SOLAR SYSTEM

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  SOLAR SYSTEM
What is in the Solar System? - pretty much everything found around the Sun, which includes the Sun, planets, satellites (moons), asteroids, comets and anything else that is the area (dust and debris is pretty much all that remains). It should be noted that the proper way to refer to a moon around a planet is the term satellite. Satellites can be natural (like a moon) or artificial (like weather satellites). This also helps to avoid confusing our Moon with others.
One thing you also need to be aware of are the great distances between the planets. This YouTube video shows just how far apart the planets are, as well as their relative sizes. Keep that in mind when you think about all of these truly distant worlds!.
Pretty much everyone learned about the planets when they were in elementary school and you learned that the order of the planets going from the Sun outwards is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. I suppose you were wondering where is Pluto on this list? Isn't it a planet? I think the best way to answer this is that Pluto should never have been considered a planet in the first place. It doesn't have the physical characteristics of any other planets, it isn't even the largest thing in its neighborhood, and if we are going to consider calling Pluto a planet, then there would be a bunch of other objects out that that should also be called planets - which means you would have more to memorize.
In the Summer of 2006, the International Astronomical Union (the largest organization for astronomers around the world) had a committee come up with the definition for a planet in an effort to decide if Pluto should be a planet. Since planets were objects that have been known of since ancient times, no one had ever thought about defining what a planet actually was until astronomers started discovering a bunch of other planet like objects in our solar system. As it turns out the committee came up with pretty lame guidelines, which were voted against, but another group came up with the criteria for being part of the planet club. These are the following:

1. Must orbit the Sun
2. Have enough mass so that it is a fairly round object
3. Is the dominant object in its orbital path around the Sun (biggest thing around)

Pluto does not meet the third criterion, so it is not a planet. The same holds true for other objects such as asteroids and comets and a whole bunch of other small objects out there. Objects like Pluto that are big, but not the only big thing in their area are now refered to as dwarf planets. Sort of a downgrade, but it does make sense. Anything that is not a planet or dwarf planet is just a small solar system body. I don't know why there was this big fuss - I decided well before the International Astronomical Union that Pluto wasn't a planet. They should have just asked me! So no matter how you view it, as far as this course is concerned there are only 8 planets in our solar system!
We'll start the study of the solar system by looking at how it came about and how it compares to other solar systems out there (yes, there are other solar systems out there). One of the most obvious aspects of our solar system is how the planets are divided up into two types.

Terrestrial Planets

Figure 1. The terrestrial planets shown to scale. Images from NASA. These are planets that are similar to the Earth (that's what the terra means) and include Mercury, Venus, Earth (duh!) and Mars. These planets have the following characteristics.

• They all have a relatively small mass and radius. Earth is the biggest one amongst this group. While you might think that the Earth is pretty large, as you'll see, it is actually pretty puny.
• These worlds have warm, solid surfaces. The temperatures range between values of 100 K to about 900 K. While 100 K is pretty cool for you and me (it's actually about -280 F), it is pretty warm compared to other places.
• These planets have dense, rocky compositions. A good fraction of their interiors have compositions of nickel and iron, making them have densities that range from 3000 - 5000 kg/m³.
• They are all found close to the Sun, since of course these are the first four planets.
• Those planets that have an atmosphere have one that is made up of heavy gases - things like N₂, CO₂, O₂, and H₂O. Mercury is the only one without an atmosphere.
• Satellites (natural ones) are very rare - only three satellites are found amongst the four terrestrial planets.

Jovian Planets

Figure 2. Jovian Planets shown to scale. Image courtesy Calvin J. Hamilton. These are the planets that are similar to Jupiter (Jove is another name for Jupiter, by Jove!). Included in this group are Jupiter, Saturn, Uranus, and Neptune. These planets have the following characteristics (which are in some ways exact opposites from the characteristics of the terrestrial planets).

