Sub-Neptune planets that jump in time with the rest of their planetary systems are less dense than those that don’t, planetary scientists have found.
Although conspicuously absent from the solar system, the most common planets in the Milky Way are known as “sub-Neptune,” or worlds between the size of Earth and the ice giant Neptune. It is 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 extrasolar planets, or exoplanets, have traditionally had difficulty measuring the density of sub-Neptunes. -Neptune. .
Depending on the techniques used for these measurements, sub-Neptunes seem to fall into two distinct categories: “inflated” and “non-inflated”. However, the question has been whether there really are two distinct populations of sub-Neptunes or whether these differences are a result of the method used to measure the density. In this regard, new research from the University of Geneva (UNIGE) and the University of Bern (UNIBE) suggests that there are actually two physically different families of sub-Neptunes. And inflated 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 it takes another planet to complete two orbits.
A remarkable recently discovered resonant planetary system 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 inner planet completes an orbit in 9.1 Earth days, the next planet to 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 external in 54.7 days.
Thus, for every orbit of the star that the outer planet completes, the inner planet completes six orbits. This means that these sub-Neptunes are said to be in a 6:1 resonance. 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 existed around the bright orange star HD 110067 for about 4 billion years, roughly the same time as the solar system has existed. Fascinating as that is, however, it doesn’t tell us why the sub-Neptunes in this system appear to be less dense.
The team behind this new research has proposed several possible explanations for the lightness of resonant sub-Neptunes; more likely it seems to suggest that the process is related to how these are formed.
It is possible, the team says, that all planetary systems converged toward a resonant chain during their early existence. However, they believe that only 5% of systems can maintain this pace.
Breaking the resonance chain can lead to a series of catastrophic events, with planets crashing together and often merging to become denser conglomerate worlds. This means that resonant chain systems can also retain their bloated sub-Neptunes, the team says, as collisions and mergers increase the density of the same planets in non-resonant systems.
“Numerical models of planetary system formation and evolution that we have developed in Bern over the past two decades reproduce exactly this trend: planets in resonance are less dense,” Yann Alibert, a professor in the Division of Space Research and Planetary Sciences at UNIBE and member of the discovery team, said in a statement. “Furthermore, this study confirms that most planetary systems have been the site of giant collisions, similar to or even more violent than the one that gave birth to our moon.”
Confusion under Neptune and discovery bias
To estimate the density of a planet, astronomers need two pieces of information: the planet’s mass and its radius. Two methods used to obtain mass measurements are the Transit Time Variation (TTV), which only works if a planet transits the face of its star from our vantage point on Earth, and the radial velocity method, which uses the gravitational pull exerted by a planet. his star to measure mass.
“The TTV method involves measuring variations in transit time. Gravitational interactions between planets in the same system will slightly modify the moment the planets pass in front of their star,” team member Jean-Baptiste Delisle, of the Department of Astronomy of the UNIGE Faculty. Science, the statement said. “The radial velocity method, on the other hand, involves measuring the changes in the star’s velocity caused by the presence of the planet around it.”
The scientists realized that the TTV method tended to show sub-Neptune planets with less density than those measured with the radial velocity technique.
By performing a statistical analysis, the team found that the radial velocity method takes longer to detect large, low-mass planets such as inflated sub-Neptunes. This means that observations using radial velocity are more likely to stop before a planet’s mass can be estimated. This results in a bias in favor of higher masses and densities for planets characterized by the radial velocity method, with less dense planets being excluded.
Further investigation showed that, not only was the TTV method more likely to pick up the least dense exoplanets, but 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 size.
With the existence of two distinct families of sub-Neptunes confirmed and the discovery of a link between bulge planets and resonant planetary systems, scientists are in a better position to understand the evolution of our galaxy’s most common type of planet.
They may also soon be able to explain, finally, why our solar system lacks such a world.
The team’s research is published in the journal Astronomy & Astrophysics.