Comet or Asteroid: what is the difference?

Because looking up the difference between a comet or asteroid drives me nuts.

They are both:

  • Small chunks of “stuff” that hang around in the solar system. Most describe some sort of orbit, but some pass once and we never see them again.
  • The vast majority can’t be seen with the naked eye
  • They can be made from variety of materials, including rock, ice, carbon-rich materials or nitrogen-rich materials
  • They are much smaller than our moon, have very little gravity and therefore an irregular shape.

Comets are:

  • More likely to be made from water ice or have a thick coating of water ice.
  • The evaporating ice is what gives them an atmosphere that is not bound by the gravity of the comet (because the comet is too small). The water vapour gives them their tail.
  • They originate in the outer solar system.

Asteroids are:

  • Mostly rocky
  • They hang around in the inner solar system (anywhere up to Jupiter) which is also where they are thought to have been formed


  • Some asteroids have also been found to outgas water, while some comets have passed the sun so many times that whatever water ice they had on the surface has gone, leaving the rocky core.

So yeah, there isn’t a clear line to separate between the two and no clear definition. For the sake of convenience, some people use the word asteroid for bodies that move in the inner solar system. The are more likely to be rocky.

Comet or Asteroid: what is the difference? was originally published on Must Use Bigger Elephants


There be telescopes!

A few pretty pictures. More here

Space tweeps and the large 72m telescope. Photo by Canberra photographer Martin Ollman

The large telecope at night.

2.55am and we’re receiving a signal from MSL in this windy, cold and rainy field. Photo by Martin Ollman.

Oops, I dropped the umbrella!

More space tweeps. I’m even in this one. Photot by Martin Ollman.

Pointed at the Moon…

Once more on the search for exoplanets, and Alpha Centauri

Excuse me for the absence of regular science posts, but my brain has turned to mush from writing fantasy. Isn’t it easy when you’re allowed to make everything up and as long as it makes sense, no one cares about accuracy? But, yes, I will return to where I left my characters in the Jupiter system, or in a space station orbiting the fictional gas giant Sarasvati, in the not-too-distant future.

This morning I came across this very interesting article on the Centauri Dreams website. By the way, Centauri Dreams, the website of the Tau Zero Foundation, is a very rich source for writers of realistic SF, especially in relation to planetary exploration and interstellar travel.

The article summarises results and speculation arising from new planets discovered by the European Southern Observatory’s HARPS spectrograph, which provides the most accurate Radial Velocity measurements we currently have (see an earlier post on how planets are discovered). Because of its increased sensitivity, HARPS can detect smaller planets. The smallest planet found at this point in time is a mere 1.5 times Earth’s mass. One of the, perhaps expected, outcomes of the spectrograph’s bevy of newly found low-mass planets (super-Earths or near Earth-mass) is that there are many of these smaller planets, a lot more than there are very large planets, and that the previous bias was merely a product of larger planets being easier to detect. The galaxy is swarming with smallish rocky planets. It is quite likely that some will be found inside the habitable zone.

We may already have found some of these planets. Much was made last year of the ‘discovery’ of Gliese 581g, supposedly in the habitable zone of an M class star. However, further analysis has so far failed to confirm the existence of this planet. But the star has two other planets which orbit at the edge of the habitable zone, and out of these, Gliese581d looks the most promising. The width of the habitable zone is not absolute, but varies with the planet’s albedo (basically, how much light it reflects) and composition and (if any) atmosphere composition (see another post on that here). So a newly discovered planet, HD85512b, at 3.6 Earth masses, may also fit the bill. It is a little close to its K class parent star, but could harbour liquid water on its surface if certain conditions of composition and atmosphere are met (see original paper by Kaltenegger et al. here).

Using the HARPS spectrograph, another group of researchers report on the search for planets orbiting sun-like stars within 40 light years from our solar system. (original paper by Pepe et al here). This work has resulted in a the discovery of a number of planets, again, most in the smaller size category. One of the main targets for the hunt is Alpha Centauri B, but there are some problems, one of the main ones being that it is part of a triple star system, and that any model the describes the wobble of the star that is caused by an orbiting planet must take into account that there are two other stars in the system, and as you could understand that is tricky business.

Image depicting an exoplanet system snarfed from NASA JPL

What if the Earth had no moon (part 2)

A while ago, in this blog post, I mused about what the earth would be like with no moon. You may hear the Moon blamed for things as diverse as reproductive cycles and people’s moods, but in that post, I argued that if all of a sudden, we’d find ourselves without a moon, not a great deal would change. The Sun causes tides, albeit smaller ones, and any stability issues of the Earth’s axis would not change overnight.

But this is not a realistic scenario. The Moon isn’t going to just disappear (the subject of Death Star-like blasting a planet apart is a subject for another blog post, but let’s just say it’s physically nigh impossible). The real question I should have asked is: what would life on Earth be like if Earth had never had a moon? If the Pluto-sized object that may or may not have collided with us to form the Earth and Moon as we know them had missed the Earth, and sailed straight on, to eventually burn up in the Sun.

