Exoplanetary System Found With 6 Worlds in Orbital Resonance

200 light-years away from Earth, there’s a K-type main-sequence star named TOI (TESS Object of Interest) 178. When Adrian Leleu, an astrophysicist at the Center for Space and Habitability of the University of Bern, observed it, it appeared to have two planets orbiting it at roughly the same distance. But that turned out to be incorrect. In fact, six exoplanets orbit the smallish star.

And five of those six are locked into an unexpected orbital configuration.

Five of the planets are engaged in a rare rhythmic, dance around the star. In astronomical terms, they’re in an unusual orbital resonance, which means their orbits around their star display repeated patterns. That property makes them an intriguing object of study and one that could tell us a lot about how planets form and evolve.

“Through further observations, we realized that there were not two planets orbiting the star at roughly the same distance from it, but rather multiple planets in a very special configuration.”

Adrian Leleu, Center for Space and Habitability, University of Bern.

Adrian Leleu leads a team of researchers who studied the unusual phenomenon. They presented their findings in a paper titled “Six transiting planets and a chain of Laplace resonances in TOI-178.” The paper is published in the journal Astronomy and Astrophysics.

In the team’s initial observations, it appeared there were only two planets, as five of them move in such a way as to deceive the eye. But further observations showed that something else was happening in the system. “Through further observations, we realized that there were not two planets orbiting the star at roughly the same distance from it, but rather multiple planets in a very special configuration,” said lead author Leleu.

In this artist’s animation, the rhythmic movement of the planets around the central star is represented through a musical harmony, created by attributing a note (in the pentatonic scale) to each of the planets in the resonance chain. This note plays when a planet completes either one full orbit or one half orbit; when planets align at these points in their orbits, they ring in resonance. Credit: ESO

TOI-178’s orbital resonance is similar to another familiar orbital resonance right here in our own Solar System. That one encompasses Jupiter’s moons Io, Europa, and Ganymede.

The orbital resonance shared by Ganymede, Europa, and Io is fairly simple. Io makes four full orbits for every single orbit of Ganymede and two full orbits for Europa’s full orbit. But the planets around TOI-178 have a much more complex relationship.

TOI-178’s five outer planets are in a 18:9:6:4:3 chain of resonance. The first in the chain and second from the star completes 18 orbits, the second in the chain and third from the star completes 9 orbits, and it continues on from there. The closest planet to the star isn’t part of the chain.

For a system to be orbiting its star in such an orderly and predictable fashion, conditions had to be relatively sedate in this system. Giant impacts or planet migrations would have disrupted it. “The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explained co-author Yann Alibert from the University of Bern.

But there’s more.

In our Solar System the small inner planets are all rocky, while the planets in the outer Solar System are large and gaseous. Beyond Neptune is a region of ice dwarf planets and Kuiper Belt Objects. Image credit: NASA/JPL/IAU

In our Solar System, the inner planets are rocky, and the planets beyond the asteroid belt are not; they’re gaseous. This is one of those instances where we might be tempted to think our Solar System represents some sort of norm. But the TOI-178 system is much different. Gaseous and rocky planets are not delineated like in our system.

“It appears there is a planet as dense as the Earth right next to a very fluffy planet with half the density of Neptune, followed by a planet with the density of Neptune. It is not what we are used to,” said Nathan Hara from the Université de Genève, Switzerland, one of the researchers involved in the study. 

“This contrast between the rhythmic harmony of the orbital motion and the disorderly densities certainly challenges our understanding of the formation and evolution of planetary systems,” says Leleu.

The team used some of the European Observatory’s most advanced, flagship instruments in this work. The ESPRESSO instrument on the VLT, and the NGTS and SPECULOOS instruments at the ESO’s Paranal Observatory. They also used the European Space Agency’s CHEOPS exoplanet satellite. These instruments all specialize in one way or another with the study of exoplanets, which are virtually impossible to detect with a “regular” telescope.

Exoplanets are a long way away from Earth, and the overpowering light from their stars makes them nearly invisible in a regular optical telescope.

