Is Jupiter Earth's protector?

Question posed by Robin

Yes and no.

Popular wisdom paints a picture of Jupiter as a gigantic vacuum cleaner, whizzing around the solar system and sucking up dangerous lumps of rock and ice before they can do us any harm. To an extent this is true: a study in 1994 showed (through computer modelling) that if Jupiter had formed to be closer to the size of Neptune or Uranus, then Earth could have expected around a thousand times as many impacts with long-period comets than we do with a Jupiter of the size that we're accustomed to.

Jupiter can clear away space debris either by impacting with it or drawing it into a wider orbit, or by slinging it out of the solar system altogether.


A more recent study, completed in 2007, went a step further than the one mentioned above, and removed Jupiter from the solar system altogether. The results showed that Earth would experience fewer collisions with no planet at all in Jupiter's orbit. Both studies agreed that having a smaller planet in place of Jupiter would leave the Earth worse off*.

Jupiter's massive gravitational field can also disturb comets and other objects further out of the solar system, possibly causing them to fall towards the inner solar system and crossing the Earth's orbit. Both of the studies mentioned so far only looked at comets. Less than 5% of historical Earth impacts have been due to comets; the rest have mostly been collisions with asteroids. Our solar system's asteroid belt lies relatively close to Jupiter and is affected by its gravity, which sometimes causes asteroids to collide and be knocked into more eccentric orbits that cross Earth's path. General feeling at the moment seems to be that if Jupiter simply wasn't there, the asteroids would be more stable, reducing the chance of impacts from asteroids.

Further reading

* But they disagreed on how worse off.


  1. Interesting.

    I suppose my question came from the thought of what conditions are needed for a planet like Earth to form. i.e. Is it the case that not only does the Earth need to be exactly right for life to form, but also the whole solar system around it.

  2. Definitely: a planet could be right in the middle of the goldilocks zone and just the right size, with all the right chemicals floating around in the atmosphere, but if it's being pelted with cosmic rubble then not much is going to have a chance of bursting into life.


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