The situation would look vastly different. The Earth-Moon system is vastly different from other known planets in terms of relative size. Only the largest moons of Saturn and Jupiter are similar in size to our Moon, but those planets are of course much more massive. The Earth-Moon system could well be referred to as a double planet. Of course Earth and Moon affect each other, and this influence comes in the form of gravity. While the gravity of the Moon affects all surfaces of the Earth equally, it’s only the oceans that can react to this gravity. Yeah, that’s how the tides are formed.

Imagine the Earth as an oblong bubble of water, with bulges both on the side where the Moon is at and the opposite side. This bulge orbits the Earth at the rate the Moon does. But the Earth itself rotates inside this envelope of water, and this creates friction at the place where water and solid surface meet. In effect, the water is forever trying to keep up with the planet. Friction creates warmth, and yes, loss of speed.

So it is that over the four-plus billion years of the Earth’s lifetime, the rotation speed of the Earth has slowed from an eight-hour day to our current twenty-four hours. Is still slowing, in fact.

Of course life on a planet with days of eight hours would be very different. We’d experience vastly more powerful weather and especially wind systems. With a rotation speed like that, there would probably not be much opportunity for much North-South weather movement, but we’d have strong bands of air movement, much like Jupiter (which has a daylength of ten hours). We’d have similar ever-lasting cyclonic systems.

And what would it do to the seashores, having the tides jump up twice in eight hours?

Or to the biology of animals evolving with that kind of daylength? Would we all have nervous tics from seeing the Sun whizz by?

What if the Earth had no moon? Part 2

In a recent post, I talked about the answer to the question: what if the Earth had no moon? The question I answered in that post really should have read: what if we suddenly took the Moon away? Or, alternatively, what functions does the Moon fulfil today? And the answer was: not terribly much.

Recently, I read the book What if the Moon didn’t exist, by Neil F. Comins, and it goes into far more detail. Today, the Moon appears in the sky like benign companion. It is pretty, but not terribly vital to us. It causes tides, but the Sun also does that. It provides a handy measure of the passage of time, and it gave us the concept of a month. However, we don’t actually need it. Even the fact that the human fertility cycle and a moon cycle are similar seems pure coincidence. If the Moon was so important to fertility, then all animals would have developed a cycle based on it.

The picture, though, looks very different when you consider the formation of the Earth-Moon system. The Moon formed from a collision of a Pluto-sized object with the early Earth. Right, there goes the peaceful reputation.

In answering the question: what if the Moon didn’t exist, the book takes the history of the Earth back far enough so that this object (remember we’re no longer allowed to call it a planet) missed Earth and left the solar system or found its own orbit around the Sun.

In this case, life on Earth would have been hugely different from the way we know it.

One of the most important things the Moon has done over those billions of years since the collision is to slow the rotation of the Earth. It does this through tides. The Earth rotates, and the Moon pulls whatever part of the Earth facing it. If this part is water, it will rise towards the Moon. But because the Earth rotates, the water is always trying to catch up with the Moon’s current position. Over many, many years, this slows the Earth down. How much? At the time of the Collision, the Earth made a full rotation in just eight hours. Without the Moon, the Sun would have slowed the Earth down, but not by far as much, probably only to about ten hours.

If the thought of ten-hour days doesn’t drive you to distraction, think about what it would have done to the weather. Current weather patterns are determined by many things, including differences in temperature and land shapes. The prevailing movement of weather from west to east is caused by the fact that the Earth rotates inside its own atmosphere, and the atmosphere is not rotating at the same speed. If the Earth had ten-hour days, those west-to-east patterns would be much stronger. In short, we would have a lot of strong wind, and violent weather, and with both weather and ten-hour days combined, a very different planet.

That is not to say that every planet without a moon the size of ours would rotate fast. Mars has a similar day, and Phobos and Deimos are puny captured asteroids that barely deserve the term ‘moon’. Venus has no moons at all and its day is longer than its year. Clearly, a planet does not need a moon to slow its rotation to 24 hours,. Neither does thist answer the question as to whether a random planet in the habitable zone needs a large moon. Earth needed the Moon, but another planet may not.

Exoplanets: could Alpha Centauri have any?

My fiction recently gave me cause to examine interstellar travel. Many writers tend to shy away from the reality that we’re a long way from anywhere. It’s too hard, too intimidating, too depressing. I, too, have done the wormhole thing, you know, where your characters can zip between worlds, but fun as it is, and I won’t stop writing space opera, it always feels like cheating to me. In my next project, I wanted a more realistic approach. How realistic? I haven’t answered that yet, but I’ve started out by looking at the facts.

Our nearest stellar neighbour is Alpha Centauri (it’s actually a multiple star, but a bit more about that later), the brightest star in the constellation Centaur, the forth-brightest star in the sky (less bright than Sirius, which is more than twice the distance), and mostly visible in the southern hemisphere. At a distance of a mere 4.22 light years, and a theoretically achievable travel speed at 10% of the speed of light, allowing for speeding up and slowing down, it would take roughly 50 years to get there. Wow. I am waiting on the arrival in my real-life mail box of some material about how to get there, and will write more about that later, but let’s assume, for the sake of the argument, we could send a ship there within a human lifetime.

