Citizen Scientists Discover an Unusual Five-Planet Solar System

Users looking through data on Exoplanet Explorers flagged four sub-Neptune planets orbiting a distant star called K2-138

K-138 Planets
Artists illustration of the K-138 system NASA/JPL-Caltech

Citizen science has become an incredible resource for gathering or sifting large amounts of data that individual scientists just can’t handle on their own—from documenting birds around the world, to sampling the human microbiome, to even transcribing century-old weather records. Now, reports Paul Rincon at the BBC, citizen scientists have added another feather to their cap, combing through reams of data to identify a five-planet system orbiting star K2-138.

The data comes from the planet-hunting Kepler space telescope. Launched in 2009, the NASA satellite completed its primary mission's search for exoplanets in November of 2012. On its hunt, Kepler would monitor the brightness of stars and then scientists would pore through the data in search of winks in that light—the telltale sign of an orbiting planet.

But after its initial mission, Kepler was struggling. By 2013, scientists had lost the ability to precisely direct the telescopes instruments, rendering its mission at an end. But engineers salvaged the telescope by designing a secondary mission dubbed K2. In the course of three years, that mission has gathered data on an additional 280,000 stars.

The Kepler database is massive, and computers can't yet take on all the dirty work. As Calla Cofield at reports, while software is able to identify stars that host planets, it still takes a human to confirm the data. So scientists turned to citizen scientists for help, developing Exoplanet Explorers, hosted on the Zooniverse platform—a system that connects citizen scientists to projects.

According to a press release, anyone can sign up for the site, and after a little online training, users then look through signals and vote on which dip in brightness seem legit. If a minimum of 10 or more users agree with 90 percent certainty that a potential planetary transit is real, it goes to researchers for confirmation. The program was a success. In just first 48 hours since the project went live, more than 10,000 users chimed in with over 2 million classifications. 

That's where the new—and unusual—solar system comes in.

Citizen scientists identified the five-planet system in early April, just two weeks after Exoplanet Explorers launched. And astronomers quickly confirmed the system, finding it is “extremely likely” for the system to contain four planets—and possibly a fifth. (There are still more hints it may even contain a sixth planet). All of the planets in this solar system are considered sub-Neptune planets, which are 1.3 and 3.3 times the radius of Earth, but not as large as Neptune. It is the first multi-planet system discovered by crowdsourcing. The research appears in Astronomical Journal.

The K2-138 system is interesting to scientists beyond the fact that it was found by people sitting on their couches at home. As Rincon reports, the planets orbit in what is called a resonance chain. In this situation, each planet takes about 50 percent longer to orbit than the next closest planet to the star.

In our own solar system, Jupiter’s moons Ganymede, Europa and Io orbit in a resonance chain. The seven Earth-like TRAPPIST-1 planets discovered last year also likely orbit in a nearly resonant chain. The pattern of that resonance chain tells researchers that there may be other planets orbiting K2-138 since the purported sixth planet seems to skip two links in the chain.

“If you keep going with the resonance chain, you skip 19 days, you skip 27 days and you end up at about 43 days,” Jessie Christiansen of Caltech, one of the developers of the project, tells Rincon. “That's a really tantalizing clue that we may be missing more planets in this system. If this chain continues, there's a gap.”

If the results hold, the resonance also gives some hints to the way planetary systems form. According to the press release, some theories suggest that planetary systems form from a chaotic scattering of rock and gas. But a nice, tightly packed, orderly system like K2-138 hints that this isn't the case. Solar system formation may be a slightly more orderly process.