Intro to Astronomy Assignment

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Investigating Astronomy
Timothy F. Slater, Roger A. Freedman

Chapter 4

Exploring Our Evolving Solar System

Welcome to chapter 4: Exploring Our Evolving Solar System. This week we're going to study our solar system in the big picture view by looking at the major components: the planets, asteroids, trans-Neptunian objects, and comets. We’ll look at how our solar system probably formed and how this suggests that planetary formation is common.

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Comparing the Planets: Orbits

  • The Solar System to Scale*
  • The four inner planets are crowded in close to the Sun.
  • The four outer planets orbit the Sun at much greater distances.

*Planets are not to scale!

This diagram shows the orbits of the eight major planets of our solar system to scale. Please note, however, that the size of the planets themselves at this scale would be microscopic, so this is only a scale of the relative sizes of their orbits around the Sun. There are four inner planets that are all very close to the Sun: Mercury, Venus, Earth, and Mars. Then there are four outer planets whose distances from the Sun are significantly greater: Jupiter, Saturn, Uranus, and Neptune.

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Comparing the Planets:
Size and Composition

  • Inner planets: rocky materials with dense iron cores and high average densities
  • Outer planets: primarily light elements such as hydrogen and helium, low average densities

Now let's compare the planets by size and composition. The important thing to note here is that the groupings remain the same, whether we compare orbit size, planet size, or composition (or even, as we’ll see in a moment, number of moons). The inner planets are all relatively small and are made largely of rock with dense iron cores. Their average densities (the mass divided by the volume) are high. The outer planets, on the other hand, are all very large and are primarily gaseous (mostly hydrogen and helium, though with some other elements). Their average densities are low.

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Moons are Natural Satellites

  • All planets have moons, except Mercury and Venus.
  • The outer planets have many more moons than the inner planets.
  • Seven satellites are almost as big as the inner planets.

The word moon is applied to any natural satellite of a planet or other solar system body (aside from the Sun). The inner planets have few if any moons. Earth, of course, has one. Mars has two. But Mercury and Venus have none. The outer planets all have many moons (numbered about in the dozens), with a few being the same size as our Moon or larger, a couple almost rivaling the size of the planet Mercury.

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Determining Composition:
Bodies with Surrounding Atmospheres

Some of these planets and moons have atmospheres. How can we determine what those atmospheres are made of? We can use spectroscopy to look for the chemical fingerprints of different elements and molecules. Saturn's moon Titan has a very dense atmosphere (so much so that we had to send a radar and a lander there to get a view of the surface… we'll see more about that in chapter 7). When we look at Titan from Earth, we are seeing sunlight reflected off of the atmosphere. If we look at Titan with a spectroscope, we see absorption lines (remember those are dark lines against a bright background of the spectrum) for hydrogen, molecular oxygen (O2), and methane (CH4). However, not all three 3 of those are actually in Titan's atmosphere. Remember that it is the core of a star that is the hot, opaque body that emits a continuous spectrum. That continuous spectrum passes through the atmosphere of the Sun, where there is hydrogen. So, those hydrogen atoms will absorb some of the light of the spectrum leaving behind their fingerprint before the light has even left the Sun. The light then travels to Titan, reflecting off of its atmosphere. In the process, some of the photons will be absorbed by the molecules of methane, leaving their fingerprint on the spectrum. The reflected light will then continue on, and some of it will arrive at Earth. But before it gets to our spectroscope, some of it is absorbed by the oxygen in our atmosphere, and so the oxygen also leaves its fingerprint.

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Determining Composition of Titan

  • Dips in the spectrum of sunlight reflected from Titan are due to absorption by hydrogen atoms (H), oxygen molecules (O2), and methane molecules (CH4).
  • Only methane is actually present in Titan’s atmosphere.
  • Astronomers must account for the absorption that takes place in the atmospheres of the Sun and Earth.

When scientists examine the spectrum, they will see something like the diagram we see here. This is another way of viewing a spectrum. Instead of seeing the rainbow of colors with dark lines, we see a measurement of intensity at each wavelength of light. Intensity is basically how many photons of that wavelength are received by the spectroscope. If more photons are received of a particular wavelength, the graph is higher at that point. The spectrum will have a curved shape, in general, with a peak matching the wavelength of maximum emission as we talked about in chapter 2. But against that general curve, we will see the dips marking absorption lines. The narrow dip at a bit under 660 nm is an absorption line marking the presence of hydrogen. But as we saw, the spectrum acquired that absorption line before the light even left the Sun. We also see a narrow dip at just under 690 nm. That is the fingerprint of the oxygen molecules in Earth's atmosphere. Then we see two wide absorption lines that are the fingerprints of methane molecules. From this, we know that Titan's atmosphere is composed of methane. If it also had hydrogen and oxygen, we wouldn't know that from this spectrum, because we have to account for the absorption that occurs in the atmospheres of the Sun and Earth during the light's journey. This is one of the reasons that we put telescopes in space and send spacecraft to other planets, as we get above the interfering effects of the Earth's atmosphere.

