Institute of Astronomy

Earth-like moons

Published on 18/02/2013 
Question: 

I wonder what would happen if neptune somehow ended up in earth's orbit and turned the earth into one of its moons? what would happen to the earth assuming neptune kept its new orbit in the habital zone of the sun and what would happen to neptune and its moons. color changes and melting for ex. and would neptune stay blue? thanks

Assuming that we could arrange a bit of magic such that one day we woke up in orbit around Neptune with Neptune occupying Earth's current orbit there would initially not be much change, aside from the rather obvious one of a large deep blue orb taking up a substantial fraction of the sky.  We'll also assume that we arranged our magic transition such that Earth is not too close to Neptune's other moons, so that there are no immediate major impacts, let's put ourselves at the nice healthy distance of a million km from Neptune, giving us an orbital period of about 30 days.  Triton is the only moon we really need to worry about in that sense as it is far larger than all of the rest of Neptune's moons combined (99.5% of all the mass in Neptune's moons is in Triton).  Some of the others are large enough that we wouldn't want to hit them from the point of view of human civilisation, but it would do any long term damage to Earth itself if we did.

The first thing we would notice after the sudden appearance of Neptune in the sky would be the dramatic increase in the height of the tides.  Although our nominal orbit is around 3 times further from Neptune than the Moon presently is from us, Neptune is much more massive, and so the tidal field strength would be around 50 times higher.  This wouldn't directly translate into a tidal range that is 50 times larger, since the much stronger tidal forces would be more effective at deforming Earth's crust as well as the oceans, so the sea floor itself would rise and fall along with the oceans.  It is difficult to say exactly how much larger the tidal range of the water would be than at present, but the mid-ocean tidal range would almost certainly be in the range of a few metres rather than half a metre as it is now.  This would mean that large swathes of low-lying coast around the world, including many of the worlds major cities, would become tidal plains flooded twice a day.  The much greater tidal flexing of the crust would also lead to a significant, and permanent, increase in earthquakes and volcanic activity.  In the long term the extra energy pumped into the Earth through tidal heating would also become a contributor to global warming, though given the other problems I don't think we would notice.  Life on Earth would take a while to get used to these changes, but it certainly would in time, the new tidal plains would become major new habitats, and although modern civilisation would take some heavy knocks I expect that humans would get used to it too.  We could potentially decrease the effect by placing ourselves in a wider orbit, but the tidal effects are always going to be rather larger than at present.

Assuming we manage to pull ourselves away from our new problems long enough to take a look around the next thing that we would notice would be the lack of our old Moon, which, since Neptune's gravity is much stronger, would become another separate moon of Neptune.

Another thing we would notice would be the monthly total eclipses that would occur every time we passed behind Neptune.  We are used to the usual solar eclipses produced by our Moon, which are only fleeting and cover only a small fraction of Earth's surface, these new eclipses however would last for hours and cover the whole world.

On Neptune it would be the dramatic increase in temperature that would be noticed, at present Neptune has a surface temperature of around -200C and emits two and a half times as much energy from internal sources as it receives from the Sun.  The huge change in the energy balance would certainly affect the Neptunian weather, which would probably become more violent.  It would also almost certainly affect the chemical balance in the atmosphere, though exactly how the colouration would change is uncertain, the methane that gives it its present blue colouration would still be present, but other compounds would likely change.

The changes for Neptune's moons would be dramatic, most of them have large amounts of water and other frozen volatile compounds and elements like ammonia and nitrogen, which would melt, but because they are rather small they would not be able to retain the liquids and gases, and probably would mostly disintegrate due to the outgasing.  Triton would be a different case, almost as large as our moon it may not have hugely strong gravity, but it is enough that it would not immediately loose it's new atmosphere to space.  The new atmosphere would probably be quite thick, composed of nitrogen, carbon dioxide and probably ammonia, and would overly a massive, deep global ocean, since water makes up about 30-40% of Triton's mass, much more than Earth.  In fact if the atmosphere is not too thick, and we could get rid of the carbon dioxide (the ammonia would be destroyed by the Sun fairly quickly) it could become quite a nice second home for us.

Page last updated: 18 February 2013 at 13:06