Scientists have discovered the remarkable exoplanetary system TOI-201. The orbits of the three planets circling the central star evolve so rapidly that it seems the star is never quite the same as it was just a few years ago.
Three completely different worlds
Astronomers from the University of New Mexico have published the results of a new study confirming the existence of three exoplanets orbiting the TOI-201 system. These include a super-Earth (TOI-201d), a hot Jupiter (TOI-201b), and a brown dwarf (TOI-201c). The study was led by Ismael Mireles, a graduate student in the Department of Physics and Astronomy at the University of New Mexico (UNM), under the supervision of Professor Diana Dragomir. The paper, titled “Uncovering the Rapidly Evolving Orbits of the Dynamic TOI-201 System,” was published in the journal Science Advances.
Super-Earth (TOI-201 d) is a rocky planet approximately 1.4 times the size of Earth and about six times as massive, completing one full orbit around its star every 5.85 days. It orbits very close to its star and is likely too hot for liquid water to exist.
“Warm Jupiter” (TOI-201 b) is a gas giant with a mass roughly half that of Jupiter, orbiting its star every 53 days. “Warm Jupiters” are situated between “hot Jupiters,” which are closer (with orbital periods of a few days), and cold, distant gas giants such as Jupiter (~12 years). They are of scientific interest because astronomers do not fully understand how they ended up in the orbits in which they were discovered.
The brown dwarf (TOI-201 c) is the most massive object in the system, apart from the star, and orbits in a wide, highly elliptical orbit with a period of approximately 8 years. Specifically, its gravitational influence accounts for a large part of the system’s dynamic behavior. TOI-201 c is also the object with the longest transit period ever detected.
“TOI-201 c is unique because of its extremely long orbital period (~7.9 years) and its location in a system with two interior planets,” said Mireles. “Most known transiting brown dwarfs orbit much closer to their stars.”
The mass of a brown dwarf is 13 times that of Jupiter, but it is still too small to be classified as a true star. It is unable to sustain a thermonuclear reaction in its core, as the Sun does.
Real-time changes in the orbits of the TOI-201 system
This is one of the few systems where active changes in planetary orbits can be observed within a human lifetime. “It offers a rare opportunity to get a real-time glimpse into the dynamic life of planetary systems,” Mireles explained. In fact, 200 years from now, only two of the three objects will still be passing in front of the star.
To prove the existence of this system, scientists employed a combination of four observational methods. The first is spectroscopy (measurement of radial velocities), which allows scientists to measure the star’s wobble caused by the planets’ rotation and helps determine their mass. The second method is transit photometry, which involves detecting a slight dimming of a star as a planet passes in front of it.The study utilized data on transits from NASA’s TESS space telescope and ground-based observations from the ASTEP telescope in Antarctica—a project led by by the Observatoire de la Côte d’Azur, Nice in partnership with the University of Birmingham and the European Space Agency. Observations of transits from the LCOGT global network of telescopes, located in Chile, Australia, and South Africa, were also included and played a crucial role in the analysis.
“Our contribution was enabled by having a telescope in Antarctica. While the logistics involved are difficult, the telescope’s unique location and access to optimal astronomical conditions are key to studying exoplanetary systems with long orbital periods such as TOI-201,” said Professor Triaud of the University of Birmingham.
The third method involved transit time variations (TTVs), which measure slight deviations in the time it takes for a planet to pass in front of its star, indicating the presence of another planet exerting a gravitational influence. Finally, the researchers used astrometry based on data from the Hipparcos and Gaia space missions to detect tiny shifts in a star’s position in the sky caused by an invisible massive companion.
Inclined orbits of exoplanets
Mireles adds that observations of exoplanets typically provide only a snapshot of a system’s evolution. In reality, most systems change only over timescales of millions of years. What makes TOI-201 special is that researchers can actually observe its changes 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,” said Mireles.
“This was a surprise, because if planets are 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. So the next question to answer for TOI-201 is how these three objects ended up with such tilted orbits,” added Dragomir.
In 200 years, the super-Earth will stop transiting. In a few hundred years, the warm Jupiter will stop transiting, and later the brown dwarf will also stop doing so. However, they will begin transiting again thousands of years in the future, as they are subject to cycles of transiting and non-transiting configurations.
The next transit of TOI-201 is predicted for March 26, 2031, which will provide a rare opportunity for further observations around the world, including by citizen scientists.
According to phys.org
