Hear The Strange Music Of Distant Planetary System TOI-178

The animation above represents the orbits and movements of the planets in the TOI-178 star system. All but one of these six exoplanets one locked in rare resonance orbits. This star is about 200 light-years away. As the planets orbit the star, they make patterns that repeat themselves rhythmically, with some planets aligning every few orbits. The European Space Agency – which announced the new finding today (January 25, 2021) – said the planets were found in data collected by its Cheops planet-watcher, a telescope in space. Their release said that the finding:

… challenges current theories of planet formation.

The results were published today (January 25, 2021) in the journal Astronomy and Astrophysics.

In the artist’s animation above – which is through the European Southern Observatory – the rhythmic movement of the planets around their central star is represented by a musical harmony:

… created by attributing a note (in the pentatonic scale) to each of the planets. 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.

So five of the worlds in the TOI-178 system create a kind of music of the spheres. In the meantime, ESA said, the planets have very different compositions from one another.

Infographic showing the orbits and densities of the planets in the TOI-178 system.
This graphic is a representation of the TOI-178 planetary system, as revealed by ESA’s exoplanet watcher Cheops. The system consists of 6 exoplanets, 5 of which are locked in a rare rhythmic dance as they orbit their central star. The 2 inner planets have terrestrial densities (like Earth) and the outer 4 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 resonance orbits. Three of Jupiter’s moons – Io, Europa and Ganymede – do something similar. For every orbit of Europa, Ganymede completes two orbits, and Io completes four. Astronomers would say this is a 4:2:1 pattern. ESA explained:

In the TOI-178 system, the resonant motion is much more complex as it involves five planets, following a 18:9:6:4:3 pattern. While the second planet from the star (the first in the pattern) completes 18 orbits, the third planet from the star (second in the pattern) completes nine orbits, and so on.

Animation of three small dots circling a center dot, showing the resonance orbits of 3 of Jupiter’s moons.
Three of Jupiter’s Galilean moons exhibit a 3-body resonance. Conjunctions are highlighted by brief color changes. There are two Io-Europa conjunctions (green) and three Io-Ganymede conjunctions (grey) for each Europa-Ganymede conjunction (magenta). This diagram – not to scale – is via Wikimedia Commons.
Astrophysicist Adrien Leleu of the University of Bern, the University of Geneva and the National Center of Competence in Research PlanetS led the research. His team found the relationships between five of six planets orbiting the star TOI-178. Leleu said:

This result surprised us, as previous observations with the Transiting Exoplanet Survey Satellite (TESS) of NASA pointed toward a three-planets system, with two planets orbiting very close together. We therefore observed the system with additional instruments, such as the ground based ESPRESSO spectrograph at the European Southern Observatory (ESO)’s Paranal Observatory in Chile, but the results were inconclusive.

When Leleu and his colleagues first proposed to investigate the system more closely, they weren’t sure what they’d find. Leleu said:

After analyzing the data from 11 days of observing the system with CHEOPS, it seemed that there were more planets than we had initially thought.

The team had identified at least five planets. They decided to invest another day of precious observation 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 quite a remarkable finding in itself, Leleu and his colleagues noticed that there might be more to the story: the system appeared to be in harmony. Leleu explained:

Our theory implied that there could be an additional planet in this harmony; however its orbital period needed to be very nearly 15 days.

A young, clean-cut man.
Adrien Leleu of the Center for Space and Habitability (University of Bern, the University of Geneva and the National Center of Competence in Research PlanetS). Image via University of Bern.
To check if their theory was in fact true, the team scheduled yet another observation with CHEOPS, at the exact time that this missing planet would pass by, if it existed. But then, an accident threatened to cancel their plans. Co-author Yann Alibert at the University of Bern 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) initiated an evasive
maneuver of the satellite and all observations were interrupted. Another co-author, Nathan Hara of the University of Geneva, said:

But to our great relief, this maneuver was done very efficiently and the satellite could resume observations just in time to capture the mysterious planet passing by. 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, orderly way the planets orbit around their star, their densities appear to be a wild mixture. ESA Project Scientist Kate Isaak said:

It is the first time we observed something like this. In the few systems we know with such a harmony, the density of planets steadily decreases as we move 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 one very similar to Neptune.

Adrien Leleu concluded:

The system therefore turned out to be one that that challenges our understanding of the formation and evolution of planetary systems.

Crescent planet seen from space with smaller planets closer to a distant star.
Artist’s concept of the TOI-178 system with the planet in the foreground orbiting most distantly around the star. Image via ESO.
Planets near a reddish star.
Another artist’s concept of the system.
Bottom line: The star system TOI-178 now is known to have 6 exoplanets, with all but one locked in rare resonance orbits. As the planets orbit the star, they make patterns that repeat rhythmically, with some planets aligning every few orbits. The European Space Agency announced the finding on January 25, 2021. It said the planets were found in data collected by its orbiting CHEOPS planet-hunter telescope.

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