Comparing the Solar System to Exo-Solar Systems

Key points: We can see all of the major planets in the Solar System straightforwardly,  exo-planet discoveries seem to indicate that the Solar System with 8 planets in regular orbit is rare;  exo-planet discoveries may be biased by search techniques

Recall Structure of the Solar System

The Planets to Scale: from the sun, the "terrestrial planets" are Mercury, Venus, Earth, and Mars, while the "giant planets" are Jupiter, Saturn, Uranus, and Neptune (Pluto was an oddity at the end*). (from http://www.adamnieman.co.uk/futurelab/)

Remember that the Solar System has small, rocky planets close to the Sun and large, gas giant planets farther from the Sun with two biggest planets. 

Also notice several things about the orbits of the planets in the Solar System: 1) the terrestrial planets are all very close to the Sun 2) the orbits of the planets all lie in a very thin plane  3)  Length of year for giant planets is a long time Planet   Length of its year Earth 1 year Mars 1.88 years Jupiter 11.86 years Neptune 164.8 years Think about what this means for detecting a planet: if you were looking at the Solar System from far away, if you wanted to detect Jupiter either by looking for changes in the Sun's velocity due to Jupiter orbitting the Sun, look at how long it would take to see the pattern twice: 2 x 11.86 years = 23.7 years which is a long time for one person to be observing a star to search for an exoplanet (and in fact, our ability to detect such changes in velocity with the required degree of accuracy has only existed since about 1995 or 20 years ago). There is a "bias" in current work against finding planets far from their parent star just due to the length of time over which one needs to collect data.  For the technique looking for velocity changes, another bias is that larger planets cause larger effects.  Take a look at http://astro.unl.edu/naap/esp/animations/radialVelocitySimulator.html to see how the planet mass affects the result.

Another issue that affects the velocity change method is the dependence on viewing angle -- if the system is viewed face-on, no velocity changes will be seen. Transits also have an inclination dependence because the planet needs to be lined up with the star. The further the planet is from the star, the less likely that it will be lined up well enough.  This bias coupled with the fact that a tranist will occur only once per orbit leads to significant bias against detecting planets far from the star. Transits also favor large planets as they block more star light. Try the simulator at http://astro.unl.edu/naap/esp/animations/transitSimulator.html

All of the biases in the main exoplanet detection methods favor large planets close to the parent star so it is not surprising that first exoplanets to be detected are all "Hot Jupiters", large planets found closer to their parent star than Mercury is to the Sun.

As will be discussed in more detail later, the concept of  a habitable zone is useful in comparing exoplanet systems to the Solar System. The habitable zone is that region around a star where a water could be liquid on a planet's surface (but beware of greenhaoud gas issues!).

The Kepler satellite has found one system that multiple planets in a plane somewhat analogous to the Solar System.