Last year, a crew of sharp-eyed citizen scientists spotted an exoplanet the size of Jupiter hidden in plain sight. It's slightly warmer than room temperature on Earth, floats 379 light-years from us and, every 261 days, completes an orbit around a star with about the same mass as our sun.
Until now, the foreign gas giant, dubbed TOI-2180 b, was concealed in data collected by NASA's Transiting Exoplanet Survey Satellite, or TESS. It was entwined so deeply even the agency's best algorithms didn't catch it.
"This is one area where humans are still beating code," Paul Dalba, an astronomer from the University of California at Riverside and lead author of a study on the find, published Jan. 13 in the Astronomical Journal, said in a statement. Rather than rely on automation, the citizen scientists looked to a more accessible tool: their own eyes, combined with hard work.
Tom Jacobs, a member of the citizen team and a former US naval officer, said he and fellow amateur astronomers "devote many hours each day surveying the data out of pure joy and interest in furthering science." So far, they've co-authored over 68 peer-reviewed science papers.
"We love contributing to science," Jacobs said. "And I love this type of surveying, knowing that one is in new, undiscovered territory not seen by any humans before."
Humans versus code
Typically, professional exoplanet hunters program computers to sift through TESS' heaps of information and analyze brightness patterns around nearby stars. When a star dims from Earth's point of view, the shift in luminosity indicates a planet in the star's system blocking stellar rays of light headed toward us.
Think of it like a sort of cosmic Morse code, through which exoplanets – and, more recently, exomoons – signal their existence to human astronomers.
But while over 4,000 exoplanets owe their recognition to such analysis, the tried and tested method faces a slight hurdle. Remember, TESS-scouring code traces brightness patterns, which means it requires several datasets to flag a possible exoplanet detection. The newly uncovered exoplanet, though, exhibited what's called a "single-transit event." It only crossed paths with starlight once, therefore offering a single piece of data.
With the help of downloadable software called LcTools, Jacobs and fellow amateur scientists personally look through TESS data to examine star luminosity in the form of light curves, or brightness adjustments over time. This level of scrutiny helped Jacobs first notice TOI-2180 b's signal on Feb. 1, 2020. Fittingly, the group calls themselves the Visual Survey Group.
"The manual effort that they put in is really important and really impressive," Dalba said. "Because it's actually hard to write code that can go through a million light curves and identify single-transit events reliably."
However, just as expert algorithms hit obstacles, the human eye does as well. TESS codes generally search for multiple occurrences of star dimming for a reason. More signals increase the likelihood of true exoplanet detection. Single-transit events, for instance, could easily be chalked up to random noise in the data.
That's why Dalba later stepped in to bolster the breakthrough from Jacobs and team.
Using the Automated Planet Finder Telescope at Lick Observatory in Mt. Hamilton, California, Dalba measured the star's "wobble" to determine the exoplanet's size, and spent 500 days and 27 hours observing its orbit.
The entire research team also organized an "observing campaign," inviting both professional and nonprofessional astronomers to set up at 14 sites across three continents, using telescopes to keep watch on TOI-2180 b. Altogether, over 11 days, they captured more than 20,000 separate images of the exoplanet's star with varying degrees of brightness.
Dalba even camped for five nights in California's Joshua Tree National Park to examine the massive exoplanet. "Discovering and publishing TOI-2180 b was a great group effort demonstrating that professional astronomers and seasoned citizen scientists can successfully work together," Jacobs said. "It is synergy at its best."
Despite the strenuous effort, Dalba, Jacobs and the rest of the astronomers say they still lack confidence to confirm TOI-2180 b's status. Still, they did discern that the planet will be transiting its host star once again in February, providing a new window for further analyses.