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Paper in Proceedings of the National Academy of Sciences Suggests Our Solar System Is Exceptional

Kepler ENV.jpg

Back in July, around the time I was busy running our Summer Seminar on Intelligent Design, biologist Jeff Schweitzer wrote a polemical article in the Huffington Post, "Earth 2.0: Bad News for God." Schweitzer, who served under the White House Science Advisor in the Clinton Administration, rails against religion, contending that the discovery of extraterrestrial life would refute the great Abrahamic faiths. I could go on about Schweitzer's simplistic and na�ve theological analysis, including the tortured logic he uses to claim that the Bible's silence on extraterrestrial life is really a denial that ETs could exist. But I'm more interested in looking at his scientific assertion that our solar system isn't special and that in fact there must be "thousands or millions or even billions of such earth-like planets in the universe."

Schweitzer is excited about the discovery of what he calls "Earth 2.0," an extrasolar planet otherwise known as Kepler 452-B. If you believe what the Huffington Post tells you, it's probably a rocky earthlike planet thought to orbit its star within the region where liquid water is possible. He calls it "possibly habitable." In reality, many parameters beyond the mere existence of water are necessary for a planet to be habitable, so it has not been established that Kepler 452-B is habitable, or even "possibly habitable," like Earth. Schweitzer wants people to believe that this suggests habitable earthlike planets are extremely common in the universe. A quick check of the technical scientific literature shows he is mistaken.

According to an April 2015 paper in Proceedings of the National Academy of Sciences, "Jupiter's decisive role in the inner Solar System's early evolution," a review of the sizes and orbital radii of known extrasolar planets shows how exceptional our solar system is:

The statistics of extrasolar planetary systems indicate that the default mode of planet formation generates planets with orbital periods shorter than 100 days and masses substantially exceeding that of the Earth. When viewed in this context, the Solar System is unusual. ... The Solar System is an unusual member of the galactic planetary census in that it lacks planets that reside in close proximity to the Sun. [emphasis added]
The problem for Schweitzer, essentially, is that the vast majority of extrasolar planets we've discovered orbit their stars much closer than even our earth orbits the sun -- and, these extrasolar planets are much larger than earth. Both of these properties make these planets uninhabitable. Moreover, large planets like Jupiter -- which in our solar system help keep earth habitable by sweeping up comets and asteroids -- almost never exist far out in the solar system, as Jupiter does. These problems are illustrated in this diagram from the paper:


(From Figure 1, copyrighted by Konstantin Batygin and Greg Laughlin, "Jupiter's decisive role in the inner Solar System's early evolution," Proceedings of the National Academy of Sciences USA, Vol. 112(14): 4214-4217 (April 7, 2015).)

Phys.org describes these findings as follows:

There's something about our solar system that appears to be unusual. For some reason, most of our bigger planets are far away from our host star, while closer in are smaller, rocky worlds, including Earth itself.

This is not the case for many extrasolar systems that have been discovered. So-called "hot Jupiters" -- huge gas giant planets that nestle close to their star -- have been found in a few examples. In other instances, planets slightly bigger than Earth are so close to their stars as to be uninhabitable.

What this means is that our solar system stands out dramatically compared to other solar systems we've discovered and that getting rocky planets orbiting near their star as Earth does, in the circumstellar habitable zone, requires a very exceptional set of circumstances. As the paper explains:
Perhaps the most important exoplanet-related discovery has been the realization that roughly half of the Sun-like stars in the solar neighborhood are accompanied by systems of one or more planets on low-eccentricity orbits with periods ranging from days to months, and masses falling in the 1M? p < 50 M? range, where M? is an Earth mass unit. This dominant population of planets (which often presents tightly packed, nearly coplanar multiple systems) contrasts sharply with the Solar System, whose inner edge is marked by Mercury's 88-d [0.4 astronomical units (AU)] orbit (see Fig. 1). An iconic example from the new planetary catalog is the Kepler-11 system, which encompasses at least six planets comprising more than ?40 Earth masses (4). In short, the exoplanetary surveys have revealed a hitherto unrecognized oddity of the Solar System. Relative to other Sun-like, planet-bearing stars, our terrestrial region is severely depleted in mass.
By "depleted in mass" they mean that nearly all known extrasolar planets orbiting at or around earth's radius are many times bigger than earth and uninhabitable. True, Kepler 452-B is an exception to this rule, but its mass is still about five times that of the earth, and may even be a gas-based planet. It is therefore thought to be a "super-earth" or perhaps a "gas-dwarf" which would make it high in volatile elements. This fits with what the paper predicts, that "the majority of Earth-mass planets are strongly enriched in volatile elements and are uninhabitable," like a mini version of Neptune.

Indeed, others have claimed that the data suggests Kepler 452-B is volatile-rich, gaseous planet and not habitable. Physicist Andrew LePage who closely monitors claims of habitable planets writes:

A recently published analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 RE (see "Habitable Planet Reality Check: Terrestrial Planet Size Limit"). While rocky planets larger than this are possible, they become more uncommon with increasing radius. Using a model based on recent work by Torres et al. (see "Habitable Planet Reality Check: 8 New Habitable Zone Planets"), I estimate that there is something like a 40% chance that Kepler 452b with a radius of 1.6 RE is a rocky planet. This is somewhat less than the often repeated claim of "greater than 50%" chance found in media reports.

Unfortunately, the chance that Kepler 452b is a terrestrial planet might not be as good as even 40%. Recent work by Dawson et al. strongly suggests that planets with masses greater than about 2 times that of the Earth (or 2 ME which would have a radius of about 1.2 RE, assuming an Earth-like bulk composition) which orbit stars with a high metallicity are more likely to be mini-Neptunes. This is because stars with higher metallicities tend to have more solid material available to form planetary embryos more quickly making it more likely for them to acquire some gas directly from the protoplanetary disk before it dissipates. Stars with lower metallicity tend to form planetary embryos more slowly and they might not reach the required 2 ME mass threshold fast enough to begin to acquire more than trace amounts of gas before the it has already dissipated from the disk. Only 1% or 2% of a planet's total mass of hydrogen and helium is sufficient to puff up its observed radius and make it a mini-Neptune. With a iron-to-hydrogen ratio about 60% higher than the Sun, Kepler 452 has a slightly higher metallicity than the Sun increasing the odds somewhat that Kepler 452b is a mini-Neptune.

The overall trend in this data makes our solar system look highly unusual, and from what we know so far, Kepler 452-B is probably not habitable. Schweitzer's back-of-the-envelope calculation invoking even billions of earth-like planets evidently pleased the editors of the Huffington Post. As science, it's less than compelling.

Image: Artist's rendering of Kepler-452b, by NASA Ames/JPL-Caltech/T. Pyle.