How are planets formed? Until recently, astronomers who study our planets and alien exoplanets thought they knew. Now, a discovery of a six-exoplanet star system some 200 light-years away named TOI-178 is challenging planetary formation theories.

Five of the system's six planets revolve around their central sun in predictable patterns that repeat, with some planets aligning every few orbits. This seemingly well-choreographed dance, called "orbital resonance" or "chain of resonance," has rarely been observed in the universe. But the curiosity doesn't stop there. Even though the exoplanets have consistent, resonant orbits, their individual densities vary widely.

"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," team member and astronomer Nathan Hara of the University of Geneva, Switzerland, said in a press statement for the European Southern Observatory (ESO). "It is not what we are used to."

As the scientists explained in the study published in the scientific journal Astronomy & Astrophysics, the TOI-178 system formed "contrary to what one would expect from simple formation and evolution models" and will provide "important clues" to the formation of other systems.

Detecting Alien Planets

To date, astronomers have confirmed the existence of more than 4,000 exoplanets, with thousands of other candidates on deck, according to NASA. The alien planet nearest to Earth is Proxima Centauri b, as EarthSky notes. It orbits the star Proxima Centauri, located 4.2 light-years away. (One light-year equals 6 trillion miles.)

Because exoplanets are so distant from Earth, astronomers cannot observe them through conventional telescopes. Most exoplanets are found by using specialized instruments on ground-based observatories and orbiting satellites that can detect light coming from alien worlds. One method involves imaging the temporary dimming of light coming from the central star as one of its exoplanets passes in front of it. Another method involves detecting slight wobbles in the central star's light spectrum as the gravity from one of the orbiting exoplanets tugs on it.

The team that investigated TOI-178, led by Adrien Leleu from the University of Geneva and the University of Bern in Switzerland, used both of these methods to study the motion of the planets, as ESO reports. Researchers analyzed data from a host of astronomical instruments, including the European Space Agency's CHEOPS satellite, the ground-based ESPRESSO instrument on ESO's Very Large Telescope array and other instruments sited at ESO's Paranal Observatory in Chile. From the data, the astronomers uncovered a trove of important details. The six planets orbit their star much faster than planets do in our own solar system. The innermost planet in TOI-178 circles the star in just a couple of days. The slowest one does it in less than an Earth month.

The sizes of the TOI-178 planets range from about one to three times the size of Earth, while their masses are 1.5 to 8 times the mass of Earth, according to ESO. Although similar to our solar system in some ways, the positions of the planets and how they behave are out of this world.

Orbital Resonance

In our own solar system, only two examples of orbital resonance occur, ScienceAlert notes. The first lies with Pluto and Neptune: For every two orbits Pluto makes around the Sun, Neptune makes three. That's a ratio of two to three, referred to as a 2:3 resonance. The three moons of Jupiter also form a chain of resonance. For every single orbit Ganymede makes around Jupiter, Europa makes two trips and Io makes four — a 1:2:4 resonance.

Elsewhere in the universe, other star systems have planets linked in a chain of resonance. The Kepler-80 system has five exoplanets in a resonance chain of 4:6:9:12:18. The TRAPPIST-1 system has a seven-exoplanet resonance chain of 2:3:4:6:9:15:24, as ScienceAlert reports.

But ESO notes that the five planets in TOI-178 "follow a much more complex chain of resonance, one of the longest yet discovered in a system of planets." The innermost of those five exoplanets completes 18 orbits for every three orbits of the outermost. The entire chain is 3:4:6:9:18.

To better comprehend TOI-178's orderly chain of resonance, ESO created an animation of the orbits that is scored with musical notes attributed to each planet. The note rings when a planet completes either one full or one-half orbit. When planets align at these points in their orbits, they chime in harmonic resonance.

Their orderly resonance provides clues about TOI-178's past, according to team member Yann Alibert from the University of Bern.

"The orbits in this system are very well-ordered, which tells us that this system has evolved quite gently since its birth," Alibert said in the ESO press release. If another object had impacted a planet in the system earlier in its life, the configuration of orbits would have been disrupted.

How Are Planets Formed?

Although TOI-178's planetary orbits display an orderly chain of resonance, the composition of those planets is anything but tidy. The rocky planets are much larger than those in our own solar system, and the gas planets are much smaller, ESO notes. And unlike our own solar system, where the planets are neatly arranged from dense inner planets to less-dense outer planets, TOI-178 has rocky planets right next to gassy ones.

"In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory," Leleu told SciTechDaily.

According to NASA, the basic theory of planetary formation goes like this: Our solar system formed 4.5 billion years ago from a dense cloud of interstellar gas and dust that may have collapsed due to some external force, such as a shock wave from a nearby supernova. As the disk of material continued to spin, it pulled in more and more material until pressure in the core forced hydrogen atoms to combine and form helium, which released a tremendous amount of energy. A Sun was born. Eventually, the disk pulled in more matter, which clumped together to form planets and moons. Nearest the Sun, only rocky material could withstand the heat of the new star, while ice, liquid or gas settled into the outer regions of the solar system.

How or why TOI-178's planets seem to break the norm is still puzzling. Other questions remain, such as why only five of the six planets orbit in resonance. Leleu and his team continue to parse the data, looking for additional clues and even additional planets.

"We might find more planets that could be in the habitable zone — where liquid water might be present on the surface of a planet — which begins outside of the orbits of the planets that we discovered to date," Leleu told the European Space Agency.

Although the discovery leaves the door open on several mysteries, it also throws into question how our own solar system formed. Did our planets once have harmonic resonance in the way TOI-178's planets have? Leleu told Phys.org that he thinks the implications could impact some of our most fundamental assumptions.

"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," he said.

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A Six-Exoplanet System Challenges Assumptions of How Planets Form

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