Institute of Astronomy


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Ever had a question about astronomy you've want answered? Have a look through the previous questions which we've been asked and if you can't find find your answer, ask us!

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Are there planets where it's always day/always night?

Published on 28/01/2015 

Have any planets been observed to rotate at the rate that would keep only one  side facing their sun continuously? Do planets rotate at some mathematical constant related to their size and length of orbit around their sun?

This is a great question, planets rotating at the same rate as their star would have one side be permanently day and one side be permanently night as you have said. We have indeed found planets outside out solar system that work in this way. This effect is known as tidal locking and already happens with the moon where one side always faces the earth. (There is a small "wobble" with the moon where it does slightly change as it rotates about its own axis but the same side always faces the earth giving us a "dark side of the moon". You can see a great visualisation here : ) In the solar system lots of moons are locked to their host planets.

If we imagine that we have a "host" body which might be a star or planet and an orbiting body around it. The main idea with tidal locking is that one half of the body that is orbiting its host is feeling more gravitational pull than the other causing a "bulge" on one side, facing the host. When the orbiting planet (or moon) is very close to its host this bulge will start to lead or lag the bodies orbit, depending on the system. This additional gravitational pull on one side serves to slow down the day-night rotation of the planet or moon until eventually it synchronises with the orbit around the host.

Only some bodies that are close enough are really susceptible to tidal locking. For it to really work you need to be close enough for the bulge to be able to be pulled off-centre the body orbits.  Tidal locking also takes time to set in. When the planets and moons initially form they have their own rotation which over time decays. The time it takes to become locked depends on a few factors including the mass of the objects and the separation of the objects. This means that very close objects that are much lighter than their hosts lock quicker. 

All in all tidally locked objects are close to their hosts. We have found some exoplanets that we believe are tidally locked. (For great examples try looking up hot Jupiters like Wasp-43b and HD209458b.)

I hope that answers your question, planets being tidally locked is definitely  possible and we believe we have found very close in planets where, as you've said, it is permanently day on one side and permanently night on the other.

Close approach of C/2013 A1 (Siding Spring) to Mars

Published on 10/04/2014 

I have been thinking about how water was once on the surface of Mars. Today there is no evidence on water on the surface only 'channels' of were water once flowed.

I guess, water is still on mars but under the ground.

Could an asteroid have brought the water up to the surface, then after the asteroid has passed the water sinks back under the subsurface of mars ?

If a massive asteroid passed close by the planet mars, could the gravitational pull of the asteroid  exert a pull on body of mars, that could have an effect on mars molten core centre, exerting heat and pressure upwards, that brings mars water liquid to the surface, then the water on mars sinks under the ground after the effect of the asteroids pull has passed.

I though of this after thinking about how the Earths moon gravitational pull effects the earth by moving water to create tides and the effects on the earths core.  And how the gravitational pull of Jupiter has effected Europa's core, creating heat and pressures at the core to exert the water to the surface.

On Oct. 19, 2014, Comet Siding Spring will pass 138,000 km away from mars.

Do you think this close encounter with Comet Siding Spring would have a gravitational pull that would bring any water up from the subsurface of mars.

There is still water on the surface of Mars, the problem is that it is all frozen rather than liquid.  The polar ice caps of Mars contain substantial amounts of water ice, along with frozen carbon dioxide.  Similarly there is water ice in the subsurface of Mars at high latitudes, much like the Arctic permafrost on Earth.

Tidal heating can be an important effect, and is indeed what keeps Europa's subsurface ocean liquid.  To have that kind of tidal effect though needs a very massive body, Jupiter and the other Galilean moons in the case of Europa, and the Moon in the case of Earth.  By comparison Comet C/2013 A1 (Siding Spring) is tiny and the tidal pull it will exert on Mars would not be noticeable.  Tides do work both ways however, and the comet will experience quite large tides from Mars, which could significantly affect the structure of the comet.

Large impacts on the other hand might be able to temporarily melt some of the permafrost.  In the case of C/2013 A1 (Siding Spring) we know that an impact is very unlikely, but Mars does get hit by large objects every so often. 

What kind of alien life to look for?

Published on 10/04/2014 

I've been wondering about this for some years now, scientists have been trying to find alien life in paces similar to Earth, they're trying to find a place with water, decent temp, etc.
   It's as if they're trying to look for humans :O, but what really confuses me is that 90% if not all of astronomers believe in evolution, so they'll agree to the fact that we (humans) are the way we are because of our environment and the way we've adapted to our surroundings in the last 100,000 years if there was no water, would that mean that we wouldn't be here right now? Through evolution we would've adapted to our surroundings and not needed water to live, wouldn't it be the something with Aliens? If there life started in a place that had no oxygen and no water, they would've just not need oxygen and water to live, so shouldn't we be looking EVERYWHERE? We are searching for places that would support us, and maybe only us, which is probably why we haven't found something yet

The argument that looking for 'Earth-like' life might blind us to finding potentially dramatically different kinds of life that might even be more common from a universal perspective is one that people have put a considerable amount of thought into.

Essentially the issue boils down this - if lifeforms arise that are completely unlike any on Earth today, then what would they be like and how would we detect their presence?  Without an example of a non-Earth-like lifeform that question is pretty much unanswerable. We can (and people have) come up with rough ideas of how organisms with completely alien biochemistries might operate, but there is no real way of testing how accurate those ideas might be.  What we do know however is what life on Earth is like, and one thing that every living thing on Earth needs is water, which only exists in a usable, liquid, form in a fairly narrow temperature range.  Since Earth-like life is the only thing we know how to look for, that is what we are looking for.  If we discover life somewhere else, such as on Jupiter's moon Europa, then we will have a better idea whether all life shares certain basic requirements or whether there are forms of life that are dramatically different from Earth-like life.

Speed of Saturn's rings

Published on 10/04/2014 

I remember reading somewhere that Saturn's rings are racing around the planet at tens of thousands of miles per hour. Does that mean that Saturn is kind of like an enormous buzz saw? Would the rings just be a blur to you if you were approaching the planet? Or did I just interpret this concept incorrectly, and are the rings just placidly floating around the planet, kind of like we always see them pictured in sci fi shows?

Orbital speeds do indeed seem very large when you first hear them, but you have to bear a couple of things in mind.  Firstly the distances involved are also very large.  For example the international space station has an orbital speed of 7.6 kilometres per second (17000 miles per hour), which sounds incredibly fast, but it still takes 90 minutes to circle the Earth.  The ISS is also pretty good for observing with the naked eye, and although you can definitely see it moving it doesn't just whizz past, it takes a good 10 minutes to make its way from one horizon to the other.  The second thing to bear in mind is that if you were approaching Saturn in a spacecraft, you would also be moving at those kind of speeds, and so by comparison the rings would seem to be moving more slowly relative to you. 

Star cluster densities

Published on 09/04/2014 

In star clusters like M55 what is the average distance between them? They look all bunched together but I am curious to know just how close together they are. 

Globular clusters like M55 are indeed very dense.  As you may know the nearest other star system to our own is Alpha Centauri, about 4.2 light years away.  In a globular cluster however the typical distance between stars is only 1 light year.  In the centre the density is higher still, the typical distance between stars can be as little as a few hundred AU, comparable to the size of the solar system.