How “citizen astronomers” in Silicon Valley and around the globe helped NASA track a distant Saturn-sized world

Pointing a silver telescope toward the evening sky, Fadi Saibi prepared for a special observation session in the backyard of his Sunnyvale home. It took just a couple of taps to pair his phone with the telescope and lock onto a cosmic object. The target: TOI-4600c, a planet near the constellation of Draco, orbiting a star some 815 light-years from Earth.

“I was concerned because I have some trees in my backyard,” Saibi said. “But after checking, it seemed like there was good visibility.”

By day, Saibi works as the chief technology officer of Artemis Networks, a wireless technology company based in Mountain View. But on the evening of Oct. 16, he indulged a childhood passion for stargazing as part of a widespread research study conducted by amateur astronomers worldwide. The project seeks to understand TOI-4600c and other worlds orbiting distant stars, known as exoplanets. It’s an example of citizen science — collaborations that involve ordinary members of the public in scientific research.

A scientific project like this is not generally why people start watching the stars — but the compelling research has drawn a worldwide network of amateur astronomers like Saibi.

“Sometimes, I long for just spending time looking at the stars, like I used to,” he said.

The project is called UNITE, for the Unistellar Network Investigating TESS Exoplanets. Sponsored by NASA and organized by the SETI Institute in Mountain View, UNITE re-examines exoplanet candidates detected by NASA’s Transiting Exoplanet Survey Satellite (TESS), which has been probing the universe for distant worlds since 2018.

The TESS survey divides the sky into 26 segments, and the satellite observes each every 27 days. Its cameras don’t see exoplanets directly, but instead record the reduction in the amount of light coming from a star as an exoplanet crosses in front of it. This transit results in a dip in the star’s “light curve” — a visual representation of the data TESS receives.

UNITE is a collaboration with the hobby telescope manufacturer Unistellar, allowing anyone who can afford one of the company’s 6-inch smart telescopes — which retail at $4,900 — to jump into cutting-edge science. It operates under a larger exoplanet citizen science program with Unistellar started by Search for Extraterrestrial Intelligence (SETI) Institute scientist Thomas Esposito in 2020.

The main program observes a range of exoplanets — from as large as Jupiter to smaller than Saturn — orbiting stars similar to our sun. Often, these planets are much warmer and orbit closer to their star. Rather than studying these hot, rapidly-orbiting planets, UNITE aims to gather data on rarer worlds that orbit farther out from their parent stars.

“We try to go after something with more than a hundred-day orbit,” Esposito said.

TOI-4600c is similar in size to Saturn, with an orbit that in our solar system would put it between Earth and Mars. Temperatures in its swirling atmosphere are thought to be -110 degrees Fahrenheit, making it just warmer than Jupiter. And like Jupiter and Saturn, it could have moons.

TOI-4600c orbits an orange dwarf star that is slightly smaller and cooler than our sun. Following the detection of an initial transit in 2020, TESS observed a second one 965 days later in 2022. This left researchers with a cosmic conundrum: Could this exoplanet really have such a long orbit, or did TESS simply miss other times it crossed its star while looking at other systems?

Running computer models to simulate the exoplanet’s possible orbital dynamics, graduate student Ismael Mireles and his team at the University of New Mexico calculated that TESS most likely missed one transit. This meant the next one would be observable in the northern hemisphere in the middle of October 2023. UNITE later received a call from NASA’s TESS Follow-Up Observing Program with the mission to further narrow down TOI-4600c’s orbital period, or year.

SETI scientists organized an international session on Oct. 16 and 17 that involved observers from East Asia to North America. All they had to do was set up their smart telescope from their location during the observation window and import the coordinates through the Unistellar app on their phone. Their telescopes would then automatically track the host star as it swept across the night sky, recording the amount of light reaching the telescope and sending the data to computer servers at the SETI Institute.

Six observers in Japan caught the exoplanet’s “ingress,” during which it began crossing its host star. Japanese observers had a slightly difficult time tracking the planet, however, because the target star hung low in the sky, near the horizon.

“To observe at a low altitude, I went to a location with a clear view of the northern sky,” Keiichi Fukui, a retired scientist who worked at the Meteorological Research Institute in Tsukuba, Japan, wrote in an email.