The seven exoplanets of TRAPPIST-1 could be alone around their star, according to observations carried out with NASA’s Spitzer Space Telescope.
A team of astronomers looked for a wobble in the star that would suggest it was being tugged by a Jupiter-like gas giant in orbit farther out from the star, but have so far found none.
TRAPPIST-1 is an ultra-cool star in the constellation of Aquarius.
Earlier this year it was announced that it is home to seven rocky Earth-sized planets, three of which lie in the star’s habitable zone where liquid water is most likely to be found.
Since its discovery, astronomers have tried to uncover every aspect of the system, including whether any gas giants also orbit the star.
“A number of other star systems that include Earth-sized planets and super-Earths are also home to at least one gas giant,” says Alan Boss from the Carnegie Institution for Science, lead author on the study.
“Asking whether these seven planets have gas giant siblings with longer-period orbits is an important question.”
An illustration showing the seven exoplanets in orbit around red dwarf star TRAPPIST-1. The outer three exoplanets might host liquid water on their surface.
The team have so far excluded the presence of a giant planet greater than 4.6 Jupiter masses and with an orbital period of a year, and a planet larger than 1.6 Jupiter masses with an orbit time of five years.
For comparison, the known rocky planets have orbital times ranging from 1.5 to 20 days.
“There is a lot of space for further investigation between the longer-period orbits we studied here and the very short orbits of the seven known TRAPPIST-1 planets,” adds Boss.
The discovery, or absence, of gas giants around TRAPPIST-1 could help to settle some long-standing arguments about the way gas giants form.
Currently there are two competing theories.
The first is that gas giants accrete a solid core in the same way as rocky planets form. When large enough, this core attracts an envelope of gas to it, turning it into a gas giant.
The competing theory is that gas giants were created from the dust and gas of the early stellar disc collapsing down to form the infant planets; more like how a star forms.
However, in the first model it’s difficult to explain the presence of gas giants around low mass stars, such as TRAPPIST-1, whereas if the second method is true, gas giants form easily.
If there are no gas giants around TRAPPIST-1, it could suggest that gas giants form according to the first method, and are born from the core out.