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Skywatching

Saturn is the moon king

Researchers using the Subaru Telescope in Hawaii have found 20 more moons orbiting Saturn.

This brings the total orbiting the planet to 82, overtaking Jupiter, for which the total discovered so far is 79.

These new satellites are all about five kilometres in diameter, and discovering them is an indicator of the quality of modern telescopes.

Our Earth has one, quite large moon. Mercury and Venus have none, and Mars has two.

However the giant planets, Jupiter, Saturn, Uranus and Neptune have large collections of moons, and as our instruments improve and our spacecraft pass by, we are finding more.

Is this due to the planets actually acquiring new moons, or is this simply due to improvements in our instruments?

Thanks to our having a number of telescopes watching for them, almost every year we notice an asteroid passing us close by, sometimes closer to us than the moon.

Even when this happens, and the Earth's gravitational attraction on that asteroid is stronger than the Earth's pull on the moon, the asteroid does not get captured; it passes by and continues on its orbit around the sun.

The reason is simple. As the asteroid approaches the Earth, it accelerates due to the Earth's pull. When it swings past the Earth, it will be going fast enough to escape into space with the speed it had before it encountered us, although it will probably be moving in a different direction.

To be captured, it needs something to slow it down a bit. The empty vacuum of space has nothing in it to do that, which is why our Earth is circled only by the moon, satellites we have launched and a growing accumulation of space junk.

There might be a tiny extra moon out there that we haven't found yet. The same applies to the giant planets, Jupiter, Saturn, Uranus and Neptune.

Anything orbiting the sun that flies past them will head back off into space. However, it has not always been like that.

The solar system formed from the collapse of a big cloud of gas and dust. It collapsed into a rotating disc, with the centre becoming the sun. In the remaining disc material, smaller discs formed,  which collapsed to form the planets and their moons. These moons orbit in the same direction as the planets' rotation.

For example, our Earth rotates eastwards, and that is the direction of the moon's orbital motion. While any remaining disc material is present, an object orbiting the sun and coming too close may be slowed down enough by that material to be captured, becoming a new moon.

This is most likely for the giant planets, because they formed from large discs, which provide particularly large "catcher's mitts.” 

These outside additions can end up in any sort of orbit and can even move in the opposite direction to the other moons. Seventeen of Saturn's moons have these reverse or "retrograde" orbits, suggesting they are external additions.

In the outer solar system, around the giant planets there are opportunities for passing objects to seem to be captured but actually aren't. If the encounter is at the right speed and the right distance from the planet, an object might make an almost complete orbit before wandering back off into space.

This apparent orbit might be many years long, so it will take time and some careful measurements to find out whether an object is a distant moon or a visitor.

The data for these new Saturnian moons was collected between 2004 and 2007, which produced enough information for some new computing algorithms to derive orbits for the new discoveries.

During coming years, as our telescopes and calculations improve, we expect we will be finding even smaller moons, maybe as small as one kilometre, and the count will go up.

  • Mercury and Venus might be visible very low in the southwest after sunset if the sky is clear.
  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will reach Last Quarter on the 21.




Black holes shred stars

The term "tidal disruption event" or TDE must be one of the biggest cosmic euphemisms around.

This is what we say is happening when a star is being shredded by a black hole. Imagine something tearing our solar system to pieces: sun, planets, and, of course, us.

The pull of gravity decreases with distance. As we get higher and higher above the Earth, the gravitational attraction decreases, but the effect is slow for small masses like the Earth.

Even at the distance of the moon, the force is enough to keep the moon in its orbit and to stop it wandering off into space.

However, the Earth's pull on the moon is a bit stronger on the part of the moon closest to us compared with the other side of the moon. This elongates the moon in our direction by a few metres.

Fortunately, the moon's gravity is strong enough to limit the effect. The moon exerts these tidal forces on the Earth too, pulling the sea up into bumps and giving us the tides.

On the other hand, black holes are a different matter. They are extremely massive and highly compact, as we get closer to them, the difference in gravitational pull between the part of us closer to the black hole and the other side of us becomes enormous, eventually enough to pull us to pieces.

