Using the Kepler Space Telescope, an international team of astronomers successfully gathered detailed information about the seventh and outermost planet in the TRAPPIST-1 system.
TRAPPIST-1 is a cool, middle-aged red dwarf star located 40 light years from Earth in the constellation Aquarius. Only slightly larger though more massive than Jupiter, it is orbited by seven closely-spaced planets, three of which are known to be in its habitable zone, where temperatures allow liquid water to be present on their surfaces.
Far dimmer than the Sun, the star is named for the ground-based telescope that discovered its orbiting planets, the Transiting Planets and Planetestimals Small Telescope (TRAPPIST).
In 2016, a research team led by Michael Gillon of the University of Liege in Belgium detected three planets orbiting the star. One year later, a second paper on the system confirmed the system has seven planets.
Because only one transit, or passage in front of the star, was seen for the outermost planet, TRAPPIST-1h, Gillon’s study was unable to obtain details about that planet’s orbit.
A subsequent study led by Rodrigo Luger of the University of Washington successfully observed four transits of TRAPPIST-1h with Kepler over a period of 79 days.
Researchers found that all seven planets are in a mathematical pattern known as an orbital resonance, with TRAPPIST-1h circling the star once every 18.77 days.
“TRAPPIST-1h is exactly where our team predicted it to be”, Luger said. “Resonances can be tricky to understand, especially between three bodies.”
Having a chain of seven planets in resonant orbits sets a new record. The two previously largest known systems with planets in a chain of resonant orbits, Kepler-80 and Kepler-223, each have four such planets.
Each planet in a group with orbital resonances influences the others, resulting in any planet somehow knocked off course pulled back into its regular orbit.
“Once you’re caught into this kind of stable resonance, it’s hard to escape,” Luger emphasized.
The orbital resonances of the planets occurred in the system’s early days before they were completely formed, indicating they migrated inward toward the star in lock-step, he noted.
TRAPPIST-1h is now too cold to be habitable, with an average surface temperature of minus 148 degrees Fahrenheit (173 degrees Kelvin).
Yet long ago, when the star was younger and brighter, its outermost planet was likely much warmer.
“We could therefore be looking at a planet that was once habitable and has since frozen over, which is amazing to contemplate and great for follow-up studies,” Luger said.
Findings of the latest study have been published in the journal Nature Astronomy.