200 light-years from Earth is a K-type main sequence star called TOI (TESS Object of Interest) 178. When Adrian Leleu, astrophysicist at the Center for Space and Habitat at the University of Bern, observed it, it seemed as if it were two planets would have orbited it at roughly the same distance. But that turned out to be wrong. In fact, six exoplanets orbit the little star.
And five of those six are locked in an unexpected orbital configuration.
Five of the planets are involved in a rare rhythm, dancing around the star. In astronomical terms, they are in an unusual orbital resonance, meaning that their orbits around their star have repeated patterns. This property makes them a fascinating object of study that could tell us a lot about how planets form and develop.
“Through further observations we have established that it is not two planets orbiting the star at approximately the same distance, but several planets in a very special configuration.”
Adrian Leleu, Center for Space and Habitability, University of Bern.
Adrian Leleu leads a research team that has investigated the unusual phenomenon. They presented their results in an article entitled “Six Transit Planets and a Chain of Laplacian Resonances in TOI-178”. The paper was published in the journal Astronomy and Astrophysics.
From the team’s first observations, there appeared to be only two planets, five of which move in such a way as to fool the eye. However, further observations indicated that something else was happening in the system. “Through further observations we have established that it is not two planets orbiting the star at approximately the same distance, but several 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 by a musical harmony created by assigning a note (on the pentatonic scale) to each of the planets in the resonance chain. This note is played when a planet completes either a full or half orbit. When planets align themselves at these points in their orbits, they ring in resonance. Photo credit: ESO
The orbital resonance from TOI-178 is similar to another known orbital resonance here in our own solar system. This includes Jupiter’s moons Io, Europa, and Ganymede.
The orbital resonance shared by Ganymede, Europa and Io is pretty simple. Io makes four full orbits for every single orbit of Ganymede and two full orbits for the full orbit of Europe. But the planets around TOI-178 have a much more complex relationship.
The five outer planets of TOI-178 are in a resonance chain of 18: 9: 6: 4: 3. The first in the chain and the second from the star complete 18 orbits, the second in the chain and the third from the star complete 9 Orbits and from there it goes on. The planet closest to the star is not part of the chain.
For a system to orbit its star in such an orderly and predictable manner, the conditions in this system had to be relatively calm. Huge impacts or planetary wanderings would have bothered it. “The orbits in this system are very well ordered, which shows that this system has developed quite gently since it was born,” explained co-author Yann Alibert from the University of Bern.
But there is 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 with ice dwarf planets and objects of the Kuiper Belt. Photo credit: NASA / JPL / IAU
In our solar system, the inner planets are rocky and the planets beyond the asteroid belt are not. They are gaseous. This is one of the times we may be tempted to believe that our solar system is some kind of norm. However, the TOI-178 system is very different. Gaseous and rocky planets are not delineated as in our system.
“It seems that there is a planet as dense as Earth, right next to a very fluffy planet half the density of Neptune, followed by a planet the density of Neptune. It’s not what we’re 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 movement and the disordered densities certainly calls into question our understanding of the formation and development of planetary systems,” says Leleu.
The team used some of the European Observatory’s most advanced flagship tools in this work. The ESPRESSO instrument in the VLT as well as the NGTS and SPECULOOS instruments in the Paranal Observatory of ESO. They also used the European Space Agency’s CHEOPS exoplanet satellite. These instruments are all specialized in one way or another for studying exoplanets that are practically impossible to see with a “normal” telescope.
Exoplanets are far from Earth, and the overwhelming light from their stars makes them nearly invisible in a normal optical telescope.
The instruments used in this study record and characterize exoplanets in a number of ways. But what matters is recognizing light. The transit method used by NGTS (Next-Generation Transit Survey), CHEOPS (Characterizing ExOPlanet Satellite) and SPECULOOS (Search for Habitable Planets EClipsing ULtra-COOl Stars) detects the ingress of starlight when an exoplanet passes in front of its star. The radial velocity method used by ESPRESSO detects shifts in the normal spectrum of starlight when an exoplanet pulls on the star and shifts its position slightly.
Using multiple instruments with different methods and skills, the team was able to characterize the system in detail. The innermost planet in the system that 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. The planet sizes range from one to three earth sizes and the masses range from 1.5 to 30 times the earth’s mass.
The orbital resonance of the planets is in exquisite equilibrium. The authors write: “The orbital configuration of TOI-178 is too fragile to withstand huge impacts or even significant encounters. A sudden change in the period of one of the planets of less than a few 0.01 days can make the system chaotic. “They also write that their data” … shows that modifying a single period axis can break the resonance structure of the entire chain. “
This discovery just means more work for astronomers. The planets’ unusual orbital resonance and position means they need to rethink some of our theories about 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. In contrast to the three Kepler systems, the density of the planets does not increase in the TOI-178 system
Function of the equilibrium temperature. The team behind this study says if they can understand why the TOI-178 system is different, it could become something of a Rosetta Stone for deciphering the solar system and planetary evolution. Photo credit: Leleu et al., 2021.
The authors write in their work: “The determination of the architecture of multiplanetary systems is one of the cornerstones for an understanding of planet formation and evolution. Resonance systems are particularly important because the fragility of their orbital configuration ensures that no significant scattering or collision event has occurred since the earliest phase of formation in which the protoplanetary parent disk was still present. “
The nebula 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 huge molecular cloud collapses by gravity, and if enough gas collects, it eventually begins to merge and the life of a star begins. Most of the material in the cloud is taken up by the star, and in our solar system the sun has the lion’s share: about 99.86%.
The remaining material forms the protoplanetary disk, which rotates around the star in a flattened pancake shape. When material clumps together in the rotating protoplanetary disk, it eventually forms planets. There are some problems with the fog hypothesis, and other theories have tried to explain it.
These are images of nearby protoplanetary disks. There is a young star in the center of each, and the gaps in the disks are caused by the formation of exoplanets. Photo credit: ALMA (ESO / NAOJ / NRAO), S. Andrews et al .; NRAO / AUI / NSF, S. Dagnello
But this system challenges that theory. The SNDM suggests rocky terrestrial planets form closer to the star. They start out as planetary embryos and through violent fusions they create planets like Venus, Mercury, Mars and Earth. According to the SNDM, gas giants form beyond the frost line of the solar system, where planetary embryos form from frozen volatile substances.
However, the TOI-178 system questions this understanding. If the planets in this system followed the SNDM, the gas planets would be farther from the star and the rocky planets would be closer. Since this is not the case, something must have bothered her. But if something bothered them, their orbits would not be choreographed in such an exquisite rhythm. It’s a mystery.
“In a single framework, it will be a challenge for models of planetary system formation to understand the apparent perturbation in planetary density on the one hand and the high order in orbital architecture on the other,” they write.
Systems like this are difficult to understand, but ultimately they drive researchers to think harder and observe more fully.
The team of scientists wrote in its conclusion: “The TOI-178 system, as is evident from the most recent observations in this article, contains a number of very important features: Laplacian resonances, density differences from planet to planet and a star’s brightness which has a number of Follow-up observations made possible (photometric, atmospheric and spectroscopic). It is therefore likely to become one of the Rosetta Stones for understanding planet formation and evolution, even more so as additional planets are discovered that continue the chain of Laplace resonances and orbit in the habitable zone. “
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