Two “Families” of Planetary Systems — and Their Host Stars Decide Their Fate

October 15 — Look at the parents and you can broadly predict how a child will grow up. It turns out the same rule applies to planetary systems. Scientists studying the “family trees” of planetary systems in the Milky Way have found two distinct “families” that determine how those systems evolve and how they end up.

The team, led by researchers at Italy’s National Institute for Astrophysics (INAF), picked 300 of the more than a thousand multi-planet systems astronomers have found in the Milky Way — the ones with the most complete data — and analysed them.

The study shows that the environment around a young star directly affects the fate of its planets, and in turn whether exoplanets — including those with the potential to host life — survive. That matters greatly to astronomers searching for life beyond the solar system, because it can help them identify Earth-like, potentially habitable systems.

Team member Diego Turrini, a researcher at INAF, said in a statement to Space.com: “Our core question at the start was: what determines the characteristics of the planetary systems we observe today? Were those characteristics fixed at birth, or shaped gradually over a long evolution? The conclusion lies somewhere in between.”

Turrini added: “The results show a kind of ‘genetic link’ between planetary systems and their host stars. Which star a planet is born around determines the fate of that system.”

The first “family” the scientists identified consists of broad, massive planetary systems, typically two to ten times the total mass of the solar system. Their planets orbit stars more massive than the Sun, and those stars contain less of the heavy elements beyond hydrogen and helium — what astronomers call metals, a confusing definition — than the Sun does.

The second “family” consists of more compact, lower-mass systems. The planets within them are similar in mass and size — the so-called “peas in a pod” configuration. They orbit stars whose mass and metallicity are equal to or lower than the Sun’s, and their orbits are usually far tighter than that of Mercury, the innermost planet of the solar system.

One of the “inherited traits” Turrini’s team used to classify planetary systems is how far the planets’ orbits deviate from uniform, coplanar circular orbits. That measure reflects a system’s history of violent gravitational disturbance, caused by events such as giant collisions or planets being flung out of the system.

The study found that systems in the first family frequently experience violent gravitational events, while those in the second rarely show such disturbance. The team concluded that how much turmoil a planetary system goes through is linked to fine differences in the environment where its planets formed, and that those differences stem from the properties of the host star itself.

In other words, the nature of the host star not only fundamentally changes how planets form but also affects how the whole system evolves later, determining whether it will go through violent, unstable phases and large impacts.

Team member Camilla Danielski of the University of Valencia in Spain said: “The hardest part of the data analysis was deciding whether these two families of planetary systems really exist, or whether they are an illusion created by the limits of current observing technology.”

“In the end we found that the observational selection effect was only the tip of the iceberg, and that behind it lies a deeper structural difference between planetary systems,” Danielski said.

The scientists verified their conclusions with several sets of simulations of planetary system evolution run from different initial conditions.

Danae Polychroni, an INAF researcher who led the project, said: “The simulations that perfectly reproduce the dynamical characteristics of broad, massive planetary systems all satisfy one condition: the system was unstable from birth. More material available around the host star to form planets produced oversized planets, so the system could not stay stable.”

The study was published on Tuesday 13 October in the journal Nature Astronomy.