Imagine a star that is on a path taking it close to a black hole. As it approaches, the black hole's pull on the side of the star facing it gets increasingly stronger than its pull on the other side.

This difference leads to the star being pulled out of shape, elongating toward the black hole. As long as the star's own gravity is stronger than this tidal force, the star holds together. However, as it approaches, that tidal force increases and the star starts to come apart.

First to go are its outer layers, where the gravity holding the star together is weakest. This forms a long streamer, flowing inward toward the black hole. As the star gets closer, more and more of its material gets stripped off, until all that is left are shreds of material streaming into the black hole.

This forms a disc around the black hole, referred to, again euphemistically, as an "accretion disc," with the black hole continually nibbling away at the inner edge, and more material spiralling inward to replace it. Some of the material pulled off the star gets flung out into space.

Fortunately, especially for any inhabitants of planets orbiting around stars, like us, these TDE's are rare, maybe one every several thousand years taking place in our galaxy, the Milky Way.

We know of several stars trapped in orbit around the massive black hole in the middle of our galaxy, moving in highly elliptical paths taking them close the black hole.

Their fates are certain, but we cannot predict exactly when. However, when it does happen, observers in other galaxies should be able to see the core of our galaxy brighten significantly. This gives us a powerful tool for looking for TDEs in other galaxies, and greatly increasing the odds of our seeing one.

These days we have telescopes observing thousands of stars looking for exoplanets, and other telescope networks all over the world monitoring large areas of sky. Jan. 29 this year one of a worldwide network of telescopes caught a TDE happening in another galaxy.

The network, known as All-Sky Automated Survey for Supernovae, abbreviated somewhat morbidly as ASASSN, is a set of small (14 cm) telescopes at multiple sites around the globe.

The event was also picked up by orbiting telescopes.

It is easy to conclude the cosmos is full of catastrophic events. However, most of the universe is much more peaceful, like our solar system. It's just that catastrophic things liberate lots of energy, which makes them easy to see.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will be full on the 13th.


Visitors from beyond

Every year, we can count on a comet or two visiting our neighbourhood in the solar system.

For a few weeks these objects can be the most spectacular objects in the sky. These are all orbiting the sun and members of the solar system. However, very rarely a comet visits us from interstellar space; one that originated in another star system.

One of these is moving into the solar system now, and is heading inward towards the sun. It has been named Comet Borisov.

Comets are the result of interactions between lumps of deep-frozen material a few kilometres across, located in Oort Cloud, in the outer reaches of the solar system. They have been orbiting there since the birth of the sun and planets.

On rare occasions two lumps pass very close together. This results in one of the lumps being thrown into a new orbit, taking it inward toward the sun.

The other body in the interaction gets thrown outward, either into a new, more elliptical orbit taking it further out, or maybe out of the solar system altogether. In this case it will wander interstellar space for millions or billions of years until it encounters another star and planetary system.

In space, there is no friction, so the visitor will dive into the system, swing by the star and head off back into interstellar space in some new direction.

Comet Borisov will pass close to the sun on Dec. 7. It is currently moving about 33 kilometres a second and will accelerate to about 44 kilometres a second as it swings past the sun.

Assuming it does not hit anything, it will then vanish back into the space between the stars.

Comets are basically lumps of dirty ice. Anyone who has seen the frozen, dirty slush, loaded with petrochemicals and other organic materials that accumulate on the edges of city streets in Canada in late winter will have seen a good approximation of what comets are made of.

When comets are far from the warmth of a star, they are frozen solid. However, if they are in an orbit taking them close to a star they start to warm up as they approach.

The more volatile materials start to evaporate. Jets of vapour and dust erupt through the comet's surface and because the gravitational attraction of such a small body is very weak, the erupted material escapes into space. 

The sun's radiation and the solar wind push that material away from the comet, forming a tail. Without the tail comets are just small, almost black lumps. With the tail, they become one of the most spectacular objects we see in the sky.

Images taken with one of the Gemini telescopes show Comet Borisov is now developing a tail. The light from the tail bears the signature of organic chemicals.

