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.
Unlike our own solar system, which has planets orbiting the sun in an elliptical orbit, the exoplanets in K2-138 were found orbiting their sun-like star in concentric circles. The size of each planet's orbit appears to be a ratio of the other orbits, suggesting that all five planets originally formed together in a smooth, rotating disc, and over eons migrated closer in towards their star.
The results were announced at the 231st American Astronomical Society meeting.
While the fact that citizen scientists were the main discoverers of these exoplanets is exciting, and perhaps a harbinger of other similar finds in the future, the discovery itself also has some value for astrophysics.
The K2 data was mostly light curves, showing the intensity of light from individual stars. If they thought the curve looked like a transit, they clicked "Yes" and if not then they clicked "No".
Another batch of 2017 Kepler data was recently uploaded to Exoplanet Explorers for citizen scientists to peer through. In contrast, K2 has been driven mainly by decentralized, community-led efforts. The most distant planet, K2-138f, is one-tenth the distance that Earth is to the Sun. Using a platform called Zooniverse, citizen scientists sifted through Kepler's data.
At first, the researchers ran a signal-detection algorithm to spot potential transit signals in the K2 data, and after that, they made those signals available for users on the Zooniverse platform. They designed a training programme to first teach users what to look for in determining whether a signal is a planetary transit.
"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", explained Christiansen in a statement.
If at least 10 users looked at any particular curve, and at least nine of them (90 percent) indicated a possible transit, Christiansen and Crossfield would analyze that specific signal further. "It is exciting, because we are getting the public excited about science, and it is really leveraging the power of the human cloud", Crossfield said.
Several months into working with Zooniverse to get Exoplanet Explorers up and running, the researchers got a call from an Australian television program that was offering to feature the project on live television.
But the way the chain is configured in K2-138 is different to Trappist-1 and unique among exoplanetary neighbourhoods. Over 48 hours, the users made almost 2 million classifications from the available light curves. Astronomers have not yet searched through most of it for planets.
The five planets are all part of the same star system located about 620 light-years from Earth, in the constellation Aquarius. They then took some additional measurements to ensure that it was indeed a single star, and not a cluster of stars. A dip typically indicates that a large object - namely an exoplanet - has passed in front of it.
As per the data collected by NASA's Kepler telescope, these five planets are called as super-Earth as their sizes range between 1.6 and 3.3 times the radius of Earth.
Since the discovery of four planets in this system was announced past year, Dr Christiansen has been working to shed further light on this distant planetary neighbourhood, dubbed K2-138. Citizen scientists and astronomers are now searching through the new data trove, indicating that more new exciting discoveries may be in the offing. It turns out the world is big enough that there's a lot of people who are interested in doing some amateur science. "And the human eye in many cases is very effective in separating the planetary wheat from the nonplanetary chaff", Crossfield said in a statement Thursday. It's an MIT-led mission that will survey the entire sky for exoplanets orbiting the brightest stars.
Before we get into the nitty-gritty of how it was discovered, let's dive in and take a look at the new star system. "We hope that the TESS mission, which MIT is leading, will also be able to engage the public in this way".