The ground breaking TRAPPIST-1 system is known to have seven Earth-sized planets that could potentially contain liquid water and thus life. But it is also possible that the Solar System is hiding some much larger celestial objects, namely gas giants orbiting the star at a greater distance than any of the currently known rocky satellites.
“A number of other star systems that include Earth-sized planets and super-Earths are also home to at least one gas giant,” said Alan Boss, first author of a new study published in The Astronomical Journal exploring the likelihood that such gas giants exist in the TRAPPIST-1 system. “So, asking whether these seven planets have gas giant siblings with longer-period orbits is an important question.”
The easiest way to detect whether or not a star is being orbited by planets is to record whether or not the light from the celestial body dips as the satellite passes in front of it. This is how the researchers were first able to determine the existence of the seven Earth-sized planets that we now know orbit TRAPPIST-1.
But if there were gas giants also orbiting the star, their orbits may be either too wide or at such an angle that they do not pass in front of TRAPPIST-1, and so would never be detected through the slight dimming of the light.
Another way is to look at the star itself, and see how much it wobbles on its axis. Large planets such as gas giants have huge masses and thus a big gravitational pull, which can exert itself on the star it is orbiting by making it jiggle slightly. By calculating the upper limit of mass that such a gas giant could achieve in the TRAPPIST-1 system and comparing it with the wobble, the researchers worked out that there are no planets 4.6 times Jupiter’s mass with a one-year orbit, and none larger than 1.6 times Jupiter’s mass with a five-year orbit.
This might seem like an incredibly short period of time for a planet to orbit a star, especially when you consider that it takes Jupiter a full 12 years to go round the Sun, but when looked at in terms of the TRAPPIST-1 system, relatively speaking it is. The seven known planets, which are all closer to their ultra-cool dwarf star than Mercury is to ours, only take between 1.5 and 19 days to complete a single orbit.
While the researchers have managed to rule out the potential existence of some gas giants, it is not beyond the realm of possibility that other do still exist, and if they do, they could shed some light on how our own gassy giants formed in our own backyard.