Giant Planets May Be “Agents of Chaos” – GWC Mag

The folks next door can have a big impact on the livability of a neighborhood. If they’re loud, pushy, and obnoxious, they can force you out of even the most comfortable of homes.

That truism applies to planets as well as people. The wrong kind of neighbor can kick an Earth-like planet out of its star’s comfort zone or out of the system entirely, or even prevent it from forming in the first place.

Although giant planets similar to Jupiter can help protect other planets by kicking away potential comets or asteroids, a pair of recent studies supports the idea that other giant planets can make it almost impossible for Earth-like worlds to remain in stable orbits inside a star’s habitable zone—the region where temperatures are the most conducive to liquid surface water. Simulations of two planetary systems show that known giant planets in or near the habitable zone would quickly boot out Earth mass planets or nudge them into orbits that would create dramatic swings in climate, lessening their chances of hosting life.

“Hundreds of factors govern the habitability of planetary systems,” said Stephen Kane, a professor of Earth and planetary sciences at the University of California, Riverside and author of both papers. “How planets interact gravitationally is one of the most important.”

Kane said he is “increasingly obsessed” with the role played by Jupiter, the Sun’s largest planet, in the evolution of Earth and the rest of the solar system. “If Jupiter had never formed and you throw out Saturn as well, what would Earth look like today?”

The key to answering that question, Kane said, is to study the architectures of exoplanet systems with Jupiter mass planets and see whether they also contain Earth mass planets in the habitable zone and whether the terrestrial planets would follow stable orbits over long periods.

An “Extremely Rare” System

Kane’s first paper resulted from his study of the planetary system HD 141399, which contains four giant planets. HD 141399, a K dwarf star, is a bit cooler but more massive than the Sun, so its habitable zone begins a bit farther from the star but is wider. Its known planets range from about half the mass of Jupiter to about a third more than Jupiter. They orbit at distances ranging from roughly the distance from the Sun to Mercury to roughly the Sun–Jupiter distance. “This is an extremely rare kind of system,” said Kane. “There are very few with four known giant planets.”

Kane’s simulations, which project the motions of the planets in the past and incorporate a possible Earth mass planet, show that the four worlds appear to be in a stable configuration. That could permit an Earth mass planet in “limited islands of stability,” the study said.

On the other hand, the giants follow a “complex pattern of resonances and gravitationally perturbed regions,” according to the paper, possibly excluding other planets from the habitable zone. Uncertainties in some of the system’s details increase the chances that the known planets could have expelled Earth mass planets from the habitable zone or stirred up the small planetary “building blocks” enough to prevent the formation of such a world. “The interesting part of the orbital dynamics is how sensitive simulations are to initial conditions,” Kane said. “A lot of things could lead to a chaotic state.”

“I certainly would not expect to find any stable orbits for Earth mass planets in the habitable zone” around HD 141399, said William Cochran, codirector of the Center for Planetary Systems Habitability at the University of Texas at Austin, who was not involved with either study. “In general, multiplanet systems tend to be ‘dynamically packed’—they have ejected any planets that formed and were dynamically unstable, leaving the most complex multiplanet system possible.”

Habitable Planet or a Wrecking Ball?

In Kane’s second study, he and coauthor Tara Fetherolf took a close look at GJ 357, which contains three planets, including one in the star’s habitable zone.

GJ 357 is an M dwarf star, roughly one third the Sun’s mass and diameter and with less than 2% of the Sun’s total luminosity. M dwarfs are the most numerous stars in the galaxy, however, and their habitable zones are close in. That makes them easier to study, so scientists pay special attention to them.

The star’s first planet, GJ 357 b, was discovered by the Transiting Exoplanet Survey Satellite (TESS) as the planet passed in front of the star, blocking a small fraction of its light. Follow-up observations using the radial velocity (RV) technique, which measures tiny back-and-forth motions of a star caused by the gravity of orbiting planets, revealed two additional worlds.

Planet d orbits in the middle of the star’s habitable zone. RV measurements revealed a minimum mass of about 6 times Earth’s mass, which would make it a “super-Earth”—a potentially rocky world that could sustain Earth-like life.

Kane and Fetherolf used additional TESS observations to confirm that neither planet d nor c transits the star. Without transits, the inclination of the planetary orbits isn’t known, which means the planetary masses are uncertain as well. Simulations showed that planet d could be up to 10 times Earth’s mass, which would make it more of a “super-Neptune,” with a thick gaseous envelope around a solid core. That configuration would make the planet less habitable.

“A higher-mass d planet would be a wrecking ball.”

In addition, the scientists simulated a possible hidden Earth mass planet in the habitable zone. They found that a larger planet d would protect its regime. It could eject a smaller planet or make the planet’s orbit so lopsided that the seasons would be extreme, limiting the possibilities for habitability. “A higher-mass d planet would be a wrecking ball,” said Fetherolf, a visiting scientist at California State University San Marcos.

Uncertainties in the system’s age could mean that even if an Earth mass planet somehow found an unlikely gravitational sweet spot, it might not be habitable. “GJ 357 is quiet now, but it could have been more active in the past, with lots of events like solar flares and coronal mass ejections,” said Fetherolf. “That could potentially blow away the atmosphere of a planet, so if it happened in the past, those planets might not have an atmosphere even if they’re in the habitable zone.”

Looking Toward Extreme Precision

The simulations in these studies covered time frames of roughly 1 million to 10 million years. So although they show that current configurations may be stable, they say nothing about their stability in earlier eras. “Multiplanet systems can be pretty chaotic,” said Nora Bailey, an astrophysicist who has studied systems with multiple giant planets but was not involved in the current work. “You can do simulations for a million orbits or more and find that [the orbits] are stable, but if you go longer into the past, they might become more unstable.”

Finding an Earth-like planet in the habitable zone of either star would require extremely precise radial velocity measurements that are beyond current capabilities, although the technology is improving, Kane said. “We’re entering an era of extreme radial velocity precision, so we’re just about there. It would be extraordinary if we were able to verify that there was a terrestrial planet sitting in one of those stability islands. That would really speak to the orbital resiliency of planets.”

—Damond Benningfield, Science Writer

Citation: Benningfield, D. (2023), Giant planets may be “agents of chaos,” Eos, 104, https://doi.org/10.1029/2023EO230485. Published on 18 December 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0
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