spaceSpace and Physics

Citizen Scientists Discover Fifth World In Multi-Planet System


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

resonant system

An artist's impression of the K2-138 system as seen from near the outermost confirmed planet. NASA/JPL-Caltech/R. Hunt (IPAC) 

A planetary system whose discovery was inspired by Professor Brian Cox's Stargazing TV show has turned out to be even more interesting than initially thought. So far the planets detected in this system are far too close to their host star to host life, but the star is highly suitable for life and the orbits have big implications for theories of planetary formation.

The Kepler Space Telescope created a wealth of data, but computers still need human assistance to process it. Professional astronomers can't keep up with all the cases flagged by algorithms as possibly indicating planets passing in front of distant planets, so the Exoplanet Explorers project is enabling non-specialist volunteers to contribute their time. These citizen scientists screen examples computer algorithms have flagged further, reducing the numbers professionals needed to check them to manageable proportions.


Stargazing Live, hosted by Brian Cox on Australia's ABC, encouraged viewers to get involved, and hit almost instantaneous paydirt with the discovery of the K2-138 (also known as EE-1) system. Initially reported as hosting four planets, in a paper published in The Astronomical Journal K2-138 has been revealed to have a fifth planet, and even hints at a sixth. All have radii between 1.6 and 3.3 times that of the Earth, with orbits from 2.35 to 12.76 days long.

A few years ago a five-planet system, let alone six, would have sent astronomers wild with excitement, but the record now stands at eight. Meanwhile, the seven-planet TRAPPIST-1 system has three in the habitable zone where water can exist. Yet aside from being the only multi-planet system discovered by citizen scientists, K2-138 is competing for astronomers' attention because of the resonant orbits of the identified planets.

In the time it takes one planet to orbit K2-138 three times, the next one in will go round twice. Similar resonances are common for moons around Jupiter and Saturn. Other planetary systems have also shown resonance, but a five-planet unbroken 3:2 chain is new, with important implications for planetary formation.

"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 first author Dr Jessie Christiansen from Caltech in a statement.


Intriguingly, there is inconclusive evidence for an additional planet with a 42-day period. If the resonance chain is continuous, there should be two between this and the outermost confirmed planet.

K2-138 is a K-type star, cooler than the G-type Sun (5200 K compared to 5700 K), but far less likely to sterilize planets in its habitable zone with X-ray outbursts, than the red dwarfs like TRAPPIST-1 or Proxima Centauri.


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