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Determining Composition:
Planets without Atmospheres

  • Spectra of reflected light is compared to known substances.
  • Infrared light from the Sun, reflected from the surface of Europa, has almost exactly the same spectrum as sunlight reflected from water ice.

We can also look at the spectra of light reflected from solid surfaces, not just those with atmospheres. When light shines on solid surfaces, some wavelengths are absorbed and others are reflected. For example, leaves on Earth look green because the leaves absorb red and violet light and reflect green light. Our eyes see only the reflected green light. The spectrum of a solid surface looks a little different than the spectrum of a gas, as the spectrum of a solid surface will show broad absorption features, rather than narrow lines and bands. We compare the spectra we see from the solid bodies we look at with known spectra from Earth. In the case of the spectrum on this slide, which is from Jupiter's moon Europa, we see that it looks pretty much like the spectrum that we know to be from water ice. So, the surface of Europa is water ice.

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The Jovian Planets Are Made of Light Elements

When we analyze the spectra of the outer planets: Jupiter, Saturn, Uranus, and Neptune, we see that they are composed mainly of the lightest elements: hydrogen and helium (mostly hydrogen). Uranus and Neptune both contain methane, also. These elements are all in gaseous form on the exterior of these planets, and so they are often called the gas giant planets. However, in the interiors of these planets, the pressure is so high that these gases are actually in liquid form. It is also thought that each of these four planets has a rocky core.

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Asteroids

  • 100,000+ rocky objects within the orbit of Jupiter
  • Most orbit the Sun at distances of 2 to 3.5 AU, in the asteroid belt
  • Orbit the Sun in the same direction as the planets
  • The largest, Ceres, has a diameter of about 900 km (560 mi)
  • Also called minor planets

Let's move on to the smaller objects in our solar system. First, the asteroids. There are more than 100,000 of these that orbit mainly between Mars and Jupiter, though some are closer to the Sun than Mars, and some are out as far as Jupiter. The area between Mars and Jupiter, at a distance between 2 to 3.5 Astronomical Units is called the asteroid belt. They orbit the Sun in the same direction as the planets, which tells us that they formed along with the planets. The largest asteroid, Ceres, now has the designation as a dwarf planet (along with Pluto and others that we'll talk about soon). The asteroid in this picture is 433 Eros (all the asteroids, also called minor planets, have both a number and a name…the number is sequential in the order of discovery, and the name is chosen by the discoverer with approval of the International Astronomical Union).

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Trans-Neptunian
Objects

  • 1,000+ small bodies orbiting beyond the orbit of Neptune
  • The largest of these are known as dwarf planets
  • Most orbit within the Kuiper belt at 30 AU to 50 AU
  • Include Pluto, Eris, Charon, Makemake, etc.

Another group of small solar system bodies is called trans-Neptunian objects (trans-Neptunian just means “beyond Neptune”). This group contains at least 1000 objects orbiting beyond Neptune. The largest of these are known as dwarf planets, and most orbit within the Kuiper belt. The most famous of these is, of course, Pluto. Until a few years ago, Pluto was considered the ninth planet of our solar system. However, as we’ve learned more about our solar system, it has become clear that it doesn’t fit into that category. As we’ve seen in this chapter, the four inner planets share characteristics such as being close to the Sun, being made of rock with iron cores, having few if any moons, and having relatively high average density. The four outer planets share characteristics such as being further out from the Sun, being made primarily of light elements with iron cores, having many moons, rings, and relatively low average density. Pluto does not fit into either of those groups. It has a much more eccentric orbit around the Sun, and its orbit is inclined (or tilted) significantly with respect to the other planets (as you can see in this diagram). It is made of ice and has four moons. It does, however, fit into the group of trans-Neptunian objects quite well. They are all icy and have eccentric, high inclination orbits. The decision to change Pluto’s classification was made following the discovery of Eris, another trans-Neptunian object. Eris was thought at the time to be bigger than Pluto, so a decision had to be made whether to classify Eris as a planet or to create a new classification to include both bodies (and others). As more large trans-Neptunian objects were found (including Makemake, Haumea, and others), it became clear that there could potentially be lots more. The decision to change the classification of Pluto was controversial, but the most important things to remember are that a) reclassification happens in science a lot when new information is acquired, and b) it doesn’t change the fact that Pluto is still there, is still a fascinating world to investigate, and the New Horizons spacecraft flew by it in 2015! The New Horizons images have actually shown us that Pluto is, in fact, bigger than Eris, and so it now holds the title of “King of the Kuiper Belt.”

