Does each star have planets that orbit it?

In 1992, astronomers discovered the first planet outside the solar system. Since then, telescopes have observed thousands of exoplanets, orbiting not only sun-like stars, but also binary star systems; small, cold stars called red dwarfs; and even ultradense neutron stars. Is it enough to make you wonder: does every star there have at least one planet orbiting it?

In a word, no, said Jonathan Lunine, president of the Department of Astronomy at Cornell University. I mean, at least not as far as we know.

Researchers estimate that there are as many planets as there are stars in our galaxy, Lunine said, but those planets are not evenly distributed. Some stars – such as the sun and TRAPPIST-1, a red dwarf star about 40 light-years away – are home to more than half a dozen planets, while others have none.

But what makes one star host so many planets while others fly alone? Scientists believe that everything starts from the way the star was formed. When young stars form, they are usually surrounded by a ring of dust particles. These particles collide with each other to form ever larger agglomerations, which can eventually form planets. But not all young stars are so lucky.

Binary star systems can form planets in some cases – as in the case of Kepler-47 and its three planets – but the conditions must be correct.

Quite rarely, the dusty agglomeration of a young star could spin so slowly that it simply collapses into a star without ever forming a disk, Lunine said. It is also possible for a star to form planets just for the intense gravity of another star to throw them out of the solar system or at least send them too far to be detected. It may have been what happened to the planet HD 106906 b, which orbits a binary star system in an orbit 18 times farther away from its star than Pluto of the Sun.

But Lunine warned that our knowledge of how many stars host planets is subject to what we can detect. This is because many planets are detected using the transit method, which uses decreases in the brightness of a star as a sign that a planet is passing in front of it.