In 1992, astronomers discovered the first planet outside the solar system. Since then, telescopes have spotted thousands of these so-called exoplanets orbiting not only sun-like stars but also in binary star systems; Cool little stars are called red dwarfs; Even infinitesimally neutron stars. It suffices to make you wonder: Does every star have at least one planet orbiting it?
No. Jonathan Lunin, chair of the astronomy department at Cornell University, said in one word: No. This, at least not as far as we know.
“It’s always a question of whether or not you can detect something,” Lunin told Live Science. “One does not know for sure. But it is certain that there are plenty of stars where planets have been searched for, and none have yet been found.”
Related: Why are galaxies different shapes?
Scientists estimate that the number of planets in our galaxy equals the number of stars, Lunin says, but these planets are not evenly distributed. Some stars – like the sun, as well as TRAPPIST-1, a red dwarf star about 40 light-years away – is home to more than six planets, while others may not have any of them.
But what makes one star host so many planets while others fly alone? Scientists believe it stems from the way the star formed. When young stars form, they are usually surrounded by a ring of dust particles. These particles collide with each other to form larger and larger clumps, which can eventually form planets. But not all young stars are so lucky.
“If you have a star that consists of a clump of interstellar cloud that spins very fast, because that clump contracts rather than spins outward to form a disk, it could split into two or even more pieces and form a binary star system or a multi-star system,” said Lunin. “And in those cases, if the disk did not form, the two- or three-star system probably wouldn’t end up as a planet.”
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 right.
“There are binary star systems where there are planets,” Lunin said. “So do these systems end up splitting matter into two blocks and then forming a disk around one of those two blocks, or maybe both of those blocks? Could something have been captured?”
In rare cases, a young star’s dust-filled mass may rotate so slowly that it simply collapses into a star without forming a disk at all, Lunin said. It’s also possible that a star forms planets only for the intense gravity of another star to fling them out of the solar system, or at least send them too far away to be detected. This may be what happened to Planet HD 106906 B, which orbits around a binary star system in an orbit 18 times farther from its star than Pluto is from the sun.
But Lunin cautioned that our knowledge of how many stars host planets is subject to what we can discover. This is because many planets are detected using the transit method, which uses dips in a star’s brightness as evidence that a planet is passing in front of it.
“We can always look at a particular star and say, ‘Okay, no planet has been detected around it, but you know, maybe there’s a planet that’s kind of small and it’s orbiting really far from the star and it’s not crossing the star and so it’s kind of stealthy. This is always a possibility. But most likely, there are stars that do not have planets around them. ”
Originally published on Live Science.