Question is: why would you? What is there? If Alpha Centauri had planets the size of Earth, or Mars in the habitable zone, wouldn’t they already have been detected?

There is a long and a short answer to these questions. Let’s have the long answer first.

What methods do we have to detect planets?

Direct Imaging:
There is no denying that emotionally the best way to determine if something is there is to see it. Humans tend to be visual creatures and have a great ‘I’ll believe it when I see it’ instinct. Surprisingly, telescopes have seen some planets. There is some debate as to whether those objects are planets or brown dwarfs, but something is definitely orbiting those stars. To see a planet, the size of the planet and brightness of the star are going to matter. The reality is that in most cases our telescopes are nowhere near detailed enough, and the above examples are exceptions.

Radial Velocity method (also called Doppler Spectometry):
When a planet moves around a star, the star wobbles a tiny bit. Light waves behave in a manner similar to sound waves with an approaching and passing ambulance. If the star moves away from us, the waves become longer, the light more red; if the star moves towards us, the waves become shorter, more blue. This is called redshift or blueshift and can be picked up with very sensitive instruments. This method tends to detect planets whose weight ratio compares favourably with their stars – relatively large planets orbiting relatively small stars. It also doesn’t take into account any part of the star’s movement that is not towards or away from us. This is where astrometry helps. These two methods combined are the most common in planet-hunting.

Transit Photometry method:
If you’re lucky enough, the plane of the orbit of a planet around a star is exactly the same as our field of vision. In other words, we view the solar system edge-on. In that case, there will be times that the light from the star dims, because the planet passes in front. Because you need more than one pass of the planet, this method obviously favours large planets with a short orbital period.

When a star moves in front of another, the closest star distorts the light of the more distant one, making it appear 1000 times brighter than normal. This effect usually lasts a few weeks, until both stars move on. When, during this time, a planet happens to pass in between, it adds to the effect. You obviously have to be pretty lucky for this to occur at a time you happen to be watching.

This method relies on extremely accurate measurements on how a star moves in the sky. The Radial Velocity method discussed above works best when a solar system is viewed edge-on; astrometry works best when the solar system is viewed face-on. This method has enormous potential, and astronomers predict that we will be able to detect Earth-sized planets. But not yet. At the moment, distortions from the Earth’s atmosphere hamper measurements, but projects like the European Gaia mission, scheduled for 2011 will change that.

Now about Alpha Centauri:
The trouble with Alpha Centauri is that it’s not one star, but three, denoted by the capital letters A, B and C (the small letters are used for any planets discovered). Alpha Centauri A is also called Rigil Kentaurus, the largest and brightest of the group, and is a G star similar to the Sun. Alpha Centauri B is an orange K type star (see here for a cheat sheet on star types). A and B form a close binary, with a distance between them of 23AU (1 AU is the distance from Earth to the Sun), about as far apart as the Sun and Uranus. This is considered to be a piddle of a distance. The star Alpha Centauri C is also called Proxima Centauri and is actually the closest star to us. It’s a red dwarf, and 14,000AU from A. You can imagine that it is hard for planets to form in a system where they are influenced by the gravity from more than one star. However, it is not impossible. Various references that are listed at the bottom of this Wikipedia page suggest that A could possibly have planets in a zone not further than 2.5AU from the Sun-like star. In our solar system, the habitable zone is considered to be roughly between 0.9 to 1.7AU (this includes Mars, but not Venus). Additionally, planets have been detected in close binaries.

The current count of extra-solar planets numbers 418, listed here (be sure to visit the rest of the site, because it is excellent). It’s an entertaining list, which includes a small visual of what the system looks like.

When you go through the list in detail, you will notice a few things. In the first place, most planets are massive. Masses are given in Jupiter masses, so a planetary mass of 1 is equal to Jupiter, and a solar mass of 1 is equal to the Sun. Many, but not all, orbit close to their parent star, and lastly, their parent star is relatively small. Strangely enough, distance from us doesn’t seem to be a factor. The furthest planet detected I could see in the list was more than 20,000ly away. Distance between the star and the planet matters only in that it increases the time necessary to be looking at the planet to reliably detect it. The planet Gliese 581g, the source of the kerfuffle earlier this year, was the result of eleven years’ study of the parent star. Astronomy is not a short-time career. We come back to the planet mass (compared to Jupiter)/star mass (compared to the Sun) ratio. For Earth, the ratio is 0.003. For Mars, 0.0003. For Uranus, it’s 0.04. The smallest ratio I could find in the table was 0.016 for a planet called CoRot 7b. Gliese 581g, which is not in the table, has a ratio of 0.029. It has three times the mass of Earth, and its star is 31% of the Sun’s mass. I expect that new methods will bring this ratio down.

That was the long answer.

The short answer is that there might be planets, but because we haven’t seen them, they are unlikely to be Jupiter-sized, and we don’t yet seem to have the technology to detect planets much smaller than Uranus ratio, not even for our closest stars.