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The animation above represents the orbits and movements of the planets in the TOI-178 star system. All but one of these six exoplanets are stuck in rare resonance orbits. This star is about 200 light years away. As the planets revolve around the star, they create patterns that repeat rhythmically, with some planets lining up every few orbits. The European Space Agency – which announced the new discovery today (January 25, 2021) – said the planets were found in data collected by its Planet Observer Cheops, a telescope in space. Their statement said the conclusion:
… Contests the current theories of the formation of planets.
The results were published today (January 25, 2021) in the journal Astronomy and astrophysics.
In the artist’s animation above – which takes place through the European Southern Observatory – the rhythmic movement of the planets around their central star is represented by musical harmony:
… Created by assigning a note (in the pentatonic scale) to each of the planets. This note is played when a planet completes either a full orbit or a half orbit; when the planets line up at these points in their orbits, they ring in resonance.
Thus, five of the worlds of the TOI-178 system create a kind of music of the spheres. In the meantime, the ESA said, the planets have very different compositions from each other.
This graphic is a representation of the TOI-178 planetary system, as revealed by the exoplanet observer from ESA Cheops. The system consists of 6 exoplanets, 5 of which are locked in a rare rhythmic dance orbiting their central star. The 2 inner planets have terrestrial densities (like Earth) and the 4 outer planets are gaseous (with densities like Neptune and Jupiter). The 5 outer planets follow a rhythmic dance as they move in their orbits. Image via ESA.
These distant planets aren’t the only objects in space known to follow resonant orbits. Three of Jupiter’s moons – Io, Europa, and Ganymede – do something similar. For each orbit in Europe, Ganymede performs two orbits and Io completes four. Astronomers would say it’s a 4: 2: 1 model. ESA explained:
In the TOI-178 system, the resonant movement is much more complex as it involves five planets, following an 18: 9: 6: 4: 3 pattern. While the second planet of the star (the first in the pattern) performs 18 orbits, the third planet of the star (the second in the model) completes nine orbits, and so on.
Three of Jupiter’s Galilean moons exhibit a 3-body resonance. Conjunctions are highlighted by brief color changes. There are two Io-Europa (green) and three Io-Ganymede (gray) conjunctions for each Europa-Ganymede (magenta) conjunction. This diagram – not to scale – is via Wikimedia Commons.
Astrophysicist Adrien Leleu from the University of Bern, the University of Geneva and the National Center of Research Skills PlanetS led the research. His team discovered the relationships between five of the six planets orbiting the star TOI-178. Leleu said:
This result surprised us, as previous observations with NASA’s Transiting Exoplanet Survey Satellite (TESS) indicated a three-planet system, with two planets in orbit very closely together. We therefore observed the system with additional instruments, such as the ESPRESSO spectrograph on the ground at the Paranal Observatory of the European Southern Observatory (ESO) in Chile, but the results were inconclusive.
When Leleu and his colleagues first offered to study the system more closely, they were not sure what they would find. Leleu said:
After analyzing the system’s 11-day observation data with CHEOPS, it looked like there were more planets than we initially thought.
The team had identified at least five planets. They decided to invest another day of valuable observing time on the system to confirm. They found that there were indeed five planets present with orbital periods of around 2, 3, 6, 10 and 20 days respectively.
While a system with five planets would have been a pretty remarkable discovery in itself, Leleu and his colleagues noted that there was perhaps more to the story: the system seemed to be in harmony. Leleu explained:
Our theory implied that there could be an additional planet in this harmony; however, its orbital period was to be nearly 15 days.
Adrien Leleu from the Center for Space and Habitability (University of Bern, University of Geneva and National Center of Research Competence PlanetS). Image via University of Bern.
To verify if their theory was true, the team scheduled another observation with CHEOPS, at the exact time when this missing planet would pass, if it existed. But then an accident threatened to void their plans. University of Bern co-author Yann Alibert commented:
Just before the time of the observation, a piece of space debris threatened to collide with the CHEOPS satellite.
Therefore, the control center of the European Space Agency (ESA) launched a
maneuver of the satellite and all observations were stopped. Another co-author, Nathan Hara of the University of Geneva, said:
But to our relief, this maneuver was performed very effectively and the satellite was able to resume observations just in time to capture the mysterious passing planet. A few days later, the data clearly indicated the presence of the additional planet and thus confirmed that there were indeed six planets in the TOI-178 system.
With that came another surprise: Compared to the harmonic and orderly way the planets orbit their stars, their densities appear to be a wild mix. Kate Isaak, ESA project scientist, said:
This is the first time that we have observed something like this. In the few systems we know of with such harmony, the density of the planets steadily decreases as we move further away from the star. In the TOI-178 system, a dense terrestrial planet like Earth appears to be right next to a very fluffy planet with half the density of Neptune followed by a very similar to Neptune.
Adrien Leleu concluded:
The system has therefore proven to be the one that challenges our understanding of the formation and evolution of planetary systems.
Artist’s concept of the TOI-178 system with the foreground planet orbiting furthest from the star. Image via ESO.
Another concept artist of the system.
Bottom line: The TOI-178 star system is now known to have 6 exoplanets, with all but one locked in rare resonant orbits. As the planets revolve around the star, they create patterns that repeat rhythmically, with some planets lining up every few orbits. The European Space Agency announced the discovery on January 25, 2021. It said the planets were found in data collected by its orbiting CHEOPS Planet Hunter Telescope.
Source: Six planets in transit and a chain of Laplace resonances in TOI-178
Via the University of Bern
Via ESA
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