Theme 7.2 -- The Complete Solar System Expected Outcomes of the Formation Process In the previous unit we considered the nebular hypothesis for the formation of the planets. At the end of the episode of planet formation, we might expect to see certain things in the solar system. First, leftover gas that had not participated in the formation process. Second, evidence of large collisions that took place late in the process as the biggest lumps were merging finally to produce the planets. Finally, leftover rubble that might date back to the time of the formation stage: unaccreted lumps that never did make it into the planets. In addition, we'd expect to find evidence that it all happened about the same time. This will require us to develop tools for measuring the age of the different parts of the solar system. Furthermore, if the nebular hypothesis is correct, we'd expect to see evidence of planetary formation around other stars. This could take two different forms: first, we might see gaseous discs surrounding young stars in which we would interpret the formation process to be continuing: planets being formed as we watch. Secondly, we might expect to find large numbers of established planetary systems comparable to our own solar system. On the other hand, we have to be aware of the possibility that things may have slowly changed since the formation process itself. The solar system may not look entirely now as it did at the time of formation. Some of the evidence may have been eradicated, or there may have been slow, gradual changes in the arrangement. These possibilities must be considered. Leftover Gas: the Earth s Atmosphere The first and most obvious of these is the question of leftover gas. We do not see the planets moving through a gaseous medium -- even the innermost planets, where we argued that not all the gas would have condensed. What happened to the leftover gas? Why is it no longer present? The solar system is largely a vacuum. Where did that gas go? Pertinent evidence comes from our study of what are known as T Tauri stars, named after a prototypical star in the constellation of Taurus. These are young stars of about the same mass as the sun, and they are observed to go through an early stage where they have enormously strong stellar winds. The present sun has a modest solar wind which is responsible for things like the Northern Lights, but the stellar winds, the T Tauri stars are enormously greater, and would have swept out all of the leftover gas, and would even have scoured off much of the earth's primitive atmosphere.
A detailed study of the composition of the earth's atmosphere reveals that it is secondar,y created somewhat later after the formation of the planet itself by outgassing from volcanos and from the gradual accumulation of the vaporization of incoming material, including grains and pebbles, comets that are rich in icy material and so on. Indeed it is even possible that much of the water now on the earth was carried in by icy bodies such as comets. Not only is the earth's atmosphere secondary, but it has also undergone considerable evolution over the billions of years since the planet itself formed. One of the main reasons for this, of course, is the emergence of life, several billion years ago, which led to the absorption of carbon dioxide from the original atmosphere and the production of free oxygen which is now present in 20% abundance and support our existence. The Bombardment History The nebular hypothesis suggests a second observable consequence of the planet formation process. In the early stages, small pieces would emerge together relatively quietly, but towards the end as the final planets are emerging, there might have been substantial major collisions between rather large lumps, and we could look for evidence of such events. On the earth, we undergo active geology and weather, and that quickly erodes away any evidence of impacts, but we can look at more primitive surfaces as on the airless moons and the planet Mercury to see what we can determine from the evidence in the form of the size, number and distribution of impact craters that write the bombardment history of the solar system. The best evidence for this comes from our study of the moon, because we've managed to bring back lunar rocks which can be age-dated very carefully to pin down the timescale of the various episodes. The diagram here shows that in the early solar system the bombardment rate was very high, gradually dwindling down as the pieces accumulated, but then about 3.8 or 3.9 billion years ago, there was an episode known as the late heavy bombardment. Since then it has levelled off, although of course, there are still many pieces in orbit that could cause catastrophic impacts yet. Major Collision Events In addition to these generalized remarks about the bombardment history in the solar system, we now recognize certain circumstances that may be attributable to some individual large impact events. For example, the fact that Venus spins in a retrograde direction maybe attributable to a late impact with an object of fairly considerable size, and likewise the fact that Uranus is tipped on its side as it spins. Indeed it is now generally believed that the formation of our own moon maybe attributable to a major impact event between the earth and a Mars-sized piece as we'll see on the next panel. The earth is unique among the terrestrial planets in having a moon that is rather large compared to its own size, and one of the ways of understanding this is in the model shown here, in which there's an
early impact between the earth and a Mars-sized object coming in a fairly high velocity. This leads to the tip of the earth, and the formation of the moon in the way that you see it here. As noted, this is a plausible but not yet compelling argument, with evidence that depends on our detailed understanding of the chemical composition of the moon and the earth itself. We won't go into the details here, however. Leftover Material A last consideration from our nebular model is to recognize that not everything would have gone into the planets. There must be leftover pieces, and indeed that's what we see: the solar system is full of meteoroids, comets, asteroids, small objects out beyond the orbit of Pluto, and so on. This will be part of our discussion in a later theme. Ages Finally, we recognize that one of the clear implications of the nebular model is that the planets, comets, meteors, asteroids, and so on should all have roughly the same age. We'll explore that in the next section where we talk about age-dating techniques and the age of the solar system. Extra-Solar Planets If the nebular model is right, then we expect other stars to have planetary systems in abundance. As noted, these could be systems that are forming now, or systems that are long-established. Let's first ask whether we see evidence of protoplanetary discs around nearby stars. Disks of Gas and Dust Remember, that this material will be cool, not glowing in the visible at all, so we will have to use infrared or even radio radiation to hunt for such discs of gas and dust around nearby stars. The first such success came more than 30 years ago, thanks to observations made by the Infrared Astronomical Satellite IRAS. Here we see the star Beta Pictoris. The light from the central star has been blocked out here, but we can see the extended disc of gas and dust glowing in the infrared. Not much detail if visible, but the detection seems clear cut. We have much better technology now and can observe in considerably more detail. Here we see the star HL Tauri, which has an estimated age of about a million years. This is as seen by the new ALMA telescope in Chile, which works at millimeter wavelengths. Notice the disc of cool material, gas and dust; there are gaps where protoplanets are inferred to be sweeping up the material as this planetary system forms. Established Planetary Systems
