spaceSpace and Physics

You Can Fit Seven Habitable Planets In A Star System If You’re Lucky


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Trappest versus sol

Artists' impression of the TRAPPIST-1 star system, an ultracool red dwarf with seven Earth-like planets, just 40 light-years away. ESO/N. Bartmann/

We know the Solar System definitely has one world capable of supporting life, and have no problem imagining another if things had gone a bit better for Mars. According to a new study, however, the limit is a good deal higher, with the potential to fit seven planets teeming with life around one star. The team behind the work are blaming Jupiter for the fact there aren't more life-bearing planets in our own Solar System.

Much of the excitement in discovering seven rocky planets around the red dwarf TRAPPIST-1 came from the fact that three of these have locations suggesting they should be able to host liquid water, thought to be the key determinant for life.


"This made me wonder about the maximum number of habitable planets it's possible for a star to have, and why our star only has one," said Dr Stephen Kane of the University of California, Riverside, in a statement. "It didn't seem fair!"

We know from both modeling and the planets already located around other stars that you can’t pack planets too close together, otherwise their gravity will disrupt each other’s orbits. Like the cats of Killkenny, instead of two planets at good locations, you’ll end up with none.

Still, the Earth would not be troubled if Venus was a little further from the Sun, and Mars slightly closer in,so Kane set about modeling how close planets can get while maintaining suitable social distancing. In an ideal star system, he concludes in the Astronomical Journal, the answer is seven, a number science fiction writers might find appealing.

Only three of the TRAPPIST-1 planets are in its habitable zone (green) despite being as tightly packed as possible, but that is because it is a low mass star with a small zone. Stars wth the mass of the Sun have a zone large enough to fit more planets, but a giant like Jupiter interferes, even from outside the zone. NASA/JPL-Caltech

To have a habitable zone large enough to fit seven planets safely a star would need to be 10-20 percent more massive than the Sun. Our own beloved star can fit six non-interfering planets in its zone. So why don't we have more habitable planets? 


Most star systems may not hit the maximum, but it appears it's common for systems to get close to packing as many planets into the space available as possible. Kane thinks we could have had four or five, were it not for Jupiter lurking further out. "It has a big effect on the habitability of our Solar System because it's massive and disturbs other orbits," Kane said.

Astronomers have been going back and forth for decades on whether a planet with a mass like Jupiter’s makes it more or less likely for life to occur closer in, for example by influencing the number of water-bearing comets that hit inner planets.

Kane’s work can’t answer all those questions, but he hopes to show that without a Jupiter-like world you at least get more planets suitably located for life. To test this theory, he is searching for nearby stars that don’t seem to have any really large planets. One such example, the very Sun-like Beta Canum Venaticorum, has been found not to have any planets larger than Saturn close enough to be disruptive, and Kane thinks it should be prioritized for seeking habitable-zone planets.


spaceSpace and Physics