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Comets

  • Objects that result when Kuiper belt objects collide
  • Fragments a few kilometers across, diverted into new and elongated orbits
  • Oort cloud comets orbit out to 50,000 AU
  • The Sun’s radiation vaporizes ices, producing tails of gas and dust particles
  • Astronomers deduce composition by studying the spectra of these tails created by reflected sunlight

Comets include both objects from the Kuiper belt and objects from even further out, in an area called the Oort Cloud. These objects can get knocked into elongated orbits as a result of collisions or other gravitational disturbances, and these elongated orbits bring them periodically into the inner solar system. Comets are made mostly of ice. When they pass inside the orbit of Jupiter, the Sun’s radiation is enough to vaporize the ice on the surface, producing tails of gas and dust particles. The solar wind, a stream of charged particles flowing out from the Sun, pushes these tails away from the Sun. So, no matter which way a comet is moving, its “tails” are always pointed away from the Sun. We can learn about their composition by analyzing the spectra of the tails. We’ve also sent spacecraft to image them up close, to collect samples from the vaporized ices and return them to Earth (the Stardust mission), and even to crash into a comet to see what materials would come up (the Deep Impact mission).

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Cosmic “Recycling”

  • The Big Bang produced H and He (some Li and Be)―still common
  • All heavier elements created by massive stars, dispersed when stars die
  • Our solar system is recycled “star dust”

How did our solar system form? Well, we’re going to cover some of these in later weeks of the course, but the Big Bang produced light elements. All of the hydrogen in the universe was produced then, as well as a lot of the helium, and some lithium and beryllium. But there’s lots more on the periodic table of the elements than just these four lightest elements. All the rest of the elements are created inside of massive stars, and are dispersed into neighboring clouds of gas and dust when those stars die. Those clouds of gas and dust then become home to the next generation of stars. These next generations of stars are then made of more heavy elements than the previous generations. After several generations, there are enough heavy elements to create planets. Our solar system, therefore, is made of recycled “star dust.”

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The Solar
Nebular Hypothesis

  • A cloud of interstellar gas and dust contracts because of its own gravity.
  • The cloud flattens and spins more rapidly around its axis.
  • A central condensation develops that evolves into a glowing protosun.
  • The planets form out of the surrounding disk of gas and dust.

The solar nebular hypothesis, which is the idea of how planetary systems form, states that a slowly spinning cloud of interstellar gas and dust begins to contract when a knot of material gets massive enough for its gravity to start pulling on nearby material. As the cloud flattens and condenses, it begins to spin more rapidly. The central region that is denser eventually evolves into a protosun. The surrounding disk of gas and dust eventually forms planets, as collisions gradually form larger and larger objects.

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Protoplanetary Disks

  • Rapid rotation flattens the nebula.
  • ~100,000 years after contraction begins, a rotating, flattened disk surrounds what will become the protosun.
  • Also called a proplyd, planets form from its material.
  • Explains why orbits all lie in the same plane, in the same direction.

The rapid rotation in this disk is what causes the nebula to flatten. This object, a flattened disk surrounding a protosun is called a proplyd, or protoplanetary disk. We see these proplyds in the sky (a lot of them in the Orion Nebula). This solar nebular hypothesis explains why orbits in our solar system all lie essentially in the same plane, orbiting the Sun in the same direction.

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Temperatures in the Solar Nebula

  • Temperatures varied across the solar nebula as the planets were forming.
  • A general decline in temperature with increasing distance from the center of the nebula.
  • Beyond 5 AU from the center of the nebula, temperatures were low enough for water to condense and form ice.
  • Beyond 30 AU, methane (CH4) could also condense into ice.

The temperature varied throughout the solar nebula during the planetary formation stage, gradually declining as distance from the center increased. Beyond 5 Astronomical Units out from the center, temperatures were low enough to form water ice (that’s why we see ice on the moons of the outer planets). Further out, beyond 30 AU, methane also condenses into ice (we see methane ice on the trans-Neptunian objects.

