Data Trends

Overview

Although a few of the known exoplanets circle pulsars, exoplanet searches focus on sun-like stars for several reasons. One is that habitable planets are more likely to be found around a star that has a long lifetime to give any possible life on the planet time to evolve. Second, sun-like stars are less prone to massive outbursts, and the intense ultraviolet radiation that is emitted from larger stars is not as severe. Third, it is easier to detect planets around stars that aren't as bright, so fainter stars like our sun are preferable.

The 193 known planets circle around 165 different stars, and there are 18 multiple planet systems, where 13 are double, 4 are triple, and 1 is a quadruple system. Because of the large number of planets, this site will no longer maintain a database of them. Rather, the data used in the information below is based upon The Extrasolar Planets Encyclopedia, which maintains an active database.

General Observations

No one really knows for sure what the planets look like, but there is no shortage of theories. First of all, we know that almost all of the known exoplanets are "hot Jupiters." This name comes from the fact that they are very close to their parent stars (many are much closer than Mercury to the sun), and most are many times the size of Jupiter.

Why so close and so big? This comes out of the biases inherent to the detection methods: The closer to the star, and the bigger the planet, the larger the movement of the star will be due to the effects of the planet. So these "hot Jupiters" are the easiest to detect, and therefore they make up the bulk of those discovered thus far. It was also, until recently, unknown whether these planets were jovian (gaseous) or terrestrial (rocky). Then, a planet was seen eclipsing its star, and the resulting image showed that it had a very gaseous atmosphere. It is assumed that most - if not all - of those known are like this.

Only in the past few years have planets as small as Saturn (approx. 30% the mass of Jupiter) been found, and this detection is due to the more sensitive equipment that is constantly being developed. Most of the planets found thus far also have very short (some a matter of a few days) orbits around their stars. This is both due to the detection techniques, and it is a statistical anomaly: Since the discoveries have been so recent, there are not enough data to find planets that have longer years.

Nevertheless, these discoveries have forced many changes upon the theories of solar system development, which had, until recently, only our solar system as a model. According to the old model, large, gaseous bodies should not have been able to form that close to their parent stars; the gas should have either accreted to the star, or been "blown away" to a much farther distance by the young star's solar wind. So, the main change that has been made to the theories of solar system formation and evolution is that the planets migrated: After they formed farther out, they moved into much closer orbits.

For more detailed information, along with several graphs of the data, please visit the Advanced section of this site.