune worlds in this system dance around each other in a precise cosmic waltz. The innermost planet completes an orbit in 9.1 Earth days, the next planet out of orbit in 13.6 days, the third in 20.5 days, the fourth in 30.8 days, the fifth in 41 days, and the outermost planet in 54.7 days.

So for every orbit of the star that the outer planet completes, the inner planet completes six orbits. That means that these sub-Neptunes would be in a 6:1 resonance. The other resonances between different pairs of planets in the HD 110067 system are 3:2, 3:2, 3:2, 4:3, and 4:3.

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This rhythmic dance has been going on around the bright orange star HD 110067 for about 4 billion years, about as long as the solar system has existed. As fascinating as it is, it doesn’t tell us why the sub-Neptunes in this system appear less dense.

The team behind this new research has proposed a number of possible explanations for the luminosity of resonant sub-Neptunes. The most likely explanation seems to suggest that the process is related to how they formed.

an orange star is surrounded by six planets, each a different color. their orbits are outlined with white circles

an orange star is surrounded by six planets, each a different color. their orbits are outlined with white circles

It is possible, the team says, that all planetary systems converge on a resonant chain during their early lives. However, they believe that only 5% of systems can maintain this rhythm.

Breaking the resonance chain could lead to a series of cataclysmic events, with planets crashing into each other and often merging into denser conglomerate worlds. This means that resonance chain systems could also retain their bulging sub-Neptunes, the team says, as collisions and mergers increase the density of the same planets in non-resonance systems.

“The numerical models of the formation and evolution of planetary systems that we have developed in Bern over the past two decades reproduce exactly this trend: planets in resonance are less dense,” said Yann Alibert, a professor at UNIBE’s Department of Space Research and Planetary Sciences and a member of the discovery team, in a statement. “This study also confirms that most planetary systems have been the site of gigantic collisions, comparable to or even more violent than the one that gave birth to our Moon.”

Sub-Neptune confusion and detection bias

To estimate a planet’s density, astronomers need two pieces of information: the planet’s mass and its radius. Two methods used to obtain mass measurements are the Transit Timing Variation (TTV) method, which only works when a planet crosses the face of its star from our vantage point on Earth, and the Radial Velocity Method, which uses the gravitational pull a planet exerts on its star to measure mass.

“The TTV method involves measuring variations in transit timing. Gravitational interactions between planets in the same system will slightly modify the moment at which the planets pass in front of their star,” team member Jean-Baptiste Delisle of the Astronomy Department of UNIGE’s Faculty of Sciences said in the statement. “The radial velocity method, on the other hand, involves measuring the variations in the star’s velocity caused by the presence of the planet around it.”

The scientists found that the TTV method revealed planets with lower densities than Neptune than those measured using the radial velocity method.

By performing a statistical analysis, the team found that the radial velocity method takes longer to detect large and low-mass planets, such as bulging sub-Neptunes. This means that radial velocity observations are more likely to be interrupted before a planet’s mass has been estimated. This results in a bias in favor of higher masses and densities for planets characterized by the radial velocity method, while excluding less dense planets.

Further work showed that not only was the TTV method more likely to pick up less dense exoplanets, but that the densities of these planets were also lower in resonant systems than their counterparts in non-resonant systems, regardless of the method used to determine their masses.


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Now that the existence of two distinct families of sub-Neptunes has been confirmed and a link has been discovered between globular planets and resonant planetary systems, scientists can better understand the evolution of the most common planet type in our galaxy.

Perhaps soon they will finally be able to explain why our solar system does not have such a world.

The team’s research is published in the journal Astronomy & Astrophysics.

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