Institute of Astronomy


Ask an Astronomer - Solar System

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. 

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. 

Flying near the edge of darkness

Published on 20/01/2014 

On a flight to Greenland on the 18th  January 2014, we were approx 30 mins from Kangerlussauq, arriving at 9.40 local time.

When we looked out of the right hand window of the aircraft the sky above was blue - daylight - underneath that there was a layer of darkness where we could see the moon and a star and underneath that the 'land' which was a cloud layer.

Out of the left hand widows the sun was rising and looked normal.

Is that possible due to flying close to the 'night and day line' on the earth?

This sort of effect is possible when flying close to the terminator (the line between day and night).  At the time you were over Greenland and saw this effect I presume you were probably at cruising altitude 10-12km up.  Line of sight and the curvature of the Earth means that from that height you could see the Sun, although on the ground below the Sun would not yet have risen. Similarly in the sky on the 'nightward' side of the aircraft the Sun would have been shining high up in the atmosphere, but not nearer the ground, and being high up yourselves you could see the effect more clearly.  On the ground this is why the sky toward the horizon where the Sun has either recently set or is soon going to rise appears lighter.

Plane of the solar system

Published on 15/01/2014 

This has somewhat been playing on my mind for a little while now, We've all seen the posters in school telling us the order of the eight planets and they're all neatly put in a straight line and it came to me, that seriously cannot be how the planets orbit the sun in a straight line some must be off in a tilt. So I went and tried to do some research and most sources do put all the planets in a somewhat near line not really varying from a straight rotation around the sun... So I was wondering is that image correct do all the planets tend to rotate around the sun on an even plane if so then our solar system must be extremely flat with huge vast spaces closely above and below planetary rotations that are never occupied.

The solar system is indeed very flat, the orbits of all of the planets are within a few degrees of the same plane.  This plane is also very close to the plane of the Sun's equator.  The flatness of the solar system is one of the pieces of evidence that suggests that the planets formed within a disc around the young Sun.

The space above and below the plane in which the planets lie is not entirely unoccupied, as many asteroids and comets have much higher inclinations.  The main part of the asteroid belt for example reaches up to about 20 degrees above and below the plane of the planets and some of the outlying groups can reach 30 degrees.  Long period comets, like Hale-Bopp, come into the inner solar system pretty much evenly from all directions, so we believe that the Oort cloud, where they originate, is roughly spherical.  All of these small bodies probably didn't form with such inclined orbits though, they were scattered by the planets (particularly Jupiter) to reach their current orbits.

Not all planetary systems show the same amount of flatness though. Some of the new planetary systems that we have been finding around other stars are somewhat different.  Some of them are actually even more flat than the solar system, but then there are others that where the opposite is true and there are huge differences between the planet orbit and the stellar equator.  We suspect that the cases where the orbits are not aligned are systems that have had much more violent histories than the solar system with close encounters between some of the planets.