• These are the big planets; they have very large masses and radii. Masses are from about 15 to 300 times larger than the Earth.
• These planets don't really have surfaces, they just have cloud layer after cloud layer. They do thicken up, but you can't really stand on these worlds.
• The gaseous compositions mean that these guys will have very low densities. In fact, these are the lowest density planets, with average densities being in the neighborhood of the density of water (1000 kg/m³).
• They are all found in the outer solar system.
• The atmospheres (or upper cloud layers) are composed mainly of light gases - hydrogen and helium are the most abundant, and there are also small amounts of hydrogen rich molecules, such as methane, water and ammonia in them, though they tend to be in very minute amounts.
• These worlds also have large numbers of satellites going around them.

Figure 3. The planets in the solar system shown to scale. Image courtesy Calvin J. Hamilton. The descriptions of the two types of planets provides another reason to exclude Pluto from the planet club. Its characteristics don't fit into either category. This only adds more support to the idea that it shouldn't be a planet. So if you aren't happy with the rather arbitrary criteria put together by the International Astronomical Union for the characteristics of a planet, the basic characteristics of the planets in our solar system leave Pluto out in the cold (literally and figuratively).

Formation of the Solar System

The current configuration and characteristics of the solar system should provide us with some clues as to how it formed. If you are going to try to understand the processes that went into the formation of the solar system and are attempting to theoretically duplicate it, then you better make sure your model duplicates these characteristics and others that are important aspects of the solar system. It may also be the case that there were rare events in the formation of the planets in our solar system that we don't expect to occur in other planetary systems. Of course, there is also the question of how likely is it that a planetary system forms around a star - so many questions, so little time!
Here are some of the features of the solar system, which can be thought of as clues to how the solar system formed.

• Most of the material in the solar system is in the Sun (actually, about 99.9 % of it is). The planets and all the other junk are pretty minute in comparison to the Sun.
• All the planets orbit the Sun in about the same orbital plane, the ecliptic (Mercury deviates from this the most with a 7 degree tilt).
• The planets orbit the Sun in the same direction, counter-clockwise as viewed from above the north pole.
• The planets' orbits are very close to being circular (Mercury is the least circular with an eccentricity of 0.21).
• Most planets rotate on their axes in the same direction. The exceptions for this are Venus and Uranus. Venus is completely tipped over, while Uranus is sort of on its side.
• The planets' satellites tend to follow the motions of the planets; they are orbiting and rotating in a counter-clockwise direction. There are actually many exceptions to this rule, but most do follow it fairly well.
• The density of the planets decrease with increasing distance from the Sun. This also indicates that the compositions of the planets change with increasing distance from the Sun. When you include things like asteroids, their compositions also follow this trend.
• The outer planets are more massive than the inner planets and are made mainly of hydrogen and helium.
• There is extensive cratering of both planets and satellites, and it is thought that most of this cratering occurred at about the same time.
  

