How Typical is Our Solar System?

Before the discovery of the first extrasolar planets, the only planetary system we could study was our own Solar System. Based on centuries of study, astronomers had a fairly neat and tidy view of what planetary systems in general should look like based on our Solar System: a nearly coplanar system of worlds in near circular orbits with small rocky planets orbiting close in where it is warm and large, volatile-rich worlds orbiting in the distant cold regions. When the first extrasolar planet orbiting a Sun-like star, called 51 Pegasi b, was discovered two decades ago, it was found to be a Jupiter-size world in a tight three-day orbit close to its sun completely unlike anything that had been expected. With the realization that giant planets can be found so close to the stars they orbit, the flood gates of extrasolar planetary discovery opened and continues to this day.

The discovery of the first extrasolar planet orbiting a Sun-like star in October 1995, called 51 Pegasi b, clearly demonstrated for the first time that other planetary systems would not necessarily resemble our own. Click on image to enlarge. (NASA/JPL-Caltech)

With thousands of extrasolar planets, confirmed or otherwise, currently known, it is now apparent that the arrangement of planets in our Solar System is not the only one possible and that there exist classes of planets not seen in our neat little family of planets with unexpected orbital and physical properties. Given the range of planets now known to exist, a natural question is how typical is the Solar System? Is it unusual in some way and how does this affect the possibility of finding other habitable worlds among the stars?

Rebecca Martin (University of Nevada – Las Vegas) and Mario Livio (Space Telescope Science Institute) address this very question in a paper that was recently accepted for publication in The Astrophysical Journal. In their paper, they review the available peer-reviewed scientific literature on extrasolar planets found in radial velocity and transit surveys as well as a handful of extrasolar planets discovered by other means such as gravitational microlensing and direct imaging to compare the statistics of orbital and physical properties of the planets of our Solar System to the current population of known extrasolar planets.

Orbital Properties

Eccentricity: There a total of 539 extrasolar planets with measured orbital eccentricities. While the eccentricity values of the Solar System’s planets (which range from 0.0068 for Venus to 0.22 to Mercury) are relatively small when compared to those typical of known exoplanets, they are not all that significantly different. Other investigators had noticed earlier that the orbital eccentricities of exoplanets in multi-planet systems tend to become smaller on average with increasing number of planets. This is only natural since multi-planet systems with circular orbits tend to be more dynamically stable than systems with eccentric orbits. Extrapolating these trends to an eight-planet system like ours, the orbital eccentricities of the planets in the Solar System does not seem to be unusual at all.

Other work not discussed by Martin and Livio suggests that planetary systems seem to be more or less evenly split into two varieties: systems with one or maybe two gas giants in eccentric orbits with high mutual inclinations, and multi-planet systems with coplanar orbits (see “Architecture of M-Dwarf Planetary Systems”). Our solar system seems to comfortably fall into the latter category.

Semi-Major Axis: There are a total of 1,580 planets that have the average size of their orbits (or semi-major axes) measured. This total swells to 5,289 if unconfirmed planet candidates found so far during the analysis of data from NASA’s Kepler mission are included. When looking at this set of raw data, it appears that Jupiter is an outlier in the distribution of semi-major axis values with its large 5 AU orbit.

This diagram shows the distribution of Kepler’s 4,696 planetary finds (confirmed and otherwise as of July 23, 2015) as a function of orbital period and planet radius. Click on image to enlarge. (NASA Ames/W. Stenzel)

But one has to be careful about drawing premature conclusions from this observation. Jupiter probably only appears to be a bit unusual because of the strong selection bias of the surveys performed to date. For example, most of the data analyzed by Martin and Livio comes from Kepler finds which have semi-major axis values typically no greater than 1 AU because of the limited time of its primary mission survey. Finding a confirmed Jupiter analog orbiting a Sun-like star would require decades of data using the transit method. In addition, radial velocity surveys have an easier time finding planets in smaller orbits and few stars have been observed long enough with sufficient precision to detect Jupiter analogs. Likewise, searches using gravitational microlensing tend to favor planets within a few AU of their star. Detailed statistical analysis attempting to correct for these selection biases seems to suggest that Jupiter is not that exceptional. Only more data from future surveys looking for worlds in more distant orbits will be able to settle this question.

Inner Solar System: Based on the population of know extrasolar planets, the inner Solar System does appear to be special. While the innermost planet in the Solar System, Mercury, has an orbital radius of 0.39 AU, other planetary systems typically have much larger planets in much tighter orbits also present. Just how unusual the inner Solar System is statistically is difficult to quantify. Differences in results when looking at exoplanets found by just the transit method or by radial velocity measurements hints that subtle selection biases are at play but our lack of any planets inside of the orbit of Mercury certainly seems to be a bit unusual. This fact says something about the planet formation process in our Solar System and the role of planetary migration (which has been invoked to explain the properties of many exoplanetary systems) might have played in our earliest history.

