Institute of Astronomy

 

Ask an Astronomer - Planets

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.

Rogue Planets

Published on 22/01/2013 
Question: 

I understand that astronomers believe so-called rogue planets were likely ejected from their solar systems early in their planetary histories, but it's never clear what event(s) could trigger such a thing. My question: What kind of a catastrophic event in our solar system could cause the Earth to become a rogue planet today?  It's ok to be speculative. I'd love to know. Thank you.

Planetary systems become unstable when the orbits of two planets cross.  By this I don't mean that the planets collide, rather, that their crossing orbits cause them to have a close gravitational encounter.  The close gravitational encounter can transfer a tremendous amount of orbital energy from one planet to the other, potentially shooting it out of the planetary system.
 
Surprisingly, our own system is barely stable!  A close encounter between the asteroids Vesta and Ceres in about 60 million years ago makes it very difficult to trace Solar system dynamics before the encounter (http://www.sciencedaily.com/releases/2011/07/110715135156.htm) and also limits our ability to forward-predict Solar System dynamics on timescales longer than about 10 million years.  For more information about the stability of the Solar system, see this Wikipedia article and its sources: http://en.wikipedia.org/wiki/Stability_of_the_Solar_System
 
In summary, to make the Earth a "rogue planet," the Solar system would have to evolve such that another large body (a large asteroid, or Venus or Mars, or eventually Jupiter) crossed orbits with the Earth.  The asteroids and Mars probably don't have enough energy to eject Earth.  A more massive planet like Jupiter would be more effective at ejecting Earth, but Jupiter is quite far away and is less likely to cross orbits with Earth.  But the Solar system is definitely stable for the durations of our lives, and this kind of ejection couldn't happen for at least a few tens of millions of years, if not billions.

Moonrise times

Published on 22/01/2013 
Question: 

Hello Im a photographer trying to capture a photo of the moon rising above the eastern coast of the atlantic ocean, particularly in central florida. I know there is a full moon next week Jan25-26, what is the best time frame to capture it at horizon?

There are various tools around on the internet that allow one to calculate times of Moonrise and Moonset and Lunar phases, www.timeanddate.com has some quite good ones.  Unfortunately in Florida Moonrise at Full Moon occurs quite close to Sunset, so the photographic conditions might not be ideal depending on the effect you want to capture, you might be better off waiting until a few days after full when the Moon is rising later in the evening, but only you can judge that.

The threat of asteroid impacts to Earth

Published on 22/01/2013 
Question: 

I am writing a blog post on the threat of asteroids on earth. I have read a number of Science and Nature articles on the matter with the general consensus being that the probability is really low of being hit by a highly destructive 300-meter and above asteroid but that if we were to find an asteroid on a collision course for earth, the current ideas of how to deflect it would highly depend on the composition but also are largely untested. Could you please comment on whether you think funding (NASA or others) should be going towards locating smaller (<1km) objects that are not often seen if they are very close to the sun OR whether the budget should be expanded to test the deflection methods on a variety of non-dangerous asteroids within reach?

The probability of an impact with an asteroid large enough to cause large scale destruction is indeed rather small.  Generally we tend to focus on objects 1km or more in size as those that are particularly dangerous, since although smaller objects could cause a lot of damage it would be more localised (a single continent say) rather than global.  Roughly speaking we think that 1km size asteroids hit Earth about every 500,000 years and the impact rate falls off rapidly for larger objects (since there are less of them), impacts with 5km size objects happen only every 10 million years or so.  The largest impact event in recorded history was the Tunguska event in Siberia in 1908, which involved an object that was probably about 60m across.  The most potentially dangerous asteroid we know of at the moment is 1.1-1.4km (29075) 1950 DA, which has a probability of up to 0.33% of colliding with Earth in 2880, so we have a good while to work out how to deal with it.

There are a number of ongoing projects searching for near Earth asteroids, such as LINEAR and the Catalina Sky Survey.  Also, at the beginning of last year the European Union, in collaboration with the United States and Russia, set up the NEOShield project whose remit is to study appropriate deflection methods, including how the choice of method depends on composition.  One of their aims is also to test some of the proposed deflection technologies, both in the laboratory, and in space.  Both aspects are important, since if we don't know the asteroid is heading our way we clearly can't do anything about it, but it isn't much use knowing about the deadly asteroid if we don't have some way of deflecting it.

