In 2016, two astronomers from the California Institute of Technology, Konstantin Batygin and Michael E. Brown, indeed provide proof that another giant planet exists in our solar system. In an article published in The Astronomical Journal, they explain that the orbits of some objects distant from the Kuiper belt tend to cluster together in terms of both perihelion argument and physical space. However, according to them, this regrouping cannot be due to chance and necessarily finds a dynamic origin: the gravitational attraction of an enormous invisible planet.
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Atypical planetary orbits
According to Brown, this planet could be a gas giant such as Jupiter and Saturn; its mass could be equivalent to ten times that of the Earth. Following the publication of Batygin and Brown, other scientists engaged in modeling the thermodynamic evolution of this planet, assuming that it was smaller than Uranus and Neptune. Their results suggest that planet 9 would have a surface temperature of 47 K (-226 ° C) and a radius equivalent to 3.7 Earth’s rays; it would be composed of a ferrous core, a mantle of ice, then an upper mantle and a gaseous atmosphere (composed of hydrogen and helium).
It would be particularly bright in the infrared. Thus, specific instruments such as the Subaru telescope, located in Hawaii, or the Hubble Space Telescope – and its successor, the James-Webb telescope which will be commissioned in the fall of 2021 – could potentially succeed in spotting it.
Thanks to Hubble, a team from the University of California has just made an important discovery: an exoplanet displaying an improbable orbit around a double star, at a very great distance from it (more than 730 times the distance from Earth). Sun !). This is the first time that astronomers have been able to measure the motion of a massive planet so far from its star and even beyond the system’s debris disk. ” It is very largely separated from its host stars in an eccentric and strongly misaligned orbit. », Says Meiji Nguyen, lead author of the study.
This image from the Hubble telescope shows the environment around the double star HD 106906. Its light is here masked for better visibility. The circumstellar disk is asymmetric and distorted, possibly due to the gravitational pull of the planet HD 106906 b, which is in a very wide and elongated orbit. Credits: NASA / ESA / M. Nguyen / R. De Rosa / P. Kalas
However, the specialist stresses that the situation is ultimately very similar to the presumed atypical orbit of planet nine in our solar system, lying well beyond the Kuiper belt. This discovery thus confirms that unconventional orbits are possible. ” This raises the question of how these planets formed and evolved into their current configuration. », Continues Nugyen. The system where this exoplanet was observed is only 15 million years old. Thus, planet nine could have formed very early in the history of our solar system, which is 4.6 billion years old.
A disc of debris marked by the passage of a planet
The exoplanet in question, called HD 106906 b, was discovered in 2013 via the Magellan telescopes at the Las Campanas observatory, in Chile. But at the time, astronomers had no data regarding the orbit of this object. By collecting very precise measurements over a period of 14 years, the Hubble telescope has shed light on this unknown. That said, the fact that the exoplanet is so far from its double star did not make the task any easier! It is therefore subjected to a very weak gravitational attraction and gravitates very slowly around the star (its orbit is estimated at 15,000 years).
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Possible orbit (dotted ellipse) of the exoplanet HD 106906 b. The planet would reside outside of its system’s circumstellar debris disk, which resembles our own Kuiper belt. Credits: NASA / ESA / M. Nguyen / R. De Rosa / P. Kalas
How did this exoplanet end up in such a strange and distant orbit? Experts believe it originally formed much closer to the double star. But the star system’s gas disk would have caused its orbit to disintegrate, prompting it to move closer to the star pair. The gravitational effects of these swirling stars would then have thrown it into an eccentric orbit, almost outside the system. Then, a star from another system would have passed nearby, stabilizing the orbit of the exoplanet which thus remained in its original system; in 2019, astronomers confirm this theory of the passing star, identifying a few possible candidates.
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As early as 2015, astronomer Paul Kalas and his team provided evidence of the original orbit of this planet. It turns out that the debris disk of this star system is very asymmetric; it even appears truncated on one side and undergoes vertical disturbances. Thus, scientists believe that each time the planet HD 106906 b is closest to the binary star, it “stirs” the materials constituting the disk.
A model that can support the existence of planet nine?
The scenario put forward to explain the atypical orbit of exoplanet HD 106906 b could also explain how the hypothetical planet nine found itself at the edge of our solar system, well beyond the Kuiper belt. ” It’s like we have a time machine […] to see what could have happened when our young solar system was dynamically active and everything was shaken up, then rearranged Said Kalas.
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Assumed orbit of planet nine. Credits: Wikimedia Commons / CC0
Indeed, planet nine could have formed in the inner solar system, and then be expelled by gravitational interactions with Jupiter. And just like in the case of HD 106906 b, this gas giant could have shipped it even far beyond Pluto! Thus, a passing star could have stabilized its orbit and kept it in our system.
>> To read also: What if the ninth planet was a black hole?
To date, scientists have only circumstantial evidence of its existence: unusual orbits spotted on a set of small transneptunian celestial bodies. Combined gravitational influence of several objects? Statistical anomaly? Planet nine? All possibilities are considered. The upcoming commissioning of the James Webb telescope will undoubtedly shed light on this mystery: these capabilities will open up new possibilities for detecting and studying these unconventional planets and systems.
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