Physical Properties

Planet Mass: The distribution of the masses of known exoplanets has two broad prominent peaks: a high-mass peak near twice the mass of Jupiter and a lower mass peak near five times the mass of the Earth. In terms of mass, Jupiter lies near the high mass peak while Saturn is on its low-mass shoulder. The low-mass peak is straddled by Venus and Earth on the low side and Uranus and Neptune on the high side. In terms of this observed mass distribution, Jupiter and Saturn do not appear to be unusual at all. However, a strong selection bias for all exoplanet search strategies towards finding large planets obviously alters the low-mass end of this observed distribution making it difficult to determine how typical the terrestrial or rocky planets of the Solar System are. Also, exoplanet surveys to date would not be able to detect analogs of the ice giants, Uranus and Neptune, in their distant orbits. However, in terms of mass, it does not appear that the rocky planets or ice giants in our solar system are unusual.

Our solar system is somewhat unusual because it lacks “super-Earth” sized worlds between Earth and Neptune in size. (NASA)

What is apparent when looking at the distribution of exoplanetary masses, even with the obvious biases in the data, is that the Solar System is somewhat unusual because of its lack of “super-Earths”. These worlds have masses of one to ten times that of the Earth and include not only large rocky planets but volatile “mini-Neptunes” as well (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). Numerous statistical analyses of various exoplanetary survey results suggest that as many as half of planetary systems include super-Earths especially in tight, short-period orbits. This lack of an important class of planets makes the Solar System somewhat special and may be an important clue about our system’s formation and early history.

Planet Densities: When looking at the distribution of planet density as a function of mass, Jupiter and Saturn fall neatly along the trend line for giant planets (see “A New Definition for Giant Planets”). For less massive planets down into the super-Earth size range, there is significant scatter in density reflecting an intrinsic variation in the bulk compositions of these worlds with various ratios of rock, water and other volatiles (see “A Mass-Radius Relationship for ‘Sub-Neptunes’“). But once again, Uranus and Neptune fall neatly within the high mass end of this population.

Shown here is a plot of density versus mass for a range of bodies from small extrasolar planets to main sequence stars. The solid line is the linear fit of density versus mass derived by the authors with the dashed line representing a model of an object with a H/He composition. Click on the image to enlarge. (Hatzes & Rauer)

The situation becomes a bit more uncertain with planets closer in mass to Earth and Venus because of the paucity of radius and especially precision mass data for these relatively small worlds. Although they are just at the edge of the sensitivity of current surveys, the evidence suggests that planets with radii smaller than about 1.5 times that of Earth follow a trend line consistent with an Earth-like composition (see “The Composition of Super-Earths”). While more data are required especially for these smaller worlds, the available evidence indicates that there is nothing unusual about the planets of the Solar System in terms of their bulk density when compared to extrasolar planets of like mass.

Conclusion

While the current population of known extrasolar planets has begun to reveal the true diversity planetary systems, it is heavily biased by a host of selection effects inherent to the methods currently available. Despite this limitation, the conclusion of Martin and Livio based on their statistical analysis is that, in general, the physical properties of the planets in our Solar System are typical compared to the other extrasolar planets currently known although the lack of super-Earths does seem to be a bit unusual.

In terms of orbital properties, the Solar System does appear to be somewhat special especially in its lack of any planets inside the orbit of Mercury. Although the orbital eccentricities of the planets in our Solar System are lower than the population of extrasolar planets as a whole, they are not that unusual especially when looking at members of other multi-planet systems. While it is still early, it does seem that there are rocky planets orbiting inside of the habitable zones of other planetary systems so that our Solar System is not unique in this respect hinting that potentially habitable rocky planets should not be uncommon (see “Occurrence of Potentially Habitable Planets around Red Dwarfs”).

This diagram of Kepler finds (confirmed and unconfirmed) shows planets with less than twice the radius of the Earth in or near the habitable zones of their suns as a function of effective stellar flux and stellar surface temperature. The green band shows the limits of various definition of the habitable zone. Click on image to enlarge. . (NASA Ames/W. Stenzel)

While this initial comparison of our Solar System to other planetary systems is enlightening, only more data, especially for more distantly orbiting planets, will be needed to gain a clearer picture. Such information should be forthcoming as precision radial velocity surveys of a greater number of stars are conducted over an expanding length of time and from ESA’s Gaia mission whose astrometric measurements will be ideal for finding Jupiter analogs. For now, however, it appears that our Solar System, while unique in its own way, is part of a continuum of diverse types of planetary systems with nothing obvious to preclude the existence of potentially habitable exoplanets.

A French translation of this article by Alexandre Lomaev is also available: “Notre système solaire il est banal?”, Extrasolar.fr – Encylopédie des Mondes Extérieurs, August 20, 2015 (in French) [Post]

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General References

Rebecca G. Martin and Mario Livio, “The Solar System as an Exoplanetary System”, arXiv 1508.00931 (accepted for publication in The Astrophysical Journal), August 4, 2015 [Preprint]