Planetary scientists have found that sub-Neptunian planets that dance in time with the rest of their planetary systems are less dense than those that don’t.
Although notably absent from the Solar System, the most common planets in the Milky Way are known as “sub-Neptunes,” or worlds between the size of Earth and the ice giant Neptune. An estimated 30 to 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 sub-Neptune densities. .
Depending on the techniques used for these measurements, sub-Neptunes appear to fall into two distinct categories: “puffed” and “non-puffed”. The question, however, was whether there were actually two distinct populations of sub-Neptunes, or whether these differences were a result of what method was used to measure the densities. In this regard, 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 bloated sub-Neptunes are more likely to be in resonance with their planetary siblings.
Waltz with planetary partners
Planets are said to be in resonance when, for example, one planet completes one revolution in the same time it takes another planet to complete two revolutions.
One recently discovered extraordinary resonant planetary system is HD 110067, located 100 light-years from Earth. The six sub-Neptunian 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 orbits 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.
Thus, for every revolution of the star that the outer planet completes, the inner planet completes six revolutions. This means that these sub-Neptunes are said to be in 6:1 resonance. Other resonances between the various 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, about the time the solar system has existed. As fascinating as this is, it doesn’t tell us why sub-Neptunes appear to be less dense in this system.
The team behind this new research proposed several possible explanations for the lightness of resonant sub-Neptunes; most likely the process seems to be related to how they were created.
The team says it’s possible that all planetary systems converge on a resonant string during their early existence. However, they believe that only 5% of systems can maintain this rhythm.
Breaking the resonance chain can lead to a series of cataclysmic events where planets collide together and often merge to become denser conglomerate worlds. This means that resonant string systems can also maintain their bloated sub-Neptunes, the team says, because collisions and mergers increase the density of the same planets in non-resonant systems.
“Numerical models of the formation and evolution of the planetary system that we have developed in Bern over the last two decades exactly reproduce this trend: planets in resonance are less dense,” Yann Alibert, professor at UNIBE’s Division of Space Research and Planetary Sciences and a member of the discovery team, said in declaration. “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 rise 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. The two methods used to obtain mass measurements are the Transit Timing Variation (TTV), which only works if the planet crosses the face of its star from our vantage point on Earth, and the radial velocity method, which uses the gravitational pull exerted by the planet. its star to measure mass.
“The TTV method involves measuring variations in transit timing. The gravitational interaction between planets in the same system changes slightly the moment the planets pass in front of their star,” team member Jean-Baptiste Delisle from the Department of Astronomy at the Faculty of UNIGE. Science, he said in a statement. “The radial velocity method, on the other hand, involves measuring changes in a star’s velocity caused by the presence of a planet around it.”
The researchers realized that the TTV method tended to discover planets below Neptune with a lower density than those measured by the radial velocity technique.
By performing a statistical analysis, the team found that the radial velocity method required more time to detect large and low-mass planets such as bloated sub-Neptunes. This means that observations using radial velocity are more at risk of stopping short of estimating the planet’s mass. This leads to a bias in favor of higher masses and densities for planets characterized by the radial velocity method, with lower density planets excluded.
Further investigation showed that the TTV method was not only more likely to pick up on 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 mass.
With the confirmed existence of two distinct families of sub-Neptunes and the discovery of a link between inflated planets and resonant planetary systems, scientists are better placed to understand the evolution of the most common type of planet in our galaxy.
They may also soon be able to explain why our solar system lacks such a world.
The team’s research is published in the journal Astronomy & Astrophysics.