Planetary systems evolve over dizzying timespans. It typically takes millions, even billions, of years for gravitational interactions to noticeably alter a planet’s orbital trajectory. This stability allowed life to take hold on Earth. It also means orbital planes are effectively fixed on human timescales.
To infer a planet’s orbital history, astronomers typically observe clues signaling processes that played out long before we existed.
That’s why a newly observed planetary system, dubbed TOI-201, is such an outlier. Surprisingly, observations of the system’s three known planets have revealed orbits that change in real time. That is to say, they shift on human timescales, meaning astronomers have observed changes from one orbit to the next.
This impressively odd system could shed light on the early formation of our own solar system – a time when Earth’s orbit was far less stable than it is today.
A planetary system with an unusual tempo
The unusual nature of TOI-201 doesn’t end with its dynamic behavior. TOI-201 is composed of three surprisingly different planetary bodies: a “super-Earth” that orbits its star every 5.8 days; a gas giant with half the mass of Jupiter and a 53-day orbital period; and a massive outer planet 16 times heavier than Jupiter, with a roughly 8-year elliptical, comet-like orbit.
Much like Halley’s comet in our solar system, that massive outer planet – a brown dwarf designated TOI-201c – roams the outskirts of the TOI-201 system before coming in for a closer approach to its star. Of course, Halley’s comet is much smaller than TOI-201c. It also takes the best part of a century to orbit our Sun, rather than the 8 years it takes TOI-201c. The brown dwarf’s enormous gravitational influence, paired with its relatively elliptical and tilted orbit, makes for a potent combination, resulting in TOI-201’s unusual dynamic behavior.
The people behind the discovery, a large international team including scientists from the University of New Mexico, the Observatoire de la Côte d’Azur, and the European Space Agency (ESA), published their findings in the journal Science earlier this month. In their paper, they claim TOI-201 behaves in a way astronomers have never observed before.
“Usually, planets are like metronomes with each transit in front of the star happening exactly one orbital period after another,” Professor Amaury Triaud, from the University of Birmingham, explained in a press statement. “However, we were following TOI-201b, and suddenly the planet started transiting about half an hour late. This sudden jump was very surprising, and we reported our observations. Other astronomers around the globe noticed intriguing signals too, and by working together, the team could start to understand this system.”
Leveraging the most remote base on Earth
The team used four observational techniques to confirm the gravitational instability of TOI-201’s unruly planets. Firstly, they turned to spectroscopy to measure a wobble in observations of the host star, caused by the planets. This helped determine the planets’ individual masses.
Then, they measured the timing of the system’s planetary transits – the time it takes for a planet to reappear across the face of its host star. This is measured by detecting a slight dimming of the star in observations. The scientists used data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) spacecraft, as well as ground-based observations from the ASTEP telescope in Antarctica.
ASTEP (Antarctic Search for Transiting ExoPlanets) is led by the Observatoire de la Côte d’Azur, Nice, in partnership with the University of Birmingham and ESA. The 40-cm telescope is located at the Concordia Research Station – one of three permanent all-year research stations on the Antarctic Plateau.
According to ESA, Concordia is 373 miles (600 km) away from any other human settlement, making it more remote than even the International Space Station (ISS). This unique observational vantage point allows for long, uninterrupted observations with minimal atmospheric interference—perfect for detecting tiny brightness changes in stars.
“This discovery was enabled by having a telescope in Antarctica,” Triaud, who leads the team at ASTEP, said. “Whilst the logistics involved are difficult, its unique situation and its access to optimal astronomical conditions are key to studying exoplanetary systems with long orbital periods such as TOI-201.”
To round off the observations, the researchers used Transit Timing Variations (TTVs) and astrometry. TTV measures tiny deviations in the time it takes a transit to occur. In this case, it allowed the researchers to confirm the effects of the outer planet’s gravitational influence. Astrometry, meanwhile, detects minuscule shifts in a star’s position. Using astrometry data from the Hipparcos and Gaia space missions, the researchers confirmed the presence of the massive, unseen outer planet via its gravitational impact on its host star.
Orbital planes with an expiry date
The data points to a single culprit: TOI-201 c is causing the commotion, tugging at its neighboring planets as they orbit nearer their star. Besides its mass and elliptical orbit, the brown dwarf’s tilted orbit also plays an important role.
“The planets' orbits are tilted relative to each other, and because of that, they're slowly pulling each other into new orientations,” Ismael Mireles, a Ph.D. candidate in the University of New Mexico Department of Physics and Astronomy, who led the study, said in a separate statement.
This is highly unusual, as planets—such as those in our own solar system—are born in the plane of the protoplanetary disk formed by their star, resulting in aligned orbits. The researchers hope to use follow-up observations to determine how these planets became so unaligned.
In fact, the TOI-201 system’s behavior is so unstable that the researchers say they will soon stop lining up in front of their star. In 200 years, they estimate that only two of the three planets will still be transiting. Such dynamic behavior is usually on display during the early formation of a star system.
“In the solar system, almost all planets are coplanar, but here, this is not the case and each planet is different,” Triaud explained. “This points to some active orbital reorganization within the system, providing us a glimpse of what happens shortly after planet formation.”
“This is one of only a handful of systems where planetary orbits can be watched actively changing on human timescales,” added Mireles. “It offers a rare real-time window into the dynamic lives of planetary systems.”
The study appeared in Science Advances this month.
