The TOI-201 system consists of a super-Earth, a warm Jupiter, and a more massive brown dwarf at 5.8-, 53-, and 2,900-day orbital periods, respectively.
An artist’s impression of the warm giant exoplanet TOI-201b and its parent star. Image credit: Sci.News.
TOI-201 is a bright F-type star located 372 light-years away in the constellation of Pictor.
The star is 32% bigger and more massive than the Sun and is around 870 million years old.
Also known as HD 39474 and TIC 350618622, it hosts at least three companions: TOI-201b, c and d.
“The goal was to characterize the TOI-201 planetary system to understand not just what planets are there, but how they interact with each other dynamically,” said Ismael Mireles, a Ph.D. candidate at the University of New Mexico.
“This helps scientists understand how planetary systems like our own Solar System form and evolve over time.”
TOI-201d is a rocky super-Earth planet roughly 1.4 times the size of Earth and approximately 6 times Earth’s mass, completing one orbit every 5.8 days.
TOI-201b is a warm-Jupiter exoplanet about half the mass of Jupiter orbiting every 53 days.
TOI-201c is a brown dwarf — the most massive body in the system besides the star, on a wide, highly elliptical approximately 8-year orbit. Its gravitational influence is responsible for most of the system’s dynamic behavior. TOI-201c is also the longest-period transiting object ever to be discovered.
“TOI-201c is unique because of its extremely long orbital period (~7.9 years) and its location in a system with two interior planets,” Mireles said.
“Most known transiting brown dwarfs orbit much closer to their stars.”
“Since the mass of TOI-201c is near the boundary separating massive planets from brown dwarfs, one mystery this system poses is whether this body formed like a planet or like a star,” said University of New Mexico’s Professor Diana Dragomir.
“This is one of only a handful of systems where planetary orbits can be watched actively changing on human timescales,” Mireles said.
“It offers a rare real-time window into the dynamic lives of planetary systems. In fact, in 200 years only two of the three objects will still be transiting.”
What makes the TOI-201 system special is that the astronomers are actually able to watch it change in real time.
“The planets’ orbits are tilted relative to each other, and because of that, they’re slowly pulling each other into new orientations,” Mireles said.
“This was a surprise, because if planets are been born in the plane of the protoplanetary disk that existed early in the life of the star, they are expected to have aligned orbits, like the planets in the Solar System,” Professor Dragomir said.
“So the next question to answer for TOI-201 is how these three objects ended up with such tilted orbits.”
In 200 years, TOI-201d will stop transiting; a few hundred years later, TOI-201b will stop transiting and later on, TOI-201c will stop transiting.
However, they will start transiting again thousands of years in the future, since they undergo cycles of transiting and non-transiting configurations.
The next transit of TOI-201c is predicted for March 26, 2031, which will provide a rare opportunity for follow-up observations worldwide, including by citizen scientists.
“It was truly a multi-year, large team effort to study this system,” Mireles said.
A paper on the findings was published in the journal Science Advances.
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Ismael Mireles et al. 2026. Uncovering the rapidly evolving orbits of the dynamic TOI-201 system. Science Advances 12 (16); doi: 10.1126/sciadv.aef2618
