Astronomers may have solved a problem that has existed for decades: distinguishing between particularly massive gas planets and brown dwarfs. Both substellar bodies have similar temperatures, gas envelopes and brightness, blurring the line between planet and “failed star”. But now researchers have found a feature that could help better define the boundary. Accordingly, brown dwarfs rotate significantly slower than their planetary counterparts, and their speed is further away from the maximum possible. The reason for this lies in the different formation conditions of both types of celestial bodies and in the stronger magnetic field of the failed stars.
Brown dwarfs are cross-border among the celestial bodies. Because they are too big and warm to be planets, but too low in mass to become real stars through hydrogen fusion. As a result, they often only glow faintly in the infrared and are similar in temperature and brightness to the largest representatives of the extrasolar gas giants. This makes distinguishing them difficult, sometimes impossible. According to the common definition, the boundary between the planet and the brown dwarf is around 13 Jupiter masses – the mass range from which deuterium fusion is possible inside. But astronomers have already discovered several celestial bodies that lie exactly on the border between the two. It was not possible to clearly determine whether they were a large gas planet or a brown dwarf.
Rotation in sight
Now astronomers led by Chih-Chun Hsu from Northwestern University may have found a feature that more clearly separates gas planets and brown dwarfs. For their study, they examined six extrasolar gas giants and 25 brown dwarfs in binary systems – this means that all test objects move in orbits around other celestial bodies and are easier to compare. The researchers targeted these 32 objects using the high-resolution KPIC spectroscope at the WM Keck Observatory in Hawaii. In the data, they analyzed, among other things, the width of the spectral lines that the gas envelope of the celestial bodies creates. This allows conclusions to be drawn about the speed of rotation of the objects: the faster a celestial body moves, the more its spectral lines broaden. “KPIC is the first instrument that allows us to measure features such as rotation that were previously difficult to detect,” explains Hsu.
When evaluating the spectral data, the astronomers found what they were looking for: “We have found the first clear evidence that gas giants differ from low-mass brown dwarfs in their rotation,” writes the team. Accordingly, planets rotate significantly faster than brown dwarfs of the same age. Their rotation speed reaches a higher percentage of the maximum possible for a given size, as Hsu and his colleagues report. According to this, gas giants that are ten million years old rotate on average at 27 percent of their so-called breakup speed, while brown dwarfs of the same age with comparable orbit inclinations only rotate at around 9 percent. Assuming random, non-equal orbit parameters, the values are 25 and 13 percent. These differences were also evident in 43 other brown dwarfs and 54 extrasolar gas giants whose rotation had been measured in previous studies.
(Video: WM Keck Observatory)
Relic of educational conditions
Astronomers attribute these differences in rotation speed to the different formation paths of planetary gas giants and brown dwarfs. “Rotation is a fossilized relic of the mode of education,” says Hsu. Gas planets form in the protoplanetary disk of their star and their angular momentum arises from interactions with this disk of matter. “The rotation rate of planets that is closer to the breakup speed may be due to the fact that they lose less angular momentum due to the braking effect of the disk during their formation phase,” the astronomers write. Like stars, brown dwarfs can be formed by the collapse of a gas cloud, but also by the local instability of a disk of material around a larger star. Because brown dwarfs, as failed stars, have a stronger magnetic field, they are exposed to a stronger braking effect, as the team explains.
In addition, other influencing factors come into play: “Our results indicate that both the mass of the planet and the mass ratio between it and its star influence how quickly it ultimately rotates,” says Hsu. Age also plays a role because both planets and brown dwarfs can speed up with age. “But we have only just begun to explore what the rotation of a planet can tell us,” the astronomer continued. “With future instruments and larger telescopes, we will be able to measure the rotation of even more worlds.” Next, the team plans to also research the rotation of unbound celestial bodies in the planetary boundary region – planets and brown dwarfs that fly alone through space.
Source: Chih-Chun Hsu (Northwestern University) et al., The Astronomical Journal, doi: 10.3847/1538-3881/ae434b