Amateur Astronomers Discover ‘Unusual’ Five-Planet System Using Zooniverse And Kepler Data

The newfound K2-138 planetary system was detected by Exoplanet Explorers volunteers and boasts a rare characteristic that sets it aside among other systems.

Artist's concept of the K2-138 planetary system, discovered by amateur astronomers via the Exoplanet Explorers project.
NASA/JPL-Caltech

The newfound K2-138 planetary system was detected by Exoplanet Explorers volunteers and boasts a rare characteristic that sets it aside among other systems.

For the first time ever, citizen scientists have identified a multi-planet system found entirely through crowdsourcing. The newfound planetary system, made up of five confirmed exoplanets and possibly a sixth one, is not only a first in terms of how it was discovered, but also due to a rare feature described as “unique among exoplanetary neighborhoods.”

The five-planet solar system was detected last year by volunteers enlisted in the Exoplanet Explorers project, an online initiative for citizen scientists hosted on Zooniverse, and which uses fresh data from NASA’s Kepler space telescope.

The amateur astronomers combed through heaps of 2017 Kepler data, collected during its K2 mission and uploaded on the Zooniverse platform. In their search for possible planet candidates, more than 10,000 volunteers worldwide sifted through Kepler observation of nearly 100,000 stars to classify their blinking patterns, and originally discovered four of the system’s five planets — a remarkable find that took place just 48 hours after the project was launched in early April 2017.

Since then, project co-founder Jessie Christiansen, staff scientist at Caltech in Pasadena, California, has been studying the newfound system — dubbed K2-138, after the K2 mission — and has identified an additional confirmed exoplanet, raising the number to five. But this multi-planet solar system may still have a lot to offer, since Christiansen’s team also found “hints of a sixth planet as well,” shows a Caltech news release issued on Thursday (January 11).

All the five confirmed exoplanets in the K2-138 system are rocky super-Earths — meaning they exceed the size of our home planet, while remaining smaller than Neptune. According to Gizmodo, the five exoplanets measure between 1.6 and 3.3 times the Earth’s radius, and orbit an orange dwarf lying 620 light-years away.

Their parent star is slightly bigger than our Sun, its “orange dwarf” classification pointing to a mass 0.45 to 0.8 times larger, and the five exoplanets are tightly crammed around it. In fact, the most distant of the five planets, called K2-138f, orbits the star at one-tenth the distance between the Earth and the Sun.

Given their close proximity to the parent star, the five K2-138 exoplanets are extremely hot, with temperatures ranging from 800 to 1800 degrees Fahrenheit, and are most likely inhospitable to life. Their discovery, announced this week at the 231st meeting of the American Astronomical Society in Washington, D.C., is detailed in a study authored by Christiansen together with Ian Crossfield, astronomer at the University of California, Santa Cruz, and the other co-founder of the Exoplanet Explorers project.

Artist's rendition on the K2-138 planetary system diagram, showing the top-down view of the five exoplanets, together with their orbits and relative sizes.
Artist’s rendition of the K2-138 planetary system diagram, showing the top-down view of the five exoplanets, together with their orbits and relative sizes. NASA/JPL-Caltech

Their project, featured on April 4-6 on ABC Australia’s Stargazing Live TV series, recruited volunteers to take a look at Kepler’s C12 dataset — star observations that hadn’t been previously analyzed by professional astronomers — and give their opinion on whether the dips detected in the stars’ brightness signaled potential planets in passing (transit) or were just false alarms.

“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,” Christiansen explained in the Caltech news release.

As a matter of fact, a new batch of 2017 Kepler data was recently uploaded on Zooniverse for the Exoplanet Explorers project, informs a NASA news release. This could mean further exciting discoveries down the road.

In the months following Stargazing Live, Christiansen and Crossfield examined the K2-138 system and found not only a fifth and, possibly, a sixth planet, but also that the exoplanets have a unique characteristic.

The five planets in the K2-138 system are linked “in an interesting mathematical relationship called a resonance,” revealed the Caltech news release. The planets take between 2.35 days to 12.76 days to complete their orbit, also known as an orbital period. The two researchers observed that “each planet takes almost exactly 50 percent longer to orbit the star than the next planet further in,” which constitutes a resonance chain.

“The [orbital] periods of the five planets are 2.35, 3.56, 5.40, 8.26, and 12.76 days, forming an unbroken chain of near 3:2 resonances,” Christiansen and Crossfield show in their study, which has been accepted for publication in The Astronomical Journal.

This makes K2-138 the only planetary system with an unbroken chain of resonances in this configuration, notes the Caltech news release. Other planetary systems also exhibit resonances, one example being the seven-planet TRAPPIST-1 system.

“But the way the chain is configured in K2-138 is different to Trappist-1 and unique among exoplanetary neighbourhoods,” notes the BBC, reporting on the discovery of the five-planet system.

This unique feature of the K2-138 system could shed more light into how planets are formed and how they migrate though the planetary system from the moment of their birth.

“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 Christiansen.

“What’s exciting is that we found this unusual system with the help of the general public,” she added.