The Observational Basis

2.1.1 The Solar System

Table 2.1 lists some of the broad features of the Solar System, most of which you met in Chapter 1. Any theory worthy of serious consideration really has to be able to account for most of these features, and for some others too. But it does not necessarily have to be able to account for them all. If there are any features that a theory cannot account for, this would not necessarily rule the theory out. For example, it might be that the theory has not yet been worked out in sufficient detail, perhaps because a physical process is insufficiently well understood, or because we do not know enough about the state of the substances from which the Solar System formed. It is however, fatal for a theory if it unavoidably produces features that are clearly unlike those

Discovering the Solar System, Second Edition Barrie W. Jones © 2007 John Wiley & Sons, Ltd

Table 2.1 Some broad features of the Solar System today

1 The Sun consists almost entirely of hydrogen and helium

2 The orbits of the planets lie in almost the same plane, and the Sun lies near the centre of this plane

3 The planets all move around the Sun in the same direction that the Sun rotates (called the prograde direction)

4 The rotation axis of the Sun has a small but significant inclination, 7.2° with respect to the ecliptic plane (the Earth's orbital plane)

5 Whereas the Sun has 99.8% of the mass of the Solar System, it has only about 0.5% of its total angular momentum

6 The axial rotations of six of the nine major planets are prograde with small or modest axial inclinations. The rotations of Venus, Uranus, and Pluto are retrograde

7 The inner planets are of low mass and consist of rocky materials, including iron or iron-rich compounds; the closer to the Sun, the more refractory the composition

8 The giant planets lie beyond the inner planets, are of high mass, and are dominated by hydrogen, helium, and icy materials, with a decreasing mass and hydrogen-helium content from Jupiter to Saturn to Uranus/Neptune

9 The asteroids are numerous small rocky bodies concentrated between Mars and Jupiter

10 There are even more numerous small icy-rocky bodies concentrated beyond Neptune in two populations, the Edgeworth-Kuiper belt and the Oort cloud. These give rise to the comets

11 The giant planets have large families of satellites that are rocky or icy-rocky bodies observed. For example, if a theory predicts that roughly half the planets should be in retrograde orbits then we can rule the theory out.

□ What about a theory that predicts that there are no giant planets? We can rule this out too!

2.1.2 Exoplanetary Systems

The number of exoplanetary systems presently known (13 January 2007) is 177, 20 with two or more planets, giving 205 exoplanets in total. Already, they have supplied valuable insights into the origin and evolution of the Solar System. Direct detection of exoplanets is at the limit of present instrumental capabilities, because a planet is a very faint object with a very small angular separation from a far brighter object - its star - and the planet's light therefore cannot be seen. Therefore, up to now detection has been almost entirely indirect.

Indirect detection techniques

Most of the exoplanets have been discovered through the motion they induce in the star they orbit. In our Solar System the planets cause the Sun to follow a small (complicated) orbit around the centre of mass of the system (Section 1.4.5). Therefore, if small orbital motion of other stars can be detected we can infer the presence of one or more planets even if they are too faint to be seen. One way is to measure repeatedly the position of the star with respect to much more distant stars. This is called the astrometric technique. An outcome is shown in Figure 2.1(a), where, for simplicity, it has been assumed that the centre of mass is fixed against the more distant stellar background. In reality, the motion of the centre of mass would add to that in Figure 2.1(a) to give a wavy stellar path. A second technique is possible if the angle i in Figure 2.1(b) is greater than zero. In this case the orbit is not presented face on to us, and therefore as the star moves around its orbit its speed along the direction to the Earth varies,

Centre

Stellar of mass

To Earth

(line of sight) Edge view of orbit

Figure 2.1 (a) The orbit of a star due to a single planet in orbit around it, in the simple case when the centre of mass of the system is stationary with respect to the distant stars. (b) The angle of inclination i of the normal to the plane of the stellar orbit with respect to our line of sight.

i.e. its line-of-sight speed varies. These speed variations cause variations in the wavelengths of the spectral lines of the star. This is due to the Doppler effect, whereby the observed wavelength depends on the speed of the radiation source with respect to the observer (Christian Johann Doppler, Austrian physicist, 1803-1853). The technique based on this effect is called the radial velocity technique. It has discovered the great majority of exoplanets to date.