Formation of Earth and life

Published on 07/11/2012 
Question: 

Hello, I was talking to someone the other day and we got onto the subject of Space as it fascinates me, thinking about space and how life began on Earth is the one thing i can't get my head round, thats why it fascinates me. So anyways, I was wondering, is it possible that the planet we know as home, Earth, wasn't always where we are are now? What I mean is, when I try and think how life began on Earth, maybe Microscopic life to begin with yet life doesn't just start. it has to have something to begin with, you cant put a rock into space, leave it for a few million years and then come back and there will be life on it. What I am wondering is, was life on Earth, whatever it was in the start, frozen on a drifting Asteroid, that Asteroid being Earth, kind of like Pluto is at the moment, just a huge planet of ice and rock, though this huge drifting Asteroid of Ice and Rock was glancing past the sun and got pulled into the circular gravitational pull and now rotates. Obviously of Earth was going on a head on course with the Sun it couldn't be pulled into a a gravitational circle as it would be too sharp a turn, though if it was glancing past the sun it could maybee have got pulled into the gravitational pull, then with the heat of the Sun being just right over the years the Sun defrosted this huge block of ice and rock and slowly thawed out the life, almost like the life was in Cryostasis and now its being defrosted, this can happen as theres a certain moth that does this in the Arctic, gets frozen over winter and defrosts and comes back to life in Spring, just wondered if that has every been wondered and what the answer was, that mabye our Earth and life didn't begin where we think it did, instead Earth actually drifted in from another part of space and got caught in the Suns Gravitational pull and the heat defrosted it and allowed life to begin, well not begin, but carry on now its been thawed out, then over the years the speed of the earth rotating combined with it spinning on all axis sort of moulded into a circular shape planet, kind of like sanding it down. Long question I know but it was on my mind, it also seems a bit more plausable, that life didn't begin in the Milky Way, instead the Milky Way is where our planet ended up and thawed out the life that was on it. Just wondered. Btw I don't study Astronomy, just interests me as its the only thing I can't get my head around so I tend to thing about things, so excuse me if this has already been answered. Thanks for your time, all the best

Thanks for your question.  To answer it, it is useful to split it into two parts; 'did Earth form in the solar system?' and 'was Earth the birthplace of the living organisms that now populate it?'

The answer to the first part of that question is almost certainly yes, Earth did form in the solar system.  The process of planet formation involves quite a bit of jostling about so all of the components that went on to form the young Earth did not necessarily come from near the current orbit of Earth but it would be almost impossible to place a rocky planet into the current orbit of Earth by capturing it.  While it is possible for planets to be ejected during formation, space is really very empty and the probability of an ejected planet passing near to another star is very low.  Even if it did happen and the planet was captured it would end up in a very wide very eccentric orbit (similar to a comet) very different to the current orbit of Earth and although orbits do change over time it would be very difficult to change a comet-like orbit into an Earth-like one.

The question as to whether the genesis of life on Earth was indeed on Earth is a very different one and one that has had a great deal of discussion in one form or another for centuries.  If you look up panspermia you will be able to find far more information than I can give you here, though it is good to be aware that there are some rather crazy ideas out there.  It can be something of a controversial topic but as yet no one has come up with a complete theory explaining how life can arise from non-living material either so it is difficult to really say which is the more likely origin of life on Earth.  Anyway the basic idea of panspermia is that life is spread throughout the universe by asteroids and comets, rather like your suggestion.  As well as the suggestion that life first arrived in the solar system by such a mechanism there is the related suggestion that life could be spread throughout the solar system in the same way if it did first arise within the solar system.  As the only place we know of at the moment that has life is Earth it is difficult to make deductions.  Research is being done into investigating how long terrestrial microbes can survive in space, the Russian Phobos-Grunt mission had a small capsule on board that was designed to test whether the microbes contained within it could survive in space for the 3 year length of the mission, but unfortunately, as you may have heard on the news, that ended up in the Pacific.  When we have more experiments like that, or if life is found somewhere else in the solar system, like Mars or Europa, we can begin to answer the question of where life on Earth arose, and how widespread life is in the universe, in more detail.  All we have to go on at the moment is that pretty much anywhere you look on Earth, no matter how extreme, you find life.