Jupiter as Janus's head

Ambiguous role of the giant planet for the impact risk of the earth

What is the risk of impact? © ESA
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Jupiter is not only the largest planet of the solar system, it was also always considered a "guard" and shield of the Inner Solar System against asteroids and comets. But a new study of the impact risk of different celestial bodies for the earth has now invalidated this theory and exposed him instead as Januskopf.

As Jonathan Horner from the Open University (OU) in the United Kingdom said last week at the Potsdam European Planetary Science Congress, the Giant Planet has a major impact on a group of potential Earth orbit cruisers: the Centaurs, a group of planetoids orbiting the sun between the orbits of Jupiter and Neptune. They are considered as a reservoir for the comets of the so-called "Jupiterfamilie", which included the comet shoemaker Levy -9, which struck in 1994 on the Jupiter.

Five scenarios examined

"The notion that a Jupiter-like planet plays an important role in mitigating the risk of impact on potentially habitable planets is very common, " explains Horner. "But there has been only one study in the past, and it has looked only at the risks of long-period comets." However, the research of the researcher and his colleagues is much more extensive.

The scientists developed a computer model that analyzes the trajectories of more than 100, 000 centaurs, planetoids, that circle between Jupiter and Neptune in the last ten million years. The simulation was conducted for five different scenarios: Jupiter, without Jupiter, and planets of three-quarters, half, and a quarter of Jupiter's mass. The results showed that the expected impact rate in a Jupiter solar system is absolutely comparable to one without it.

Highest risk for planet sizes smaller than Jupiter

Interestingly, however, planets with masses smaller than Jupiter seemed to have a much more negative impact than the two extremes: in all of these cases, the rate of impact of comets of the Jupiter family on Earth increased. "If a planet of about the mass of Saturn or a little bigger takes the place of Jupiter, that increases the number of impacts on Earth, " Horner explains. "But if there is nothing at this point, there is no difference to the current impact rate. Instead of a clear case of a Jupiter as impact protection, the planet seems to give with one hand to take with the other. "Display

The cause of this behavior is the distraction of comets from their orbit between Jupiter and Neptune. Without a planet in place of Jupiter, they will not be diverted from their orbits, so they will not be able to fly to Earth. However, if a saturn-sized planet sits in this position, its gravity directs the comets towards the earth. Unlike Jupiter, however, the gravity of this smaller planet is not enough to hurl these "deviants" completely out of the solar system. They therefore remain on their Erdbahnkreuzenden trajectory.

Jupiter, on the other hand, directs the comets towards the earth at first, thus increasing the risk of impact, but in doing so it can throw them out altogether, lowering the risk to the earth again. He thus assumes a dual role of danger and protection at the same time. In the next step, astronomers now want to investigate the risk of long-period comets.

(European Planetary Science Congress, 27.08.2007 - NPO)