How is a solar system made? What are the ingredients that we need to make one? Actually, you could answer these questions by asking how the Sun was made since it is the main component of the solar system. Remember, it has most of the mass of the solar system. If someone were to do a survey of our solar system, they'd most likely say that there is a G2 Main Sequence star in it and not much else. Maybe they'd also mention Jupiter, but beyond that, everything else is pretty minuscule. If you are going to make a solar system, you are going to primarily be involved in making a star (the Sun in this case). We have already gone over this process of how stars form, but we did not look at how the other stuff - planets, satellites, etc. form since they are such small components of the whole process.
To make a solar system you need to start out with a gas and dust cloud composed of about 70% hydrogen, 27% helium, and 3% all else. We're talking about a gas cloud that will form into the solar system, so we usually refer to it as the solar nebula. Now you know where all of this stuff has come from, don't you? At least, if you have been paying attention the past 11 weeks or so, you should know. First of all, hydrogen and helium are the most abundant elements in the Universe and were formed in the Big Bang (even though some He is formed in other stars, that doesn't contribute a lot to the overall abundance). All of the other stuff (the stuff that makes up 3% of the mass) came from secondary sources, through the fusion of elements in the cores of stars. These stars then spew this material out through various mechanisms such as stellar winds, planetary nebula phases, and the ultimate littering of the galaxy, supernova explosions. The most common elements found in this 3% are things like oxygen, carbon, nitrogen, silicon, iron, and so forth.
You have a solar nebula sitting out there that will start collapsing down to make the Sun and some other stuff. We don't want it to form in any general way, do we? We have to have it ending up as a solar system like we see today, with all the planets spread out around the Sun in the ecliptic. How can we do that? One thing that will help the process is to have the nebula rotating a little bit. What good is that? This has to do with that concept of angular momentum that we have run into before (see the stuff about pulsars). As something collapses, it spins faster. The gas cloud starts collapsing, with most of the stuff collapsing into where the Sun will be. Does that mean there will only be a Sun formed in the middle? No, that's not what's going to happen. If you have something spinning really fast, there is a tendency for it to stretch out. Have you ever seen a person make a "real" pizza? I don't mean someone taking a pizza out of a freezer and putting it into a oven, I mean someone making a pizza the old fashioned way. One thing they do is toss the dough up into the air. Is that all they do? No, they also spin it. Why? The rotation (angular momentum) helps to spread out the material (dough). The collapse speeds up the rotation, which in turn spreads stuff out into a flat disk (like a pizza or a solar system). This explains why all of the planets and most of the material in the solar system are found in this disk (the ecliptic) and are moving around the center in the same direction.
Now that you have all of the stuff in a nice, neat, flat disk, you can start making planets - well, not right away. Material will first come together in little bits and some bits will come together more easily than others. Why? The temperature of the disk will play a role into how quickly things form into bigger chunks of material and if it is even possible for some things to form. Near the Sun it is very difficult for the lightweight gases and ices to come together since the strong radiation from the newly forming Sun will tend to break these things up easily. Only stuff with strong gravitational attraction (heavy stuff) will be able to come together and stay together. This would include the heavy elements or what you might think of as the stuff that makes up rocks and metals. You have to remember, this high density stuff is sort of rare in the solar nebula compared to the lightweight stuff, so you aren't going to be making many of these rocky/metallic chunks or very large chunks of rocky/metallic material. Further from the Sun, the temperature of the cloud is cooler and gases and ices can coalesce easily - as well as the less common metals and silicates. There are more of the light weight (low density) elements, so they will far outnumber the heavier metals and rocks. You can see right away that the densities of the planets vary according to the heat (distances) from the newly forming Sun. The sizes of the planets are influenced in a similar way; since the inner planets are made up of the less abundant heavy elements, they will end up having lower masses, while the outer planets are made up of the most common materials, hydrogen and helium, and therefore they will be much bigger. Another aspect of this temperature dependency on the formation of the planets is the size of Jupiter. Jupiter is located closer to the Sun than the other Jovians and is about as close as a Jovian type planet can get - it is also the most massive. When Jupiter formed it sort of gathered up most of the available mass, since it was close to the location where all the abundant lightweight stuff was able the coalesce. It is closest Jovian to the Sun, so it is closer to where more of the mass is. The end result is that Jupiter is by far the most massive planet in the solar system.