The nebular hypothesis also tells us that there should be established planetary systems around many nearby stars. Here, we consider various detection techniques that could lead us to those discoveries. We might hope to get direct images of planets as dots of light. This is very challenging for planets shining by a reflected light next to a bright parent star but it is possible for a few. As the planet orbits thanks to Newton's Laws we know that the star itself should wobble in response. The wobbling of the star can be measured in the fore-and-aft direction as the star moves towards and away from us, thanks to the Doppler shift in the spectral lines, but it is not yet possible for us to measure the side-to-side motion of the star in response to the planet's presence. And finally, if a planet should pass briefly in front of a star in a transit, we expect to see a dimming of the star, and of course if the planet is in regular orbit we would expect that to be a repeated periodic effect. Direct Imaging Direct imaging of planets is very challenging, as noted. It's comparable to looking for a firefly hovering beside a searchlight. The planets are shining by reflected light and only a little will be cast in our direction compared to the bright star. Here, though, we see a system in which the light from the star itself has been masked off and three planets are indeed visible. Such examples are so far very rare though. Velocity Wobbles In an earlier unit we talked about how we detect the presence of planets through the velocity wobbles as measured by the Doppler shift in the absorption lines. On the left, we see a reminder of why this happens. The planet is making a large orbit around the common centre of mass, the star wobbles a little and is sometimes approaching us, sometimes receding. On the right, we see the observed change of velocity periodically as the star moves back and forth from our perspective. Transits Earlier, we explored the use of transits in the same respect. On the top left we see what is happening, with the planet moving across the face of the star and blocking off some of the light. We do not see that detail but we notice the dimming as shown on the bottom left where there's loss of a little over a percent of the light for a short period of time. On the right-hand side we see how much better we do from space where we don't have the problem of the variable earth's atmosphere confusing the measurements. The bottom part of that diagram shows the measurements that are made with the Kepler telescope in space that has been studying many hundreds of thousands of stars for a long period of time.
Here indeed is a reminder of the operation of the Kepler space telescope, which was put into space to look fixedly in one direction to study more than 100,000 stars continuously to try and detect transits. On the right, we see several transits of particular stars showing the duration and the dimming of the light, and the link at the bottom leads you to a very nice brief animation in which we see how Kepler can detect multi-planet systems, where the flickering of the star tells us about the presence of 5 or 6 planets in a single system. The discovery of so many planetary systems is very strong evidence that the nebular hypothesis is correct, and that most stars will have abundant planets surrounding them. In this panel, we see some particularly interesting planets: those that are not very much bigger than the earth and may be in regions around the parent star in which they could provide habitable zones for life forms to exist, where it's neither too hot nor too cold. This is a topic we'll return to when we talk about the search for extraterrestrial life. Selection Effects So, we've discovered planetary systems around many stars but we should be aware of very strong selection effects -- that is to say certain biases that are going to influence the kinds of planets we can detect and constrain our ability to draw general conclusions. For example, planets that are big in size are the easiest to find because they block off more light during transit. Planets that are large in mass are likewise easiest to find because they make the parent star wobble more in velocity. Finally, planets that are closer to the star are easiest to find because they're more likely to produce a transit, and they also have stronger gravitational effects if they're close. This means that big, massive planets close to the parent stars are the most easily picked up, at least to begin with in these studies. Given all that, it's not surprising that many of the planetary systems that we've detected so far contain large, massive planets quite close to the parent stars. It will take many years and improving technology to allow the confirmed detection of a solar system very much like our own. In the top panel we see a summary -- now a little dated -- of the early detections of extra-solar planets. We see that indeed the big planets are very much in the dominance here, but that is slowly changing with time, as we discover more and more small planets, which requires high sensitivity and time. In the bottom panel, we see a particular system known as Upsilon Andromedae where there are three planets that are comparable to or larger than Jupiter in size, and yet they're all relatively close to this parent star. At the bottom of that panel we see our own solar system for comparison. Planetary Migration It's not surprising that we are able to find the big planets, but the real question is why are there so many Jupiter-like planets so close the parent stars? Consider Upsilon Amdromedae, the system we looked at on the last panel, for example. In the nebular model, we expect only small rocky planets to form in that region. It's so hot there that only a fraction of the initial material can condense, so the inner planet should not be huge -- and yet here we find three planets
comparable to or larger then Jupiter very close to the parent star. How can that be explained? From the study of such systems, a new understanding has emerged. We now realize that complex gravitational interactions between planets can cause them to migrate, that is to say, to move around in a planetary system over the passage of many millions of years. Small planets may even be ejected from the system entirely. So, the system as we see it now is not as it was at the time of formation: planets have drifted in their positions. Computer modelling now suggests that effects of this sort may have been important in our own solar system as well. The big outer planets may have migrated to some extent, and there would have been important effects on the orbits of the asteroids, the many small objects beyond Neptune, and possibly Uranus and Neptune themselves. By good fortune, however, the earth's orbit has been relatively stable, and life on earth has survived, of course, we would not be here to discuss it if the situation were otherwise. Other planetary systems may not be so fortunate. So, this raises yet another consideration in our hope of detecting extraterrestrial lifeforms elsewhere. They may, like us, need to be lucky survivors. An important goal in our study of extrasolar planets is to be able to isolate the light of a single planet orbiting a nearby star and look for spectroscopic signatures of atmospheric components that suggest that life may be present -- for example, oxygen. The new James Webb Space Telescope (the replacement for the Hubble Space Telescope) has set this detection as one of its challenging aims. The link at the bottom of the page will take you to a site that allows you to explore the possibilities.