Since the Kepler space telescope began its search for exoplanets in 2009, it has uncovered a strange pattern among these alien worlds: Their orbits follow a consistent rhythm. For example, if a planet in a system takes twice as long to orbit its star as its predecessor, the next planet should take twice as long, and so on. But some strange planetary systems don’t follow this “pea-in-a-pod” rule.
During a January 7 presentation at the 247th meeting of the American Astronomical Society in Phoenix, Arizona State University (ASU) graduate Federico Noguer revealed a particularly curious exception to this trend. The star TOI-1873—about 600 light-years from Earth—hosts three giant planets just slightly smaller than Neptune. All three were discovered in data from the Transiting Exoplanet Survey Satellite (TESS), and two of the planets were observed for the first time through a volunteer science project called Planet Hunters TESS.
Observations show that the two innermost planets take seven and 25 days, respectively, to orbit their host star, which suggests that the orbit of each planet in this system should be about 3.6 times longer than the orbit of the previous planet. Then, it should follow that the third planet passes the host star once every 90 days. But between the three times TESS observed the third planet, a staggering 900 days had passed.
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It may well be that TESS has missed some flybys of the planet. If so, its orbit would likely be faster than 900 days, but still not less than 128 days. “Regardless, it’s a strange difference,” says Molly Simon, an ASU astronomer who worked on the study.
More careful observations will be required to fully uncover this strange star system. The team has already measured the masses of the inner two planets and confirmed them as exoplanets, but researchers still need to deal with that dangerous outer planet. They are hoping to win time at ground-based facilities to focus on the mysterious orbit of the third planet.
“I’m always thinking about what telescopes we can propose,” says Noguer. “But we’re trying to work very hard. Even if it’s the shortest (possible) period, if there are clouds in the sky, you have to wait another 128 days to try again.”
Courtney Dressing, an astronomer at the University of California, Berkeley, who was not involved in the research, says the results so far show the importance of crowdsourced science. “It’s always good when the public gets to see something that might have been missed by the algorithms run by the science team,” she says.
Astronomers are curious whether other planets also orbit the star system, which could explain the large differences in orbits. “This is an important question when you look at systems that don’t follow the pea-in-a-pod paradigm,” says 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 soon as they get all their observations in order, scientists are working to formulate possible explanations for how this system came into existence. Perhaps there are hidden planets there, or maybe other bodies have interacted with the three planets, binding them into this mysterious arrangement. The team plans to submit a paper this summer officially confirming the newly discovered solar system.
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