Since the Kepler Space Telescope began its prowl for exoplanets in 2009, it has uncovered a strange pattern among these alien worlds: their orbits follow a consistent rhythm. If one planet in a system takes twice as long to orbit its star as the planet before it, for instance, then the next planet should take another two times as long, and so on. But a strange few planetary systems don’t abide by this “peas-in-a-pod” rule.
During a January 7 presentation at the American Astronomical Society’s 247th meeting in Phoenix, Federico Noguer, an undergraduate at Arizona State University (ASU), revealed one particularly curious exception to this trend. The star TOI-1873—about 600 light-years from Earth—hosts three large planets just a bit smaller than Neptune. The trio was discovered in data from the Transiting Exoplanet Survey Satellite (TESS), and two of the planets were first noticed through a volunteer science project called Planet Hunters TESS.
The observations show that the two innermost planets take about seven and 25 days to circle their host star, respectively, suggesting that the orbit of each planet in this system should be about 3.6 times as long as that of the last. It should follow, then, that the third planet passes by the host star once every 90 days or so. But between each of the three times TESS observed the third planet, a staggering 900 days had passed.
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It well may be that TESS simply missed some of the planet’s flybys. If so, it would likely have an orbit faster than 900 days but still no shorter than 128 days. “Regardless, this is a weird spacing,” says Molly Simon, an astronomer at ASU, who worked on the study.
It’ll take more careful observations to fully unravel this strange star system. The team has already measured the masses of the inner two planets and confirmed them as exoplanets, but the researchers still need to tackle that pesky outer planet. They’re hoping to win time at ground-based facilities to zero in on the third planet’s mysterious orbit.
“I’m always thinking of what telescopes we can propose for,” Noguer says. “But we’re trying to do something very hard. Even if it’s the shortest [possible] period, if there are clouds in the sky, then you have to wait another 128 days to try again.”
The results so far show the importance of crowdsourced science, says Courtney Dressing, an astronomer at University of California, Berkeley, who was not involved in the research. “It’s always neat when members of the public find something that might be missed by the algorithms run by a science team,” she says.
Astronomers are curious about whether other planets maybe be looming in the star system, which may explain the large gap in orbits. “That’s an important question when you look at systems that don’t follow the peas-in-a-pod paradigm,” adds Paul Robertson, an astronomer at the University of California, Irvine, who did not participate in the study. “You have to ask, ‘Well, what happened?’”
As they get all their observations in order, the scientists are working to model possible explanations for how this system came to be. Perhaps there are hidden planets, or maybe other bodies interacted with the three planets to string them into this puzzling arrangement. The team plans to submit a paper officially confirming the newly discovered solar system this summer.
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