WEEK 53: DECEMBER 27-31

TOPIC: PUTTING IT ALL TOGETHER

     

When I first began reading about the solar system as an elementary school student in the mid-1960s, it seemed to be a much simpler place than what we know today. There was the sun, around which orbited nine planets; some of these planets had moons, Jupiter having the most with 12. Between Mars and Jupiter was the asteroid belt, and occasionally some small asteroids might pass closer to the sun, and meanwhile there were also the occasional comets, some of these being of short-period while others were of longer period, that passed through the inner solar system. While some of the planets showed features that could be examined telescopically, for the most part they – and certainly their moons – were little more than dots of light in the sky, and that was definitely true for the asteroids as well. All told, everything was relatively quiet and orderly.

The solar system that I know today is much different than that. I can certainly attribute part of this to my own maturing and development as a person, to all the increased reading I have done since those early days, and to a career as a scientist where I have made some of my own contributions towards the study of the solar system. However, a large part is also due to our tremendously increased body of knowledge of the solar system, which has been fueled to a large extent by the development of telescopes and imaging technology which has made our observations much more extensive and proficient, and to computer technology which not only aids that endeavor but also allows us to develop mathematical models of the physical processes involved. Then there are all the spacecraft missions that have flown during those decades, many of which have visited all the planets as well as several of their moons, and have transformed those objects from point of light into unique and detailed individual worlds. Additional spacecraft missions have visited several of the asteroids and comets, and have enormously increased our knowledge and understanding of these objects as well.

One of the major steps in this process of our increased understanding has been the discovery of other planetary systems outside our own; the census of these now numbers several thousand, with more being discovered all the time. As was covered in last week’s “Special Topics” presentation, we have also detected numerous stars that are accompanied by surrounding disks of material that are in the active process of forming planets. The combined study of all of these objects has told us that planetary systems are very commonplace in the Galaxy (and presumably the universe as a whole), and while each planetary system is unique in its characteristics, the underlying processes by which they form appear to be the same. Thus, by examining all these, we are able to glean significant understanding into our own origins.

    LEFT: The solar system (not to scale!) as I knew it in the mid-1960s. Courtesy NASA. RIGHT: A field of stars, in and around the constellation Centaurus. The globular star cluster Omega Centauri is at left, and the bright stars Beta (upper) and Alpha (lower) Centauri are at right. Exoplanet discoveries over the past couple of decades suggest that most, if not almost all, of the stars in this image – and everywhere else – are accompanied by planetary systems, formed by the same processes by which Earth and the other planets in our solar system formed.

Last week’s “Special Topics” presentation discussed this basic underlying process, including the role played by the objects called “planetesimals” and the larger “protoplanets” (and how these objects came into existence in the first place). While many of these objects indeed went on to become incorporated into forming the planets we see today, those that didn’t ended up having different fates. There were a lot of these objects in the solar system during that era, and some of them, due to gravitational influences of already-formed planets – especially Jupiter – were sent into the sun. Many others were ejected into the far outer solar system, and still others were ejected from the solar system altogether and into interstellar space. Yet others have remained within the inner solar system as “leftovers” from the planet formation process, and it is likely that Jupiter’s gravitational influences prevented the formation of additional planets in these regions, especially in the part of the solar system we now call the “main asteroid belt.”

These “leftovers” are the “small bodies” that we see today. The planetesimals that are made up primarily of “hard” materials like silicates and metals are what we now call “asteroids,” while those that are made primarily of more “volatile” substances like ices and frozen gases are what we now call “comets.” There are even a few leftover “protoplanets” around: the large main-belt asteroids (1) Ceres and (4) Vesta are likely such objects, and the large metallic asteroid (16) Psyche that has been discussed in a few previous “Ice and Stone 2020” presentations may well be the remnant metallic core of a protoplanet.

Comets cannot survive in the warm regions of the inner solar system for very long before their ices disperse into interplanetary space. The fact that we continue to see them, 4½ billion years after the solar system formed, means that there must be reservoirs of them in the colder regions of the outer solar system that continuously replenish the supply of them. The Kuiper Belt beyond Neptune and the Oort Cloud in the far outer solar system – both of which are subjects of respective “Special Topics” presentations – are these reservoirs, and via various gravitational influences that are discussed in those presentations there continue to be significant numbers of them that are brought into the inner solar system.

Since they are the “leftovers” from the planet formation process, asteroids and comets are believed to be relatively pristine and “unprocessed” objects from the early days of the solar system. There is thus much interest among scientists in studying these objects, both from the ground and, when feasible, by visiting spacecraft, since we can then gather insights into the conditions that existed when the planets – including, certainly, Earth – were forming. Some of the substances that have been detected within these objects match those that have been detected within interstellar gas and dust clouds, strengthening the idea that our solar system had its origins in such an environment. Comets, and quite a few asteroids as well, contain significant amounts of water, and thus it would appear that the early Earth received its water from these objects as it was forming. Many of these objects, including some of the meteorites that have fallen on Earth, contain various organic substances, including amino acids and other organic compounds associated with biological activity, and it accordingly would seem that Earth – and, presumably, other planets as well – received the basic raw materials of life from these objects early in their history.

    Some scenes of Earth. The raw materials that eventually produced these scenes were originally brought to Earth by comets and asteroids early in its history. Photographs courtesy Vickie Moseley.