Astronomers discovered a system that includes six planets and is unlike anything they have seen before. The planetary system could challenge scientists’ theories about how planets form and change over time.
Our solar system is just one of many planetary systems. So far, no two are the same. Systems vary in the number and type of planets they contain.
About 200 light years from us is the star TOI-178, which is located in the constellation Sculptor. The research team initially thought that there were only two stars orbiting the planet.
A closer look revealed something completely different.
‘Through additional observations we realized that there were no two planets orbiting the star at approximately the same distance from it. Rather, they were several planets in a very special configuration, ”said lead study author Adrien Leleu in a statement. Leleu is a member of CHEOPS at the University of Bern, Switzerland. (CHEOPS stands for Characterizing Exoplanet Satellite of the European Space Agency).
The study was published this Monday in the journal Astronomy & Astrophysics.
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Five of the six planets are essentially stuck in a rare rhythmic orbit around the stars, creating a resonance. This means that some of the planets actually do line up every few orbits and that there are discernible patterns as their orbits complete them.
This happens around Jupiter in our solar system when three of its moons – Io, Europa and Ganymede – orbit the gas giant. Io is the closest of the three moons. For every four orbits it completes around Jupiter, Europa completes two and Ganymede completes one. This creates a 4: 2: 1 pattern.
The resonant orbits of the TOI-178 system are more complicated. It is one of the longest resonant chains found in a planetary system, the researchers said.
The five outer planets of the system follow this pattern: 18: 9: 6: 4: 3.
That means that for every 18 orbits of the second closest planet to the star, the third planet completes 9.
A chain of resonance can reveal information about how a planetary system forms and evolves. Also in what way it was in the past.
‘The orbits in this system are very well ordered. Which tells us that this system has evolved quite smoothly since its inception, ”study co-author Yann Alibert said in a statement. Alibert is an affiliate professor of astrophysics at the University of Bern.
Planetary systems can be volatile places during their early days. And disturbances caused by the gravitational influence of large planets can disrupt and drive others out. Other times, impacts between planets or other objects can disrupt systems.
This system has been preserved, hence the resonant orbits. However, the densities of the planets are not well ordered, the researchers noted.
“It appears that there is a planet as dense as Earth right next to a very spongy planet with half the density of Neptune, followed by a planet with the density of Neptune. It’s not what we’re used to, ”study co-author Nathan Hara, a postdoctoral researcher and CHEOPS fellow at the Université de Genève, said in a statement.
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The planets of our solar system are organized with those rocky and with higher density closer to the Sun. Meanwhile, the gaseous planets of lower density are further away.
“This contrast between the rhythmic harmony of orbital motion and disordered densities certainly challenges our understanding of the formation and evolution of planetary systems,” Leleu said.
Several telescopes were used to study the system. Among them, the CHEOPS satellite and several ground-based telescopes at the European Southern Observatory in Chile.
Exoplanets are difficult to observe directly, but the scientists used two methods to achieve this. These methods include radial velocity, or observing starlight for telltale oscillations as planets move around an orbiting star. The other method is the transit, when they are immersed in the light of the stars as the planets pass in front of them.
Both techniques revealed that the planets are much closer and in faster orbits around their star than Earth is from the Sun. For example, the planet closest to the star completes a full orbit in just a couple of Earth days , while the most distant takes about 10 times longer.
Planets vary in type, including those that are rocky and larger than Earth, known as super-earths. There are also the gaseous planets smaller than those in our solar system, called minineptunes.
While planets are one to three times the size of Earth, their masses are 1.5 to 30 times that of Earth.
None of the planets are considered to be in the star’s habitable zone. That is, at the perfect distance from the main star where these planets could harbor liquid water or life on their surfaces.
However, more observations of this system could reveal more others orbiting the star that are in that area. Future telescopes will be able to directly image some of these exoplanets and observe their atmospheres. Which will reveal more secrets of the TOI-178 system.