Pedagoguery

There has been quite a lot of news in recent months about new exoplanets. Scientists have now discovered enough exoplanets to uncover some interesting patterns. However, some difficulties still exist.

One of the main difficulties have to do with the types of stars around which we find planets. Some characteristics of stars make it harder or easier to find planets using various techniques. For example, using the radial-velocity method (where you measure the slight back and forth Doppler shift of a star as it is tugged by a planet orbiting it), you want a star with numerous, sharp absorption lines. Such stars tend to be cooler and less massive. However, you still want to check the more massive stars. The problem is, stars of spectral type A, like Sirius, tend to rotate very fast, giving them broad absorption lines, and their hot temperature leads to fewer lines. This can be overcome by looking at stars of a similar mass (about twice that of the Sun) at a later stage in their evolution. When the star just starts to leave the main sequence, it becomes a K subgiant. The star swells, slowing its rotation, and cools, leading to more absorption lines. Analysis of such stars has led to the discovery that that they tend to be more than twice as likely to have a Jupiter-mass or larger planet.

By contrast, looking for smaller planets using the occultation method (where you look for the slight dip in starlight when a planet crosses the face of the star) is likely to be more successful for dimmer, red M-class stars. This is because the amount of the dimming is greater when the difference in angular area occupied by the star and the planet is smaller. In other words, the planet blocks out more of the star's light. This is the case with M-class stars, as is seen in the results from Kepler.

Another area where looking at the discovered exoplanets can help us is to help figure out how planets are formed. There are two competing theories of planet formation. The first is the core accretion model. In this model, small dust grains in the stellar nebula collide and stick together, gradually forming larger and larger objects. Eventually, the object becomes large enough to draw gas to itself by its own gravity. This model depends quite strongly on the elemental composition of the nebula – the more elements heavier than helium (termed “metals” by astronomers) there are, the more likely the star is to have planets. The second theory is the disk instability model. In this model, parts of the stellar nebula gets dense enough to attract nearby gas due to its own gravity. This model depends on the ratio of masses between the star and the nebular from which it is formed. The key difference is that if the core accretion model predominates, then if a star has a high metallicity, it should be more likely to have planets. What does the data say? It definitely tends toward the core accretion model. A star with 3 times the sun's ratio of iron to hydrogen is two and a half times more likely to have planets than one with the sun's iron to hydrogen ratio.

As more data comes in, we will be able to determine even more about planets, how they form, and whether any of them may be able to support life.

Next time, a look at the quantum gaps in the Big Bang theory.