Physics World: Formation of Isolated Planets

Planets typically form in swirling disks of gas and dust around young stars. But that’s not always the case, as researchers using the James Webb Space Telescope have now discovered. In the young star cluster NGC 1333 in the constellation Perseus, they’ve discovered six previously unknown objects with masses five to 10 times that of Jupiter. They appear to have formed independently of the star, the scientists report in The Astronomical Journal.

It has been known for a quarter of a century that not all planets orbit stars. So loners also travel far from their stars. But until now, most astronomers assumed that these planets once orbited stars. It was assumed that they were only later ejected from their home system by other nearby celestial bodies.

Previously Unknown Formation Path of Large Planets

During observations with the James Webb Space Telescope, Ray Jayawardhana of Johns Hopkins University in the United States and his team discovered several planets that do not orbit a star. The researchers noticed them when they were looking for the lightest objects that form like stars in the star-forming region NGC 1333, which is about a thousand light-years away. Among the celestial bodies discovered were dwarf stars and six particularly light objects: planets with a mass between five and ten times that of Jupiter, the largest planet in our solar system.

The results show that planets can form in two different ways, Jayawardhana says: “On the one hand, like Jupiter in a disk of gas and dust around a young star, and on the other, like a star through the independent collapse of a young star.” “A gas cloud,” he says. However, the team couldn’t weigh the planets they found, even if the space telescope was sensitive enough to do so. The researchers concluded that objects like stars weighing at least five times as much as the gas giant Jupiter could form.

Overall, about one in ten objects in NGC 1333 is not a star at all, but a large planet, the study concludes. The lightest of these was particularly surprising: It itself is surrounded by a disk of gas and dust. Apparently, “their own small planetary systems can form around these small objects with large gas planet masses,” says Alexander Scholz of the University of St Andrews in Great Britain, who also participated in the study.