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A discovery by citizen scientists has led to the confirmation of a system of five planets orbiting a far-off star.

Furthermore, the planets' orbits are linked in a mathematical relationship called a resonance chain, with a pattern that is unique among the known planetary systems in our galaxy.

Studying the system could help unlock some mysteries surrounding the formation of planetary systems.

The results were announced at the 231st American Astronomical Society meeting.

The system was found by astronomy enthusiasts using Zooniverse, an online platform for crowdsourcing research.

"People anywhere can log on and learn what real signals from exoplanets look like, and then look through actual data collected from the Kepler telescope to vote on whether or not to classify a given signal as a transit, or just noise," said co-author Dr Jessie Christiansen, from Caltech in Pasadena.

Since the discovery of four planets in this system was announced last year, Dr Christiansen has been working to shed further light on this distant planetary neighbourhood, dubbed K2-138. This led to the discovery of the fifth planet and hints of a sixth.

All the worlds are a bit bigger than our own planet, ranging between 1.6 and 3.3 times the radius of Earth.

The collected findings have now been accepted for publication in the Astrophysical Journal.

The raw data used in the discovery was provided by Nasa's Kepler space telescope, which identifies potential planets around other stars by looking for dips in the brightness of those stars when planets pass across their face - or transit them.

Links in a chain

Being in a resonance chain means each planet takes almost 50% longer to orbit the star than the next planet further in.

Intriguingly, the sixth planet skips two slots in that resonance chain - taking much longer to orbit the star than it should if it was simply the next planet up from number five.

"If you keep going with the resonance chain, you skip 19 days, you skip 27 days and you end up at about 43 days," said Dr Christiansen.

"That's a really tantalising clue that we may be missing more planets in this system. If this chain continues, there's a gap."

It's not the only system to exhibit resonances. The orbital motions of seven planets in the Trappist-1 system are linked in a complex chain. In the time that it takes for the innermost planet to make eight orbits, the second, third and fourth planets revolve five, three and two times around the star, respectively.

But the way the chain is configured in K2-138 is different to Trappist-1 and unique among exoplanetary neighbourhoods. The near-resonant chain in Trappist-1 is thought to have formed as the planets migrated inwards toward their parent star after formation, early in the system's history.

So K2-138 could yield further clues to the process by which planets form and then migrate from their original positions. This is a particularly contested area, with several competing ideas.

"Some current theories suggest that planets form by a chaotic scattering of rock and gas and other material in the early stages of the planetary system's life. However, these theories are unlikely to result in such a closely packed, orderly system as K2-138," said Dr Christiansen.

"What's exciting is that we found this unusual system with the help of the general public."

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