Astronomers have filled in a missing piece of the picture of TRAPPIST-1, the planetary system discovered 40 lightyears away in the constellation of Aquarius. The discovery of exoplanets around star TRAPPIST-1 was announced in February this year by NASA.

The star has seven exoplanets orbiting it, designated b, c, d, e, f, g and h.


While the three innermost planets - b, c, and d - are Earth-sized and orbit their star in the so-called ‘habitable zone’, the outer planets are said to be too cold to host life as we know it.

An animation simulating the orbits of the planets around TRAPPIST-1 for 90 Earth days. Image Credit: NASA/JPL-Caltech
Not much was known about planet h, the outermost, until data captured by the Kepler spacecraft revealed that it orbits its star once every 19 Earth days at a distance of ten million kilometres.

The amount of energy it receives from TRAPPIST-1 is about the same as dwarf planet Ceres in our Solar System receives from the Sun.

The team had already predicted the orbital period of TRAPPIST-1h by studying the orbits of the six innermost planets.

Using data from the Spitzer Space Telescope, they noticed a mathematical pattern in the frequency at which the six planets orbit the star and the gravitational tug on the star caused by the orbits.

They used this information to infer the orbital properties of planet h.

"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," says Rodrigo Luger, lead author of the study.


"Nature usually surprises us at every turn, but, in this case, theory and observation matched perfectly."


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.