The instruments used in this study detect and characterize exoplanets in a couple of different ways. But it all comes down to detecting light. The transiting method used by the NGTS (Next-Generation Transit Survey), CHEOPS (Characterizing ExOPlanet Satellite), and SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) detect the dip in starlight when an exoplanet passes in front of its star. The radial velocity method employed by ESPRESSO detects shifts in the starlight’s normal spectrum when an exoplanet tugs on the star and shifts its position ever so slightly.

By using multiple instruments with different methods and capabilities, the team was able to characterize the system in detail. The innermost planet in the system, which is not in resonance with the others, moves the fastest. It completes an orbit in just two Earth days. The slowest planet moves ten times slower than that. The planet sizes range from one to three Earth sizes, and the masses range from 1.5 to thirty times Earth’s mass.

The orbital resonance of the planets is in an exquisite balance. The authors write that “The orbital configuration of TOI-178 is too fragile to survive giant impacts, or even significant close encounters… a sudden change in period of one of the planets of less than a few .01 d can render the system chaotic.” They also write that their data “…shows that modifying a single period axis can break the resonant structure of the entire chain.”

This discovery just means more work for astronomers. The unusual orbital resonance and positions of the planets means they need to rethink some of our theories around the formation and evolution of planets and solar systems.

This figure from the study compares the density, mass, and equilibrium temperature of the TOI-178 planets with other exoplanet systems. In Kepler-60,
Kepler-80, and Kepler-223, the density of the planets decreases
when the equilibrium temperature decreases. Contrary to the three Kepler systems, in the TOI-178 system, the density of the planets is not a growing
function of the equilibrium temperature. The team behind this study says that if they can understand why the TOI-178 system is different, it could become a sort of Rosetta Stone for deciphering solar system and planetary development. Image Credit: Leleu et al, 2021.

As the authors write in their paper: “Determining the architecture of multi-planetary systems is one of the cornerstones of understanding planet formation and evolution. Resonant systems are especially important as the fragility of their orbital configuration ensures that no significant scattering or collisional event has taken place since the earliest formation phase when the parent protoplanetary disc was still present.”

The nebular hypothesis, also called the Solar Nebular Disk Model (SNDM), is the working theory for the formation of our Solar System and others. According to the model, a giant molecular cloud undergoes gravitational collapse, and when enough gas gathers together, it eventually begins fusion, and a star’s life begins. Most of the material in the cloud will be taken up by the star, and in our Solar System, the Sun has the lion’s share: about 99.86%.

The remaining material makes up the protoplanetary disk, which rotates around the star in a flattened pancake shape. As material clumps together in the rotating protoplanetary disk, it eventually forms planets. There are some problems with the nebular hypothesis, and other theories have tried to explain them.

These are images of nearby protoplanetary disks. At the center of each one is a young star, and the gaps are in the disks are caused by forming exoplanets. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

But this system challenges that theory. The SNDM suggests that rocky, terrestrial planets form nearer the star. They start out as planetary embryos and through violent mergers create planets like Venus, Mercury, Mars, and Earth. Gas giants, according to the SNDM, form out beyond the Solar System’s frost line, where planet embryos form out of frozen volatiles.

But the TOI-178 system challenges that understanding. If the planets in that system followed the SNDM, then the gas planets would be further from the star, and the rocky planets would be closer. Since they’re not, something must have disrupted them. But if something disrupted them, their orbits wouldn’t be choreographed in such an exquisite rhythm. It’s a conundrum.

“Understanding, in a single framework, the apparent disorder in terms of planetary density on one side and the high level of order seen in the orbital architecture on the other side will be a challenge for planetary system formation models,” they write.

Systems like this are challenging to understand, but ultimately, they drive researchers to think harder and to observe more fully.

As the team of scientists write in their conclusion: “The TOI-178 system, as revealed by the recent observations described in this paper, contains a number of very important features: Laplace resonances, variation in densities from planet to planet, and a stellar brightness that allows a number of followup observations (photometric, atmospheric, and spectroscopic). It is therefore likely to become one of the Rosetta Stones for understanding planet formation and evolution, even more so if additional planets continuing the chain of Laplace resonances is discovered orbiting inside the habitable zone.”

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Evan Gough

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