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Found a new gaseous planet with a record orbit, produced by a very powerful gravitational drive.

WHAT WOULD HAPPEN if Jupiter suddenly changed its orbit and started darting among the other planets of the Solar System? We would with risk a potentially disastrous planetary carom for our planet. This scenario, highly unlikely for the Solar System, resembles what happens around the HR 5183 star about 100 light-years from us.

The orbits of the outer planets of the Solar System, from Jupiter (red) to Neptune (blue) compared with those of HR5183 (Astronomical Journal)
The orbits of the outer planets of the Solar System, from Jupiter (red) to Neptune (blue) compared with those of HR5183 (Astronomical Journal)

An international team of astronomers has in fact discovered a giant planet with a strongly elongated orbit, which leads it to move very far away from its star, at a distance greater than that between Neptune and the Sun. In the point of maximum approach to the star the planet is then “launched” again outward from the slingshot effect, often exploited even in space missions. As explained in The Astronomical Journal, such an elliptical orbit is very different from those of the known planets and therefore represents an unprecedented result that will help us better understand the formation and evolution of extrasolar planets.




A slow stellar dance
“The planet is different from the planets in our Solar System,” said Sarah Blunt, a Ph.D. student at the California Institute of Technology and the article’s first signature, “and it does not resemble any exoplanet discovered so far. The other planets revealed at great distances from their stars they tend to have very low eccentricities, which means that their orbits are more circular “.

The Keck observatory on the top of the volcano Mauna Kea, used to study the planet HR 5183b (Caltech)
The Keck observatory on the top of the volcano Mauna Kea used to study the planet HR 5183b (Caltech)

The new planet, called HR 5183b, has a mass equal to three times that of Jupiter and was discovered with the radial velocity method. Blunt and colleagues have therefore monitored the motion of the main star, highlighting small variations in speed due to the gravitational interaction with the planet. By measuring these small deviations it is possible to highlight the presence of a planet and determine its characteristics and orbit. Blunt is part of the California Planet Search, a project led by Andrew Howard of Caltech that deals precisely with monitoring the motion of the stars on a scale of tens of years to reveal the presence of these planets with large orbits. For the planets that orbit away from the star, these observations require a lot of time, because to make an orbit they can take tens or hundreds of years. To study HR 5183, data from the Lick Observatory in California, the Keck Observatory in Hawaii and the McDonald Observatory in Texas were used.

Slingshot effect
Analyzing the data, astronomers have shown that the orbit of the planet has a period between 45 and 100 years. If it were in our Solar System, HR 5183b would in fact orbit between a minimum distance similar to the one that separates the Sun from the band of asteroids, while at the farthest point it would be beyond the orbit of Neptune.




And it is here, at a great distance from its star, that the planet spends most of its time. But once the maximum distance is reached, the planet returns to accelerate towards the star until it reaches the minimum distance, to which it receives a gravitational thrust that slides it back away. By virtue of this slingshot effect, the planet deflects its orbit thanks to the gravitational field and gains speed by “stealing” it from the star. This is a well-known mechanism in physics, and aerospace engineers use it a lot in space missions, in order to accelerate the probes without having to carry large amounts of fuel onboard. And it is precisely in this phase of gravitational slings that astronomers have revealed the planet. “We have seen the planet approach the star and is now moving away,” Blunt continues, “This creates such a distinctive signature that we are sure that it is a real planet, even if we have not seen a complete orbit.”

A lost companion?
Usually, the planets form inside discs of dust leftover from star formation, so their orbits should be almost circular. If this planet has such a bizarre orbit, in the past it must have received another gravitational pull from another body, perhaps another planet.

This is the most plausible explanation for researchers, according to which another equally massive gas giant orbited around the star. Following a close encounter, HR 5183b would have pushed the other out of the star system. The Jupiter giant would have remained around the star, but its orbit would have brought the signs of that distant planetary accident.

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