Free-floating gas giants

MYSTERIES OF THE UNIVERSE

Astronomers are baffled by the presence of planetarymass objects without a host star

Reported by David Crookes

Just when scientists believed they had a handle on how stars and planets form, along come celestial bodies that might turn existing theories on their head. In this case, around 540 planetary-mass objects roughly the size of Jupiter that are freely floating in space. Discovered in the Orion Nebula using the James Webb Space Telescope, astronomers have found they’re not tied to a star. What’s more baffling is that some of them happen to be moving in pairs. In theory, they shouldn’t exist. “There’s a fair degree of scepticism about them because the find was totally unpredicted,” says Mark McCaughrean, senior advisor for science and exploration at the European Space Agency (ESA), who worked with Samuel Pearson, also at the ESA, on the study. But they, and many others, are convinced they’re real.

To better understand the significance of this discovery, it’s important to know the established rules of star formation which take place when giant clouds of dust and molecular gas in a nebula cool, fragment and then slowly collapse under their own gravitational attraction. It’s common knowledge that the universe is filled with stars of different masses. There are relatively few large ones, but there are many more as you go to lower masses, a trend that continues into the regime of substellar objects known as brown dwarfs. The fragmentation process tends to generate more small objects than large ones, and that keeps happening as long as the gas and dust can cool down as it becomes more dense.

Stars in the range of 0.08 to 0.6 times the mass of the Sun are called M dwarfs, and they’re everywhere. Like the Sun, they fuse hydrogen into helium in their cores and reach a steady-state brightness that lasts for billions of years. But below 0.08 solar masses, an object doesn’t have enough pressure in its core to fuse hydrogen – we call these failed stars ‘brown dwarfs’, and they cool down and get fainter. But the starformation process involving fragmentation can make even smaller objects. “The clouds are still breaking into little chunks and they can create bodies with less than eight per cent the mass of the Sun, but the question is, where does it end?” McCaughrean asks. “Does fragmentation have a lowest possible mass? Since the 1970s, basic physics has said it does.” At this point, it’s useful to start talking in different units. “Jupiter has roughly one-thousandth the mass of the Sun,” explains McCaughrean. “So Jupiter is 0.001 of the mass of the Sun, and the Sun has 1,000 times the mass of Jupiter. More precisely, the mass of the Sun is 1047.57 times the mass of Jupiter, but 1,000 is close enough for current purposes.”