However, most of us will not be able to observe this comet. The brightest we expect this visitor to be is about magnitude 15, which will place it within the reach of amateur astronomers with fairly large telescopes.

Of course, we could be wrong!.

The presence of these interstellar visitors suggests that, as we would expect, other stars also have Oort Clouds of construction debris surrounding them. 

With all these systems throwing comets inward and outward, there must be a good number of comets moving around in interstellar space, especially since the number is constantly being added to and their chance of destruction tiny.

That raises the surprising issue of why so few of them are visiting us.

One suggestion is that many stars are members of binary or multiple systems, where two or more stars orbit one another, and in the highly disrupted environment caused by the formation of multiple stars, the Oort Clouds would have been disrupted and absorbed by the stars.

This suggests that interstellar visitor comets will be rare, which so far is what we have been seeing. However, since the right search tools have only recently become available, we don't really know.

  • Jupiter and Saturn lie low in the southwest after dark.
  • The moon will reach First Quarter on the 5th.


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A warm, wet world

Bizarre exoplanets — planets orbiting other stars — are interesting in that they help us understand how planetary systems form and how they work.

However, what we seek more than anything else are planets like ours: planets that could have life at least something like ours.

The sheer size of the universe compared with our speck of a world makes it logical that we are not alone, and a terrible waste of space and resources if we are.

One of the better candidates for an exoplanet bearing life as we know it has just been discovered. It has been named, not exactly creatively, as K2-18b.

It lies about 111 light years away; its surface temperature lies somewhere between 10 and 40 degrees Celsius and its atmosphere contains lots of water vapour.

It is little more than twice the diameter of the Earth, and has around eight times its mass.

On its surface we would weigh a bit less than double what we do on the Earth. It would not be healthy for us but we could probably tolerate it for a short while.

Of course, we did not evolve for life on such a world. If there are oceans on that world, things swimming in them would be almost weightless, just as they are here on Earth.

Planet K2-18b is orbiting a dim, red dwarf star. These stars are really miserly in the amount of energy they radiate.

The range of distances from a star where planets can have surfaces warm enough for liquid water to be present is known as the Goldilocks Zone. For dim stars, this zone is narrow and lies close to the star.

Planet K2-18b lies in the zone and is indeed close to its star, taking only 33 days to orbit it once. Our Earth's trip around the Sun takes a year.

Considering how far away this star is, and that nobody has actually seen the planet as even a dot in a telescope, how can we have deduced all these things?

The main way we detect planets beyond our solar system is to look for minute dimmings in the brightness of distant stars as their planets pass in front of them. Obviously this only works for systems where we happen to be looking in the plane in which the planets are orbiting their star. 

By measuring how long it takes to transit across the disc of its sun and the time between two successive dimmings, we get estimates of how far it is from its sun and how long it takes to complete each orbit.

In addition, from the time taken for the light to dim and then the time it takes for the starlight to rise again to its undimmed condition, we can deduce the size of the planet.

We can go further. Stars are very hot, so when we look at them we see bodies made of atoms, not molecules. Some cool stars have molecules in their atmospheres, but none that have any biological significance.

If the planet has an atmosphere, as it passes in front of its star, some of the light reaching us has passed through the planet's atmosphere, and in the process has impressed on it the signatures of what the atmosphere is made of.

In most cases, we can be sure that if we see signatures of molecules, those molecules are present in the planet's atmosphere.

We get a double check by analyzing the light of the star when the planet is not passing in front of it. The atmosphere of this new planet has a strong signature of water vapour, so life is a possibility.

With the new space telescopes coming on line in the next few years, it will be possible to dig deeper. Life on a planet changes its atmosphere; for example our planet's atmosphere is rich in oxygen.

This gas is so reactive it would vanish quickly if it were not continually topped up by our plant life.

It could be that alien creatures might not be fans of oxygen, but if we detect any unusual chemicals that would vanish if not continually produced, we could be well on our way to proof we are not alone.

  • Jupiter and Saturn lie low in the southern sky after dark.
  • The moon will be new on the 30th.


More Skywatching articles

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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