A Bizarre Exoplanet Breaks Solar System Rules
Since the Kepler space telescope began searching for exoplanets in 2009, it has discovered a strange pattern among these alien worlds: Their orbits follow a constant rhythm. If a planet in a system takes twice as long to orbit its star as the planet before it, for example, then the next planet should take twice as long again, and so on. But strangely, some planetary systems do not respect this “pea in the pod” rule.
In a Jan. 7 presentation at the 247th meeting of the American Astronomical Society in Phoenix, Arizona State University (ASU) undergraduate Federico Noguer revealed a particularly curious exception to this trend. The star TOI-1873, located about 600 light years from Earth, is home to three large planets only slightly smaller than Neptune. The trio was discovered in data from the Transiting Exoplanet Survey Satellite (TESS), and two of the planets were first noticed thanks to a volunteer science project called Planet Hunters TESS.
Observations show that the two innermost planets take about seven and 25 days, respectively, to orbit their host star, suggesting that the orbit of each planet in this system should be about 3.6 times longer than that of the last. It should therefore follow that the third planet passes in front of the host star about once every 90 days. But between each of the three times TESS observed the third planet, 900 days passed.
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It could well be that TESS simply missed some flybys of the planet. If so, it would likely have an orbit faster than 900 days but still no shorter than 128 days. “Either way, it’s a strange spacing,” says ASU astronomer Molly Simon, who worked on the study.
More careful observations will be needed to fully unravel this strange star system. The team has already measured the masses of the two inner planets and confirmed that they are exoplanets, but researchers still need to tackle that pesky outer planet. They hope to gain time in ground facilities to focus on the mysterious orbit of the third planet.
“I’m always thinking about what telescopes we can offer,” says Noguer. “But we are trying to do something very hard. Even if it is the shortest [possible] period, if there are clouds in the sky, then you need to wait another 128 days to try again.
The results so far show the importance of citizen science, says Courtney Dressing, an astronomer at the University of California, Berkeley, who was not involved in the research. “It’s always nice when members of the public discover something that might escape the algorithms run by a scientific team,” she says.
Astronomers are curious if other planets are looming in the star system, which could explain the large gap between orbits. “This is an important question when considering systems that don’t follow the pea-in-a-pod paradigm,” adds Paul Robertson, an astronomer at the University of California, Irvine, who was not involved in the study. “You have to ask, ‘Well, what happened?’ »
As they put all their observations in order, scientists work to model possible explanations for the creation of this system. Perhaps there are hidden planets, or perhaps other bodies have interacted with the three planets to string them together in this confusing arrangement. The team plans to submit a paper this summer officially confirming the discovery of the solar system.
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