The planets in the solar system are a diverse lot. A new study indicates that extrasolar planetary systems are not.1
The eight planets in the solar system range in size from 3,400 miles in diameter (Mercury) to 86,882 miles in diameter (Jupiter). The mass range is much more dramatic, from 0.055 times Earth’s mass (Mercury) to 318 times Earth’s mass (Jupiter). Densities range from 0.687 times that of liquid water (Saturn) to 5.514 times that of liquid water (Earth).
The list of extrasolar planets with well-measured characteristics now numbers more than 3,700.2 The number of known multiple planet systems now stands at 621.3
A team of thirteen astronomers from Canada and the United States earlier this year studied 909 planets in 355 multi-planet systems discovered by the Kepler spacecraft.4 They found that “planets within a single multi-planet system tend to be closer in size than planets drawn randomly from the collection of multi-planet systems.”5 That is, where there are multiple planets in a planetary system, those planets tend to possess similar diameters. They also found that “adjacent planets within a given system tend to be spaced in a regular geometric progression.”6
Now, three astronomers at Yale University investigated whether this similarity in planetary size is accompanied by similarity in mass for planets in the same planetary system.7 For planets discovered by the Kepler spacecraft, their masses are much more challenging to measure than their diameters. The three astronomers applied a special technique developed by astronomers Eric Agol (Eric is a Christian who attended my Paradoxes Sunday class when he was at Caltech) and Daniel Fabrycky8 to use the transit time variation of planets to determine the planets’ masses. Using this technique, the three astronomers assembled a large database of measured masses for the “Kepler multis.” Kepler multis are planets discovered by the Kepler spacecraft with known planetary partners in the same system.
The team first noted that the majority of Kepler multis are either super Earths or sub-Neptunes. That is, they range in mass from 1.1–14 times Earth’s mass. This statistic is startling, given that none of the Sun’s planets fall in this mass range.
Second, the researchers noted that Kepler multis have much more tightly spaced orbits than do the solar system planets. Third, they affirmed their preconception that Kepler multis in the same planetary system tend to possess similar masses.
The three astronomers did observe that the Kepler multis manifest a wide range of masses. In fact, they manifest a much wider range than the solar system planets. For the solar system, the range is 0.055–318 Earth masses. For the Kepler multis, it is 0.4–8,586 Earth masses. The astronomers demonstrated, however, that within each multi-planetary system, the planets exhibit similar masses. Inter-system planet mass variability is large, while intra-system planet mass variability is remarkably small.
In the words of the research team, the similar diameters and masses of intra-system Kepler multis make them all “peas in a pod.”9 These peas in a pod are strikingly different from the eight planets in the solar system. The three astronomers have added to the ever-accumulating evidence that the solar system is unique and special in its exquisite design features that enable it to sustain advanced life on one of its planets.