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Planetesimals Become Protoplanets,
then Rocky Planets

Since we can’t actually watch a solar system forming, scientists can use computer simulations to test out hypotheses. These diagrams show the results of one such simulation. It starts with 100 planetesimals (small bodies) orbiting the Sun. As these objects collide, more massive objects form. After about 30 million years, there are 22 larger planetesimals, and after a total of 441 million years, there are four planets. Our solar system’s protoplanetary disk started out with many more than 100 planetesimals, but the simulation does show the hypothesis stands up.

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Outer Planet Formation:
Capturing an Envelope of Gas

Cold, slow moving gases were gravitationally attracted to the Jovian planet cores.

How did the outer planets become so different from the inner planets? Out at their distance, cold, slow moving gases were attracted to the planetesimals there by gravity. The planetesimal continued to attract both rocky and gaseous material until the rocky mass and gaseous mass were about equal. At that point, it would actually have been able to much more rapidly attract more gas, eventually growing significantly with a very thick hydrogen-rich atmospheric envelope.

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Final Stages of Solar System Evolution

  • Our unstable young Sun ejected its thin outermost layers into space―a brief but intense burst of mass loss called a T Tauri wind.
  • The T Tauri wind swept the solar system nearly clean of gas and dust.
  • The planets stabilized at roughly their present-day sizes.

In the final stages of the evolution of our solar system, the Sun was still quite unstable, throwing bursts of material out into space. This is called a T Tauri wind. The name of this is based on the first such object seen, a protostar in the constellation Taurus the Bull, called T Tauri. In a young solar system, this T Tauri wind sweeps the rest of the solar system clear of leftover gas and dust (leaving behind more massive objects). It was at this point that the planets stabilized at about their current sizes. Check out page 108 in your text for an overview of this whole process: from a rotating cloud of gas and dust into a solar system. Since we see things like proplyds and T Tauri stars elsewhere in our galaxy, it reinforces the idea that planetary formation may be common. Further enhancing this idea is the discovery over the past decades of extrasolar planets: planets orbiting stars beyond the Sun.

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Searching for Extrasolar Planets (or Exoplanets)

Astrometric Method

Radial Velocity Method

The first planets around other stars were found in the mid-1990s. Since then, the number of exoplanets (also called extrasolar planets) has grown significantly. The number of candidate planets (not yet confirmed) is in the thousands, and the number of confirmed planets is growing too rapidly to bother mentioning a number here, as it’ll be outdated before you read this. Check out the website www.exoplanets.org for the current numbers.

For the most part, these planets have been discovered because of their influence on their parent stars. The first exoplanets were discovered because of their gravitational interaction with their star. A star with a planet (or planets) orbiting it will orbit a common center of mass. We can’t see the planet (or planets), as they do not give off any light of their own and the light of their star washes out any view of reflected light off the planets. But we can see the star appear to wobble. If the motion is across our line of sight, we can detect this by seeing the star visibly wobble as we measure its position in the sky very precisely. This is called the astrometric method. If the star’s motion is towards and away from us, then we can use the radial velocity method and look for alternating redshifts and blueshifts in the spectrum as the star moves. Remember that an object moving closer to us will have spectral lines that appear blueshifted (shifted towards shorter wavelengths), and an object moving further away from us will have spectral lines that appear redshifted (shifted towards longer wavelengths). So, both the astrometric method and the radial velocity method are measuring the gravitational influence of planets on their parent stars, but they measure a different component of the motion. These methods have been mostly finding very large planets (around the size of Jupiter and bigger) that are relatively close to their stars (remember that the gravity is stronger with large objects close together).

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Searching for Extrasolar Planets (or Exoplanets)

Transit Method

Image credit: http://www.euhou.net/

Another method for finding extrasolar planets is the transit method. If we are looking at a star system edge on, sometimes the planet and its star might line up from our perspective. When the planet moves in front of its star, it blocks a tiny portion of the star’s light. It might be a tiny portion, but we are still able to detect the small dip in light. If we see this happen multiple times, we can confirm that there is a planet (or planets) there, and we can also calculate the orbit(s). This is the method that was used by the Kepler spacecraft, which stared continuously at more than 100,000 stars in a small patch of the summer sky, waiting for that tell-tale dip in a star’s light. Thousands of planets have been found in the Kepler data. Back when I was in college, we had one solar system to look at and could only speculate about finding more. We could guess that others had formed, but we had no proof. We now know that planetary formation is common, and we are likely to continue to find many more extrasolar planets, especially as the technology improves, allowing us to find smaller planets and also planets that are further away from their parent stars. In early 2013, the Kepler satellite even found a planet smaller than Mercury (about 1/3 the size of Earth)!

Speaking of Earth…next week in chapter 5, we will look at the Earth itself. That’s it for chapter 4. See you next time.

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