From either technique we can obtain the mass of the planet(s) and some of the orbital elements. The details will not concern us except to note that whereas the astrometric technique gives the mass of the planet m , the radial velocity technique gives mp x sin(i). This is because we detect the component of the star's orbital velocity towards us and not the total orbital velocity. Thus, if i is unknown we obtain only a lower limit on the mass of the planet corresponding to i = 90°, as if we had an edge-on view of the orbit.

There are some other techniques for indirect detection of exoplanets, and though so far they have delivered a very small yield, this will rise, particularly in the case of the transit technique. This relies on the slight diminution of the light we receive from a star, if one of its planets passes between us and the star.

□ If, from a distant observer's vantage point, Jupiter were to transit the face of the Sun, what decrease would it cause in the light received? Jupiter's radius is about a tenth that of the Sun's, and so Jupiter would appear as a disc with an area about one-hundredth that of the solar disc. Therefore, the decrease would be 1%. For a transit to occur the orbit of an exoplanet must be presented edge on to us, or nearly so.

A much fuller account of the techniques for finding exoplanets can be found in books in Further Reading. We now consider what the exoplanets teach us about the origin and evolution of the Solar System.

Some characteristics of the known exoplanetary systems

The first exoplanets were discovered in 1992 in orbit around a pulsar. A pulsar is the remnant of a star that has suffered a catastrophic explosion - a supernova explosion. Such an explosion would surely have destroyed any planetary system, and so the planets are presumed to have formed subsequently. But interesting though these pulsar planets are, pulsars are rare objects, quite unlike the Sun. It was in 1995 that the first exoplanet was detected in orbit around a star other than a pulsar - the star was 51 Pegasi. Table 2.2 summarises the characteristics of the known exoplanetary systems that are of particular relevance to this chapter - the small number of pulsar planets is excluded.

Table 2.2 shows that, at present, the lowest known exoplanetary mass is 0.017 times the mass of Jupiter, or 5.4 times the mass of the Earth (this happens to be its actual mass, not the minimum mass). However, most exoplanets have (minimum) masses between a tenth and 10 times that of Jupiter. Table 2.2 also shows that the minimum semimajor axis is only a = 0.0177 AU, much less than Mercury's 0.387 AU. Indeed, nearly half of the exoplanets have a < 0.387 AU. Many of these have masses between 0.5 and 1.5 Jupiter masses, and are called 'hot Jupiters'.

The distance range in Table 2.2 needs to be put into perspective. Our Galaxy is about 100000 light years across its disc, and contains roughly 2 x 1011 stars (light year, ly - see Table 1.6). The 300 ly in Table 2.2, compared with the size of our Galaxy, thus puts most exoplanetary systems in our cosmic backyard. This is because the closer a star, the brighter it appears and the easier it is to make observations. This is an example of an observational selection effect.

The stars in the great majority of the exoplanetary systems are main sequence stars not very different in mass from the Sun. Such stars have been the star of choice for observers, mainly because they have many narrow spectral lines suitable for the radial velocity technique, and because they are much brighter than low-mass main sequence stars. Higher mass main sequence stars are even brighter, but are rare and have short lives.

Only a few of the known exoplanetary systems are like the Solar System, with the giant planets several AU from the star. Is therefore the Solar System a rare type of planetary system? Not necessarily. This is because the easiest planets to detect with the radial velocity technique are those that induce the greatest orbital speed of the star, and these are massive planets close to the star - another example of an observational selection effect. Moreover, the orbital period increases with semimajor axis, and therefore data have to be accumulated for longer times to discover planets further out. In the case of Jupiter, with an orbital period of 11.86 years, an

Table 2.2 Some characteristics of the known exoplanetary systems"

Characteristic

Data

Comment

Stellar mass

0.34-1.5Mo

A substantial majority are main sequence stars

Stellar distance

10.5 ly and up

Very few are beyond 300 light years

Planet massb

0.017-13cm7

Most are in the range 0.1-10 mJ

Planet semimajor axis

0 0

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