Let's get back to making planets. What is driving the whole solar system formation process? Gravity is the force behind it all (as you should know by now, "Gravity Rules!"). The material is coming together, forming larger and larger chunks. Condensation and accretion will cause concentrations to develop in the solar nebula until the chunks reach sizes where they dominate their areas - they are now planetismals. These are the basic building blocks for a planet. They vary in sizes from tiny fractions of a centimeter to several kilometers. They are just basically big enough (and therefore have enough gravity) to pull in more material so that they can get bigger. Of course, if you combine enough of these planetismals together you will make your basic planet.
Figure 4. A planet that comes together gradually will have a mish-mash composition. Once it gets heated up, the layers will sort themselves out, with the high density stuff sinking to the middle and the low density stuff rising to the surface.
Now the planets that you make will depend upon what material is in the area. The inner solar system is full of rocky and metallic planetismals, while the outer solar system is dominated by gaseous and icy planetismals. The material comes together and makes bigger and bigger chunks. Now you have a great big glob of different planetismals - so why don't planets look like big mixed up globs of material? As these things come together and make up the planets, there is a lot of heat released by radioactive elements (I'll talk more about this stuff later), and this helps to heat up the interiors of the planets. When the rocks, gases and metals are heated and become sort of squishy they rearrange themselves, so that the high density stuff (the metals) sink to the center and the lower density stuff (the gases) rise to the surface. This is just like how some salad dressings separate their material - high density stuff sinks, low density stuff floats. To be really technical, we can say that the planet becomes differentiated once the stuff gets sorted out. That just means that the different elements get arranged according to density - high density in the center, low density on the surface.
While the planets were forming, some of the big ones, like the Jovians, may have done the same thing that the solar nebula did. By spinning fast, the clouds of material that would become something like Jupiter would form into a disk and then parts of the disk could form into small objects like moons. The big Jovian planets can be thought of as mini-solar systems as they make their satellites. They have so much mass that it is easy for them to form satellites. It is also possible that they can catch stray satellites - objects that formed elsewhere but get too close to the Jovians so they are trapped by their strong gravity into an orbit.
As planets (and satellites) start to take shape and their surfaces start to solidify, they will experience a rather nasty episode of the solar system cleaning up. During the first 500 million years of the solar system's history, there would have been a lot of big planetismals floating around that weren't incorporated into planets. When the planets and their moons had pretty much formed, these miscellaneous planetismals would have slammed into them. The earliest part of the solar system's history would have involved a lot of damage to the planets and satellites. This era is generally referred to as the Heavy Bombardment era. This would have been the time that the really big craters currently seen on the Moon and Mercury were made. It should be remembered that all planets, satellites, asteroids and other objects were hit by these planetismals at this time as well, but in many cases these impact craters were covered up or erased. The most intense part of the heavy bombardment era is thought to have occurred around 3.8-4.1 billion years ago. By this time the modern structures and physical characteristics of the planets were pretty well established and they ended up as targets of big planetismals. I should also mention that we think the formation of the solar system probably started around 4.6 billion years ago - this is a number that most astronomers and geologists are pretty happy with (I'll explain why in a little while).
Another cleaning up process involved the Sun. While planets and planetismals were doing their thing, there would still have been some stuff floating around that wasn't really part of a planet, or perhaps it was material that was ejected by a planet. This would have included a lot of hydrogen and helium that couldn't be parts of the terrestrial planets since they were too hot to hold onto them. Most of these light gases in the inner solar system were cleared out due to the strong solar wind as the Sun became a true star. The outer planets, however, are far enough away from the Sun that the solar winds didn't do much damage. Also, because the Jovian planets are so huge, they are able to hold onto their outer layers better (more mass, more gravity).
Figure 5. The various steps in the formation of the solar system. Top left - Gas and dust cloud - the solar nebula starts to contract. Top center - The protosun starts to form, pulling in most of the mass. The rotation of the disk increases due to angular momentum. Top right - The disk forms around the protosun, which is getting hotter and hotter due to contraction. Bottom left - The temperature of the Sun reaches a point where it will influence the area around it and start to blow the lightweight material from the inner solar system, leaving behind only the heavier dust and metals (mainly). Bottom center - The largest planetismals in the various locations will start to pull in more material, clearing out the areas around them. Bottom right - Eventually everything comes together to make the solar system we see today.
