Astronomers have been speculating for a long time whether our solar system might have an as yet undiscovered large outer planet. Now, for the first time, astronomers have discovered an exoplanet whose orbit is as unusual as that of this hypothetical “Planet 9”. Because even the exoplanet, named HD 106906 b, orbits on an inclined, eccentric and very far outward orbit, as observation data from the Hubble space telescope show. The existence of this exoplanet thus proves that such distant outer planets exist and provides valuable information on how they can arise.
The impetus for the debate about the hypothetical ninth planet in the solar system began in 2016, when astronomers analyzed the orbits of some trans-Neptunian objects more closely. It turned out that some of these icy celestial bodies on the outer edge of our solar system deviated from the expected orbit in conspicuously the same way. Since then, researchers have speculated whether these anomalies might be caused by the effects of gravity on an as-yet-undiscovered planet. According to models, this “planet 9” is about the size of Neptune and orbits on a strongly eccentric and inclined orbit that extends up to 1500 astronomical units away from the sun. But whether this planet really exists is controversial – also because it is difficult to explain how a planet can get into such an unusual orbit on the basis of current planet formation models. Hypotheses on this range from Jupiter’s disruptive effects to a captured exoplanet.
Exoplanet on an exotic outer orbit
Now astronomers working with Meiji Nguyen from the University of California at Berkeley have discovered an exoplanet that could rekindle speculation about Planet 9. The planet HD 106906 b, 336 light-years away, is orbiting on a similarly unusual orbit. The exoplanet, which weighs around eleven Jupiter, is part of a binary star system that is 15 million years old. According to the first recordings in 2013, however, HD 106906 b is around 730 astronomical units away from its two central stars. Transferred to our solar system, this planet would be 25 times farther away from the sun than Neptune. It would lie far outside the Kuiper Belt, a reservoir of icy comets and minor planets on the outer edge of the solar system. In addition, HD 106906 b is not in the plane of its system, but 21 degrees above it.
However, the first observations of this exotic exoplanet could not yet clarify whether HD 106906 b was still part of its system. Theoretically, it would also be possible that it was thrown out and thus no longer orbits the central stars. To answer this question, Nguyen and her team have now evaluated 14 years of observation data from the Hubble space telescope and data from the Gaia star catalog. These analyzes show that HD 106906 b orbits its stars in a real orbit that is still bound to the stars by gravity. Because of its great distance, however, it takes 15,000 years to complete one revolution. Similar to the forecast for Planet 9, the exoplanet follows a strongly eccentric orbit that is inclined by 36 to 44 degrees to the orbit plane of its system. Even on its closest approach, it never gets closer than about 500 astronomical units, as astronomers have determined.
Parallels to Planet 9
According to the astronomers, HD 106906 b is the first evidence that planets can exist with such unusual and extremely distant orbits. “HD 106906 b is the only known and directly imaged exoplanet that is so displaced and far from its central stars,” says Nguyen. “At the same time, it is the first planet whose orbit corresponds to that of the hypothetical planet 9.” However, this raises the question of how this exoplanet got into this unusual orbit. One possibility would be that HD 106906 b was once created much closer to its stars – probably only about three astronomical units away, as the researchers explain. Then, however, the friction with the dense gas disk caused it to get too close to its central stars and to be ejected by gravity turbulence. An additional influence of gravity, for example through the close passage of another star, could then have prevented the exoplanet from being catapulted completely out of the system.
Evidence of such significant disturbances in the star system of HD 106906 b is shown by the images from the Hubble space telescope. Accordingly, the outer dust disk and the comet belt are deformed in this system in a strikingly asymmetrical manner. How exactly this deformation came about is still open: “It’s a bit like coming to the scene of a car accident and trying to reconstruct what happened,” says Nguyen’s colleague Paul Kalas. “Have passing stars disturbed the planet and this disturbed the disk? Or did the passage of stars cause both effects at the same time? This is really astronomical detective work. “
The exact course of events around the exoplanet HD 106906 b is still unclear. Nevertheless, there are already some parallels to the scenarios through which the hypothetical planet 9 could have entered its exotic outer orbit. Accordingly, it also originated in the inner solar system, but was thrown out of its orbit by interactions with the gas giant Jupiter. Its “push” could have thrown Planet 9 far into the outer regions of our system. There, its orbit was then stabilized by the sword force of a nearby star. It is still unclear whether Planet 9 really exists and whether it was created that way. In any case, the existence of HD 106906 b proves that there are planets with such unusual orbits.
Source: Meiji Nguyen (University of California, Berkeley) et al., Astronomical Journal, 2020; doi: 10.3847 / 1538-3881 / abc012