Is Jupiter getting bigger? © Getty Images

Is Jupiter getting bigger?

With constant collisions from asteroids, comets and other space junk, you would think so…

Asked by: Louis Goodwin, Lewes

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Jupiter’s large gravitational field makes it prone to impacts from asteroids, comets and other Solar System flotsam. This was spectacularly demonstrated in 1994 when the comet Shoemaker-Levy 9 plunged into the atmosphere of Jupiter, adding at least one trillion kilograms to the planet’s mass in a single event. It is estimated that Jupiter’s rate of mass increase from impacts or accretion is up to 8,000 times that of the Earth’s. Events like Shoemaker-Levy 9 are rare but even adding up the smaller bombardments of Jupiter that continuously rain down on the planet, the increase in mass is still a tiny fraction of Jupiter’s overall total.

However, this does not mean that Jupiter’s mass is increasing. Jupiter’s atmosphere is warm; so warm that gas molecules are moving fast enough to escape the gravitational pull of the planet. Furthermore, the solar wind actually ionises many of the atoms in the Jovian atmosphere. Since these atoms become neutrally charged, they can easily escape Jupiter’s magnetic field and float off into space. This slow but constant loss of mass from Jupiter’s atmosphere is actually greater than the gain in mass from collisions so, overall, Jupiter is shrinking not growing in mass.


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