It appears from our analysis of this process that it seems to have occurred rather quickly, on the order of a few hundred million years. We also have some hints about this process by looking at other young star systems. If you go back to the star formation part of the course you might remember that we often see disks of material around young stars. These disks could be indicators of the start of the planetary system formation process. The disks are pretty big, so it isn't too difficult to see them, especially with techniques that let us block out the light from the star or viewing them with infrared telescopes. Quite a few stars have been found with some very tantalizing looking disks. Many have been observed with infrared telescopes like IRAS and Spitzer, while in some cases they can be seen with the Hubble Space Telescope. In some cases it appears that similar things like asteroid belts are also seen around other stars, as can be seen here. It also appears that the formation of planets can also lead to their destruction, which is the case seen here. A recent observation by the infrared Herschel telescope has revealed a water vapor rich disk of material around a star that is in the process of forming. Of course some of this water will form into icy objects, some will stay in a gas form and some can become liquid water - that will depend upon where in the disk the material is located.
Some recent observations of young stars find evidence of planet formation, in these cases around stars that are only a few million years old. The planets that appear to have formed would be the jovian types, which makes sense due to their large mass and gravitational pull. Also, observations of very small stars (brown dwarfs) appear to also show indications of the planet formation process, with dusty disks of material surrounding them. As more data comes in, we find that the process of solar system formation is pretty widespread and in some cases, fairly easy to do.
Figure 6. (click on to see larger image) Some examples of disks or rings of material around some stars, as seen by the Hubble Space Telescope. In both pictures, the light from the star is blocked out so that the dusty disk is easier to see - the blocked area is indicated by the large circle, while the star itself is pretty small. A scale for comparison is provided so that you can see how these systems compare to our solar system. Notice how the rings look sort of like the drawings in Figure 5! AU Microscopii Image Credit: NASA, ESA, J.E. Krist (STScI/JPL), D.R. Ardila (JHU), D.A. Golimowski (JHU), M. Clampin (NASA/GSFC), H.C. Ford (JHU), G.D. Illingworth (UCO-Lick), G.F. Hartig (STScI) and the ACS Science Team. HD 107146 Image Credit: NASA, ESA, D.R. Ardila (JHU), D.A. Golimowski (JHU), J.E. Krist (STScI/JPL), M. Clampin (NASA/GSFC), J.P. Williams (UH/IfA), J.P. Blakeslee (JHU), H.C. Ford (JHU), G.F. Hartig (STScI), G.D. Illingworth (UCO-Lick) and the ACS Science Team.
Now you are probably wondering how we know when all of this stuff happened. It's due to the presence of radioactive material - I told you I was going to explain this stuff. There are some things out there that haven't changed since they formed in the early days of the formation of the solar system. These are rocks that sometimes fall to the Earth and are picked up as meteorites. Meteorites can tell us a lot about the early solar system (unlike Earth rocks, which have been reprocessed many times and aren't "pristine"). When you look inside of a meteorite, you'll find, on occasion, some radioactive material. What's so important about this? First of all, radioactive material that is trapped inside of a rock must come from an energetic source, and the best candidate for that is a supernova. Also, the fact that the radioactive material was trapped inside of the rock fairly early in the history of the solar system indicates that the formation process was pretty quick. We know this because we find within meteorites short lived isotopes of aluminum 26 and iron 60 - if the supernova happened a long time ago, these isotopes would have decayed before being trapped in the meteorites. Most importantly of all, radioactive material can be used to determine the age of the solar system.
How is this done? We use the radioactive material's half-life to get the age of the material. The half-life is the time it takes for 1/2 of the radioactive material to decay, often into a non-radioactive form. For example, Iodine 129 decays into Xenon 129 with a half life of 17 million years. If there is a significant amount of Xenon 129 in a meteorite, then there must have been a lot of Iodine 129 trapped in the sample to begin with and there should still be some left. One thing about half-lifes is that you are always cutting the radioactive material in 1/2, so you never actually get down to there being no radioactive stuff remaining. If the rock were originally all Iodine 129 and only 1/8 of it is still Iodine 129, you know that it went through three half-lives (it was cut in half three times, which gives you only 1/8 of the radioactive material left). The rock would be about 3x17 = 51 million years old.
Not only is the radioactive material useful in determining the age of the rock, it also provides some valuable information by its very presence -

