Astronomers work out details of ‘least understood’ exoplanet

The orbital period of the TRAPPIST-1 system’s most remote planet, TRAPPIST-1h, has been uncovered


This artist depicts the TRAPPIST-1 system, with steam, water and frost representing how H2O would exist on a planet. Image credit: NASA/R. Hurt/T. Pyle

After many hours spent analysing the TRAPPIST-1 system, an ‘orbital resonance’ for all the planets has been deduced. From this resonance, the previous unknown orbital period for TRAPPIST-1h has been discovered.

News of the TRAPPIST-1 system swept across the globe this year, and astronomers have worked non-stop to unveil all they can on the remarkable structure. On 22 February 2017, NASA announced that seven Earth-sized planets are orbiting the ultra cool dwarf star known as TRAPPIST-1. The announcement became more exciting when the NASA’s astronomers stated that three of the planets lie within the habitable zone – the region around a host star where water could exist as a liquid. For three Earth-sized planets to possibly have liquid water is a tantalising prospect. However a tremendous amount more research must be conducted to conclude anything.

Since then, astronomers have studied the data collected from NASA’s Kepler Space Telescope, and deduced an ‘orbital resonance’ for the seven planets. An orbital resonance is when at least two bodies in orbit around a common body share a regular, periodic orbit because of the gravitational effect on each other. For example, the orbital resonance for Jupiter’s moons Io:Europa:Ganymede is 1:2:4. Meaning for every two Io orbits, Europa completes a single orbit, and for every four Io orbits, Ganymede also completes a single orbit.

This diagram compares the TRAPPIST-1 system to our Solar System. Image credit: ESO/O. Furtak

Astronomers proposed six possible orbital resonances for the TRAPPIST-1 system, with six different orbital periods of the least known planet, TRAPPIST-1h. When compared to the Kepler data, only one resonance fitted the observations, which eventually predicted the orbital period of TRAPPIST-1h to be 19 days.

“It really pleased me that TRAPPIST-1h was exactly where our team predicted it to be. It had me worried for a while that we were seeing what we wanted to see – after all, things are almost never exactly what you expect them to be in this field,” Rodrigo Luger, doctoral student at University of Washington in Seattle, United States says. “Nature usually surprises us at every turn, but, in this case, theory and observation matched perfectly.”

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