Astronomers have discovered six planets orbiting the star TOI-178 in an astonishing orbital "resonance" – meaning their orbits are synchronized in a precise rhythmic dance that has persisted for billions of years.
Surprise Discovery by TESS Spacecraft
The discovery was made by NASA’s exoplanet-hunting TESS mission and followup observations by the European CHEOPS spacecraft. Scientists were surprised to detect six temperate sub-Neptune sized worlds orbiting the star TOI-178, located around 100 light years from Earth in the Pisces constellation.
But they were even more shocked to find the planets’ orbits in an extremely stable configuration, oscillating with a precise tempo likened to musical harmony or clockwork. This resonance indicates the planets gravitationally interact with each other as they orbit their star in a cosmic waltz that has somehow persisted since the system formed 4 billion years ago.
"This work goes beyond just finding other worlds – it is a leap forward in trying to understand them," said lead author Dr. Adrien Leleu, an astrophysicist from the University of Geneva.
Planets Line Up Like Clockwork
The outermost planet orbits TOI-178 every 13.6 days, while the second, third, and fourth planets orbit every 20.5, 27.4 and 34.8 days respectively. The two innermost worlds complete an orbit in just 8.5 and 11.4 days.
Remarkably, the amount of time between one planet completing an orbit and the next planet "lining up" behind it is almost exactly an integer ratio – with that regular clock-like tempo maintained over billions of years.
"The orbits in this system are very well ordered, which tells us they have evolved undisturbed for a long time," explained co-author Dr. Nathan Hara from the Université de Genève. "Understanding why this system is as orderly as it is will give us unique insights into planetary system formation."
Clues to Planet Formation Mysteries
The discovery provides clues to enduring mysteries about how planetary systems take shape after the collapsing gas and dust cloud from which their star forms.
Planets are thought to arise from a rotating disk of dense gas and dust orbiting a young star. But how material clumps together into embryonic planets, and how nascent planets migrate through the disk and settle into stable configurations, remains unclear.
The resonant harmony seen in the TOI-178 system will constrain astronomers’ models of how material in disks aggregates into planets. It also suggests the worlds migrated very early on into a stable resonant configuration and remained relatively undisturbed for billions of years.
"Resonant chains like this provide valuable clues about the past evolution of the planets, as well as their interactions with the disk from which they formed," said co-author Dr. Yann Alibert from the University of Bern.
Prospects for Habitability
While too large to likely be rocky like Earth, the temperate conditions on the TOI-178 worlds put them in a "habitable zone" temperature regime where water could exist in liquid form on a solid surface. Further study will be needed to determine if they have atmospheres and if they could sustain life.
But resonant systems also encourage tidal heating – where planetary cores heat up due to gravitational interactions during close approaches. If substantial, this tidal heating could sterilize planets believed otherwise capable of bearing life.
Observations by next-generation telescopes such as the James Webb Space Telescope and Extremely Large Telescope will explore the atmospheric and surface conditions on these alien worlds.
"With extended time on the world’s largest telescopes, we may even be able to probe the structure and atmospheric composition of these worlds, revolutionizing our understanding of distant temperate planets," said Dr. Leleu.
Unraveling the Planetary "Mystic Crystal Revelation"
The name TOI-178 comes from the MIT-Run TESS Object of Interest catalog, but Dr. Leleu’s team also calls it the "Mystic Crystal Revelation" system after a mystical relic holding a secret message in the mobile game Final Fantasy XIV.
Like the resonant crystal, astronomers believe unlocking the orderly mysteries of TOI-178’s formation and evolution may reveal key insights about how cosmic bodies come together to create worlds capable of developing sentient observers like us.
"This system announces what is possible," said Leleu. "In resonance, planets line up, unfold mysteries, and unlock revelations."
The discovery is published November 30th in the journal Astronomy & Astrophysics.
Orbital Configuration Tables
| Planet | Orbital Period | Resonance Period Ratio | Distance from Star | Size |
|---|
| b | 8.5 days | 1:6 | 0.08 AU | 3.6 Earth radii
| c | 11.4 days | 3:8 | 0.12 AU | 2.8 Earth radii
| d | 13.6 days | 2:5 | 0.16 AU | 2.1 Earth radii
| e | 20.5 days | 3:5 | 0.27 AU | 2.7 Earth radii
| f | 27.4 days | 8:9 | 0.38 AU | 2.7 Earth radii
| g | 34.8 days | 4:3 | 0.50 AU | 3.7 Earth radii
Resonance Period Ratios
- Planets b to c: 11.4/8.5 = 4:3
- Planets c to d: 13.6/11.4 = 8:9
- Planets d to e: 20.5/13.6 = 3:5
- Planets e to f: 27.4/20.5 = 4:3
- Planets f to g: 34.8/27.4 = 5:3
What Happens Next
Astronomers are excited to further study signals from the TOI-178 system to refine their measurement precision and glean insights about the planets’ masses, densities, and atmospheric conditions.
Resonant systems are prime targets for observational follow-up as their predictable orbits simplify when to view mutual planetary transits. The next few years will see an acceleration in their examination by leading telescopes like CHEOPS, ESPRESSO, JWST, and ELT.
In unlocking the mysteries of resonant worlds by studying their history encoded in harmonic motion, astronomers move closer to unveiling the hidden cosmic rhythms guiding planetary origins across the galaxies.
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