1. The formation of rocks took place rather quickly since a large amount of radioactive stuff was locked into these early rocks. If there was a significant time lag, then more of the stuff would have decayed and less radioactive material would be included in the rocks. This tells us that the solar system formation process was rather quick, maybe taking only a few million years.
2. There must have been a high energy source for the radioactive elements to be produced in the first place, before the solar nebula started the planetary formation process. The most likely source would be a type II supernova (based upon the composition of meteorites).

It is also possible that a supernova explosion occurred near the current location of the solar system. It is also possible that the explosion could have triggered the formation of our solar system! Remember, you need something to start a gas cloud collapsing - some sort of shock wave - and a supernova is a good source of not only the radioactive material but also the push needed to start the solar system forming. It is possible that the creation of our solar system was due to the death of another star! A recent study of the material expelled by supernova shows that there is more than enough material produced to create many, many planets like the Earth.

Other Solar Systems

Now that we've looked at how our solar system formed, we need to ask whether we got it right. Are we correct in our theories? The only problem with that is that we don't have much information about other solar systems (actually, we should call them planetary systems, since solar refers to our Sun). Are there any other planetary systems? Yes, there are. Actually, astronomers have found many planets outside of our solar system.
How do you do that? Actually, it isn't very easy. It is very difficult to actually see a planet next to another star, mainly because the light from the star would be so bright that it would be nearly impossible to see any little object next to it. The main method of finding planets is to see if the stars they orbit have small velocities visible in their spectra. This is sort of like the way that spectroscopic binary stars are detected - by looking at their changing spectra. The only difference here is that the star will not move very fast if it is being pulled by a puny (when compared to the star) planet. Astronomers look for velocities that are only a few m/s. In a spectrum, this corresponds to a blueshift or redshift of a fraction of an angstrom - and such a small shift is very difficult, though not impossible, to see.
Figure 7. Click to see full size image. A chart showing the orbital sizes and masses of some of the planets discovered outside of our solar system. The yellow dots represent the stars they orbit, and the blue dots represent the planets, though they are of course not to scale. The extra-solar planets' masses are given in terms of Jupiter's mass. The distances of the planets from the stars they orbit are given in A. U.s. Image from the Exoplanets Website.
Astronomers are so clever that they've come up with equipment and ways to accurately measure such small redshifts and blueshifts. There have been over 750 planets discovered so far (as of August 2013 - odds are it will be up to 800 by the time you read this). However, we should be careful in calling them planets. It is possible that some of the objects could actually be very small stars (brown dwarfs), but they usually have masses that are very similar to planets in our solar system. Most often the planet's masses are similar to Jupiter or Saturn (hundreds of times the Earth's mass) and are found within 5 AU of the star they orbit. If you click here you'll see a graph of many exoplanet masses plotted versus their orbital distances. It also shows the masses and distances of planets in our solar system for comparison. Only a few planets have been found with masses similar to the Earth or smaller. The smallest one discovered so far has a mass that is about 100 times less that of the Earth's. The masses of these objects aren't known precisely, since the tilt of their orbits makes it difficult to measure the actual size of their orbits. At this time, this method will only tell us about the most massive planets, since they will have greatest influence on the stars they orbit.
There was a breakthrough in the planet hunting area when the first picture of a extra-solar (outside of our solar system) planet was obtained. A planet was found around the star 2MASSWJ1207334-393254 (or 2M1207 for short). In this case the planet is large enough to be visible to earth-based telescopes. Images of the planet were obtained using the VLT as well as the Hubble Space Telescope. In a couple of other cases planets have been "seen" when they have passed in front of or behind the star they orbit, causing a very small eclipse of the star's light. In 2008 the Hubble Space Telescope spied a planet around the relatively bright star Fomalhaut, which also has a dusty disk around it. And at the exact time that discovery was announced, the folks at the Gemini observatory proved an image of 3 planets around another star!
