Some Planetary Systems Are Born for Chaos. Astronomers Think They Know Why

Alien planetary systems appear to fall into two distinct families, with some enjoying relatively calm orbits while others bear the scars of a violent past.

For most of human history, planetary systems were a sample size of one. Ours. Then people learned about Mercury, Venus, Mars, Jupiter, and Saturn since antiquity, tracking their movements for generations. Uranus joined the planetary roster in 1781 thanks to the telescope, followed by Neptune in 1846, but every planet we discovered still belonged to the same family orbiting our Sun.

Then the exoplanet floodgates opened, and the tally now sits at more than 1,070 stars known to host more than one planet. The trouble is that these worlds don’t look much like home. Many are crammed into orbits tighter than Mercury’s. Others host gas giants on wildly stretched, tilted paths. It has been hard to see any order in the chaos.

An international team analyzed approximately 300 well-characterized systems and found two broad families. The researchers say a star’s properties are linked to the architecture and later evolution of the planets around it. The team calls it a “genetic link” between a star and its planets.

It’s a fitting metaphor. Like a family resemblance, it shapes how a system forms, evolves and stays stable over billions of years.

“The fundamental question we started from was: What determines the characteristics of planetary systems as we see them today? Are they established at birth or sculpted by evolution over time? The truth, in the end, turned out to lie somewhere in the middle,” said Diego Turrini, Italian National Institute for Astrophysics (INAF) researcher and co-first author of the article published in Astronomy.

“Our results show that there is a genetic link between planetary systems and their stars, and that, depending on the star around which they are born, this link also determines their fate.”

One family contains wide, massive systems. Their planets can have a combined mass two to 10 times that of the Solar System’s planets. These systems tend to orbit stars more massive and richer in “metals” than the Sun. In astronomy, metals are elements heavier than hydrogen and helium.

The other family is compact and lower in mass. Its planets tend to be similar in size and mass, an arrangement astronomers call “peas in a pod.” They orbit stars with a mass and metallicity similar to or lower than the Sun’s, often at distances much closer than Mercury’s orbit.

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The researchers compared systems using several measures, including their overall planetary arrangement and orbital motion. One measure, called “dynamical temperature,” describes how far a system has shifted from a neat arrangement of circular, aligned orbits. Despite the term, it does not refer to the planets’ surface temperatures. Higher dynamical temperature can point to gravitational disturbances, collisions or planets ejected from the system. The wide, massive systems showed more signs of such upheaval.

Nature or nurture? A bit of both

Planetary scientists have long argued over whether a system’s final shape is fixed at birth or carved out over time.

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“The truth, in the end, turned out to lie somewhere in the middle,” Turrini said.

In other words, the star sets the starting conditions, and those conditions then steer what happens next. Even modest differences in the disk of gas and dust around a young star can radically change the kind of system that forms. They can determine whether that system will later go through bouts of instability and giant impacts.

To test the idea, the team ran numerical simulations of planetary systems starting from different initial conditions. The study found that the scenarios that best reproduced the massive family were those in which the systems were unstable from the start. Around massive, metal-rich stars, there’s simply more planet-building material to go around. The planets end up too big too early, and the system can’t hold itself together. Some of the original planets get lost along the way.

Astronomers cannot detect every planet equally well. Large planets close to their stars are generally easier to find than smaller or more distant ones, raising the possibility that the two families reflect observational limits. In other words, there’s currently a major bias in our data of exoplanets simply because some are much easier to detect with today’s technology than others.

Camilla Danielski, a University of Valencia researcher and co-first author, said determining whether the divide was real or an artifact of current detection methods was the analysis’s main challenge. She said selection effects proved to be “the tip of the iceberg,” pointing to bigger structural differences between the systems.

The sample included 300 of the best-characterized multi-planet systems among more than 1,000 known in the Milky Way. Undetected planets could change how some systems are classified as observations improve. And these findings may help astronomers choose where to search for planetary systems resembling our own and for potentially habitable worlds. But a system’s architecture alone cannot establish whether a planet can support life.