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From Planets to Stars

Now let's examine some of the larger objects that one can find in a solar system: ice giants like Neptune, gas giants like Jupiter, and companion stars.

Gas Giants

Gas Giants are planetary bodies with a very large and thick atmosphere of gases. In our solar system we have two examples: Saturn, at 95 times Earth's mass, and Jupiter, at 318 times Earth's mass. Jupiter is heavier than the entire rest of our solar system combined, and is as wide as 11 Earths side by side. Uranus and Neptune are sometimes also considered gas giants, but may more properly be considered ice giants (see below).

We know from observations that hydrogen and helium dominate the atmospheres of gas giants. These are the two lightest elements in existence, so how can they make up the heaviest planet in our solar system? The answer is volume: anything in bulk gets heavy. The gasses deep down in these planets are also very highly compressed, and can be much more dense than the gasses we breathe every day - even more dense than water.

The structure of the gas giants is also different than that of the terrestrial planets. If you consider Earth's atmosphere relative to the size of our planet as a whole, the atmosphere is about as thick, proportionately, as the skin on an apple. Conversely, gas giants have fairly small cores surrounded by very thick atmospheres. Computer modeling indicates that their cores are likely to be surrounded by thick layers of hydrogen that is so densely compressed that it becomes a liquid metal.

People sometimes refer to Jupiter as a "failed star," but this is incorrect. Brown dwarfs are the objects commonly referred to as failed stars, and are heavier than gas giant planets, but lighter than true stars. The distinction between brown dwarfs and stars is that stars have central temperatures and pressures high enough that hydrogen fusion occurs in their cores. Brown dwarfs are not massive enough for hydrogen fusion to occur. The line between gas giant planets and brown dwarfs is a bit hazier. Several definitions have been proposed, but it is generally agreed that brown dwarfs must be greater than about 13 Jupiter masses.

Many exoplanets have been found that are more massive than Jupiter - in fact, these "large, hot Jupiters" or "super-Jovian" planets were among the first exoplanets discovered. There seems to be a continuum of gas planets from the size of Saturn, through Jupiter and the super-Jovian planets, to the smallest of brown dwarfs. As you will see later, it is easier for us to find heavy planets than it is to find light ones.

Ice Giants

We used to think that all gas giants were essentially the same: balls of hydrogen and helium, perhaps with a rocky core on the inside like the terrestrial planets. As we learned more about the planets in our solar system, we found that the outer gas giants, Uranus and Neptune, were a little different from Jupiter and Saturn.

Uranus and Neptune have substantially different atmospheres from Jupiter and Saturn. While hydrogen and helium are still common at high altitudes, lower altitudes include more methane (which gives the planets their blue color), as well as ammonia, water, various sulfides, and other heavier gases.

The cores of the ice giants may be even more interesting. With their higher percentage of carbon-bearing molecules like methane, these planets may have a core with a much higher carbon content than other planets. Based on the melting properties of diamond at the extreme pressure of Neptune and Uranus, one hypothesis is that their interiors contain oceans of liquid diamond, with solid diamond "icebergs" floating on them.

Companion Stars

Our sun is a fairly ordinary star for its type, except for one thing: it has no companion star. Most stars of its type in our galaxy (and elsewhere in the universe) are part of a binary or trinary star system, where two or three stars orbit one another.

Most stars, perhaps over 90% of them, are red dwarfs much smaller than our sun. Recent research indicates that dimmer stars, such as red dwarfs, seem to be singletons more often. Astronomers are still trying to verify this research and create models to explain the observations. Nevertheless, among brighter stars, double- and triple-star systems seem to be more common. This research is still under much debate.

Whether multiple-star systems are more or less likely to have planets is still an open question - but with more and more planets discovered every day, we will likely be able to answer this question in the next ten years. There have been many observations of circumbinary planets in recent years.

Now that we've had a brief survey of the different sorts of planets and minor bodies that one finds in our solar system, you will have a better basis for understanding the exoplanets that we talk about later.

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