While most exoplanets have been found by looking at the changing spectra of a star, it is possible to find some planets by looking for them to eclipse the star they orbit. In 2009 a spacecraft was launched which had the job of looking for such objects. The Kepler mission has already shown that it can find small objects around stars by carefully measuring the light variation caused by a planet's passage in front of the star it orbits. In August 2009, one of the first tests of the spacecraft showed that it could observe passages of even small earth-like planets in front of stars. The press release about that event is here. In March of 2011, the Kepler project announced the discovery of 1,235 candidate planets around various stars. The graphic available here shows the stars to scale, with the black dots representing the relative sizes of the planets that were possibly discovered. The lone star near the upper right is the Sun with the black dot representing Jupiter. One of the most surprising aspects of the Kepler mission is that it is only looking at a small part of the sky - about the area of your hand held at arm's length. In 2012 the gyroscopes that help stabilize and point the telescope failed, and it looked like the mission was over. But NASA is pretty clever, and they've found a way to extend the Kepler mission. They will be pointing the telescope at various places in the sky, using the light from the sun to help point it - yes, that is something that can be done. So the mission may not yet be over, and more planetary discoveries from Kepler will likely be announced.
The results from these planet searches are a bit confusing, though. Many of these large planets are found in orbits that are much closer than the Jovian planets in our solar system. How is that possible? Shouldn't only small planets be found near a star? In most cases all we know about the objects are the masses - not what they are made out of. In one case astronomers were able to estimate the density of the planet and found a value that is between that of water and rock. It is thought that this object (Gliese 436) may be made of rock, gases, and liquids in a manner that would survive around the star that it orbits. Also, many of the planets have very elliptical orbits. Why aren't their orbits circular like the planets in our solar system? There must be some way that these large planets can form close to a star. We're still working on that one. Regardless of these little dilemmas, it is still sort of neat knowing that there are quite a few planetary systems out there, so that it is possible that we are not alone!
Probably one of the most intriguing result to come from the planet searchers was the discovery of a planet around Gliese 581. This planet is located at a distance around its star that is within the star's habitable zone. This zone marks the region where water on the surface of a planet can exist as a liquid. In our solar system the zone goes from around the area of Venus out to around Mars, with the Earth right in the middle. Scientists generally view the existance of water in a liquid form as a requirement for life to exist. Personally I would have thought that coffee was more important, but that's just me. Remember, just because the planet around Gliese 581 is located within the habitable zone though doesn't mean there is life on it - it just means that water could exist as a liquid. Be careful when you read such announcements in the news about major discoveries - just because they say water could exist, doesn't mean it actually does exist on the surface of this planet. The Kepler mission has fournd nearly earth-sized planets that are located in Habitable Zones of stars similar to our Sun, but of course just because a planet is found in the correct location doesn't mean it has water on the surface. Just be on the look out for more news in the future about such systems.
While liquid water has yet to be confirmed on a planet outside of our solar system, it does appear that water in other forms exists on planets or around other stars. The first indication of this was in the spectra of a planet around the star HD209458b, which appears to show water in the planet's atmosphere. Several other worlds have been observed with water vapor in their atmospheres. While this may seem exciting, it isn't really unexpected, since water is a fairly common molecule in the Universe and we would expect to see in many locations in its various forms. Observations of a white dwarf GD 61 show evidence of asteroids containing water around it. While asteroids aren't planets, this does seem to indicate that water-based objects are present around this star, and perhaps before the star became a white dwarf there may have been planet like objects around it. Who knows? Obviously you wouldn't want to be on a planet around a white dwarf since they give off so little light, unless you like to be frozen.
No matter how you look at it, this is certainly an exciting time for planetary exploration! 2014 was a major year for discoveries, as this graph shows, but expect many more discoveries in the future.

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Now that you've read this section, you should be able to answer these questions....

• What are the currently defined criteria for an object to be considered a planet?
• What type of object is Pluto?
• What are the different characteristics of terrestrial and jovian planets?
• What observational characteristics of our solar system provide us with clues to how it formed?
• What caused the density gradient in the solar system?
• What role does radioactive material play in the formation of our solar system?
• How do astronomers search for other planetary systems?