How Common Are Earths? How Common Are Jupiters?

Among the billions of planetary systems that fill the Universe , we would like to know how ours fits in. Exoplanet data can already be used to address the question: How common are Jupiters? Here we discuss a simple analysis of recent exoplanet data indicating that Jupiter is a typical massive planet rather than an outlier. A more difficult question to address is: How common are Earths? However, much indirect evidence suggests that wet rocky planets are common. 1. How Common are Jupiters? The statistics of massive detectable exoplanets Long before we detect Earth-like planets we will have a good general picture of the variety of massive planets in planetary systems. Since Jupiter is the most prominent feature of our planetary system, and our knowledge of other planetary systems is still rudimentary, we may, with current data reasonably hope to answer the less ambitious question: How typical is Jupiter? The relevant analysis is now possible because a statistically significant sample is starting to emerge from which we can determine meaningful distributions in mass and period. To quantify these distributions as accurately as possible, we have identified a subsample of exoplanets that is minimally affected by the selection effects of the Doppler detection method (Fig. 1). Within this subsample, after a simple completeness correction, we quantify trends in mass and period that are less biased than trends based on the full sample of exoplanets. Straightforward extrapolations of these trends, into the area of parameter space occupied by Jupiter, indicates that Jupiter lies in a region densely occupied by exoplanets (Lineweaver & Grether 2002). Our analysis suggests that Jupiter is more typical than indicated by previous analyses. For example, instead of M Jup planets being twice as common as 2M Jup planets, we find they are 3 times as common. The latest exoplanet data (detected between January and August 2002) supports and strengthens this conclusion (Lineweaver, Grether & Hidas 2003). Our claims for Jupiter being a typical massive planet are well-defined in terms of M sin(i) and period but can not yet include orbital eccentricity since the eccentricity of most exoplanets is larger than the ∼ 0.1 typical of our Solar System. The frequency of Jupiter-like planets may have implications for the frequency of life in the Universe. A Jupiter-like planet shields inner planets from an otherwise much heavier bombardment by planetesimals, comets and asteroids 1