# Which impacts are to be expected, if an Earth-like planet orbits a Jupiter-like one?

In a fictional setting, an Earth-like planet is orbiting another, Jupiter-like planet. For explanation see my previous question Which astronomical or cosmological event would explain periodical low-grav-effect on earth-like planet? and Will's answer.

My planet doesn't have its own orbit around the sun, but instead is constantly meandering inside the habitable zone, which is the consequence of orbiting around another, more massive planet. The binary (Jupiter-like - Earth-like) system´s distance should be 1 AU from the sun. And furthermore we have to take into account that a periodical low-gravity effect occurs every 15 months, when the Earth-like planet approaches the Jupiter-like.

In our system, Jupiter has a big effect on the way comets and meteors move through the solar system. What impacts are to be expected on the Earth-like planet? Is there any chance that the intensity of impacts prevents intelligent life from developing on this planet?

• Could you please be more precise about their orbits you have planned? Nov 22 '17 at 11:52
• @Molot: No I can´t. This would be pure guessing in my case. I lean on Wills answer to my previous question, although he just focused on the gravitational solution, as asked. Nov 22 '17 at 12:59
• I narrowed the question significantly, to only focus on the impacts themselves. A separate, later question can talk about the effects on atmospheric composition. Nov 22 '17 at 13:55
• Isn't a 1 AE radius orbit a bit too little for a Jupiter-sized planet? I almost feel like a binary white dwarf or dwarf-red giant system is more feasible. Nov 22 '17 at 14:29
• I just want to mention in the situation you proposed, where the planets orbit like that, the Earth like planet will likely be extremely volcanic. If I have to guess, it's surface would probably be molten. The tidal energy and stress the planet endures is pretty much unimaginable, I wouldn't be surprised if the planet breaks apart from the stress alone. Nov 22 '17 at 18:55

# A moon's impact history will be different from a planet's

First off, let us clarify that the Earth-like planet in your answer is indeed a moon. The mathematics of calculating impact frequency curves for a moon are complex. They can be found Le Feuvre and Wieczorek, 2011, Appendix A. I'm not particularly interested in going through all these calculations for this problem, but fortunately that paper has some simplified scenarios we can use.

The most applicable is the ratio between impactors on the Moon and Earth. These two bodies are in the same AU-zone as your proposed planetary pair, so the result may be at least superficially similar. The linked paper has an impact frequency distribution for Earth in Fig 1 and the Moon in Fig 2. A graph of the ratio (Earth/Moon) is in Fig 3; note this ratio is of crater diameter, not impactor radius like the first two figures. In short, craters < 200 km are less likely on the Moon, while craters > 200 km are more likely.

# How would that affect life?

The Chicxulub crater is just about 200 km in diameter, so so this is a good reference size for extinction level events. Wikipedia has a list of confirmed craters on Earth; note that there are only two others in the 200 km+ range, and both of them preceded multicellular life. There are several unconfirmed craters, the most likely culprits being clustered around 250 MYa in age, corresponding with the Permian extinction.

We can say with relative certainty, that there were no more than two impact craters > 200 km formed on Earth in the last 250 My; a rate of about 125 My$^{-1}$. A large impactor during the development of mammals could have caused a mass extinction that prevented the development of intelligence, at least for hundreds of millions of years. Given that mammals took 65 million years to develop in humanity, the chances of an asteroid interrupting that development on Earth can be calculated using the Poisson distribution.

There is about a 60% chance that there will be no such impact in 65 million years given a rate of 125 My$^{-1}$. If, according the Fig 3, we double the rate of catastrophic impacts to 62.5 My$^{-1}$, then there is about a 35% chance that there will be no catastrophic impacts in a 65 My period. That is to say, impacts are such low probability events that survival of your planet is mostly due to chance. While the moon of a planet has a lower chance of making it than a planet, there is still a chance.

# Conclusion

The change in impact rate would raise the probability of a catastrophic impact, but it would not raise it so much that such an impact would be a near-certainty. You could reasonably develop intelligent life on a planet such as you describe.