Scientists detect exoplanets by detecting the wobble of a star due to the exoplanet's gravity or the dimming of the star as the exoplanet passes in front of it. When scientists first began to detect exoplanets around other stars, they assumed that they would find many solar type planetary systems and Earth’s around other stars; however, observations of disks and exoplanets around other stars, as well as models of our own solar system, indicate that the circumsolar disk around our sun was unusual.
We might pose the question as follows: how common are circumsolar disks? There are two ways to answer this question: observation and theory. The ability to answer this question is constrained by the fact that the only planet in the solar system that is observable by current technology is Jupiter. The inner terrestrial planets are not detectable because their gravity is too low to alter the path of the sun, and they are too small as they pass in front of the sun to alter the brightness of the sun. Astronomer Sean Raymond listed the factors that can be measured and their probabilities as follows:[1]
10% chance of yellow sun
10% chance of existence of Jupiter
10% chance of Jupiter in wide and circular orbit around star
50% chance of no super-Earths near the star.
Multiplying these factors together, there is a 1 in 2,000 chance of finding another Sun-Jupiter system. This does not mean other Sun-Jupiter systems would have the solar system architecture, but they would have the potential for a Sun-Earth-Jupiter system; however, there is a reason to suspect that there may be few Sun-Earth-Jupiter systems. All observed exoplanet systems have similarly sized planets. Dr. Raymond made the following comment “Typical exoplanet systems have large planets close to the star, and the planets in each exoplanet system usually are of similar sizes. We have dramatically different sized planets, which may be the oddest thing of all.” [2] Jupiter is 318 times more massive than Earth. Planetary scientists refer to the difference in sizes as the great dichotomy.
Kevin Walsh made the following statement.
“We have no idea why our solar system doesn’t look like these others, and we would love an answer,” said planetary scientist Kevin Walsh, of the Southwest Research Institute in Colorado. Since the time of Copernicus, scientists have slowly moved Earth out of its originally-conceived setting as the center of the Universe. Today, scientists recognize that the Sun is an average star—not too hot, not too cold, not too bright, not too dim—situated at a random spot in a typical spiral galaxy. So, when Kepler began its planet-hunting mission in 2009, scientists anticipated finding planetary systems that resembled our solar system. Instead, Kepler mostly discovered planet types that our solar system lacks. With bodies like “hot Jupiters” (Jupiter-sized planets that orbit their star in only a few days) to “super-Earths” (massive rocky planets far larger than our own), exoplanet systems have a knack for surprising observers. Of the 1,019 confirmed planets and 4,178 planetary candidates identified to date, only one system resembles our own with terrestrial planets near the star and giant planets set at a distance.” [3]
Planetary scientists Batygin, Laughlin, and Morbidelli made the following statement about the architecture of the solar system in comparison to exoplanets in the local part of the galaxy.
“The solar system's configuration of small inner rocky worlds and large outer giants is anomalous in comparison with most other planetary systems, which have different architectures.” [4]
Scientists have thought that Jupiter caused the small inner planets and inner solar system architecture, possibly by intercepting materials that are headed from the outer solar system to the inner solar system. The transition disk surrounding TW Hydrae has a dusty inner section that is approximately 4 AU and a gas disk section that begins at 4 AU. Astronomers think that a giant planet like Jupiter in the TW Hydrae system might have formed a gap at 4 AU distance from the star and prevented inward migration of ice and dust pebbles into the inner solar system. In the following video, Morbidelli proposes that the Jupiter dust barrier hypothesis explains the dry inner planets; however, as described in chapter 5, recent data on chondrite meteorites might contradict this hypothesis because radioactive isotopes in the meteorites indicate that the inner solar system planetesimals formed prior to Jupiter and that the inner terrestrial planets formed wet, but then their inner cores were hot and removed the water.
To fully evaluate the commonality of the solar system architecture in the universe and particularly of a life supporting planet like Earth will require technology that can detect an Earth-sized planet orbiting the sun. The reason that super-Earth’s are not helpful in the search for life is that larger planets with greater gravity (super-Earths) would hold on to their hydrogen atmosphere, which would eliminate the possibility of an oxygen and water atmosphere. They are also generally closer than Mercury to their star, having migrated inward to the inner edge of the disk from the outer disk (MOJO video 7/11, https://youtu.be/mG374opbH_8, ). Scientists hope to directly observe planets with the James Webb Space Telescope, which is scheduled to launch soon.
One possibly odd aspect of the solar system is that the outer planets did not spiral into the inner solar system. Computer models of solar system formation indicate that hot Jupiters and super-Earths that form far from the star normally spiral inward into orbits near their star. Thus, they wipe out earthlike planets that form near the star.
Another surprising characteritics of the solar system is the amount of water on Earth. According to computer models of solar system formation, the snow line in the circumsolar disk should have been 0.6 AU from the sun (between the Earth and the sun). The Earth should have formed as an icy and eventually water planet. [5,6] The Earth should have had a concentration of water that is comparable to the outer water giant planets, which have greater than 40 percent water by weight.[7] There would be no continents. Planetary scientists, R. Machida and Y. Abe described the problem of dry Earth formation:
“Models of terrestrial planet formation have been based on the assumption that the formation of planetesimals occurs in a transparent (optically thin) nebula, in which H2O ice is unstable at the formation region of the terrestrial planet due to direct stellar irradiation. However, in the astronomical context, it is confirmed by both observations and numerical models that protoplanetary disks are initially opaque (i.e., optically thick) owing to floating small dust particles, and the interior of the disk is colder than the transparent disk. If planetesimals are formed in opaque cold nebula, they should be mainly composed of H2O ice, even at the formation region of terrestrial planets.” [8]
The position of the snowline is not only a function of the decreasing radiation intensity vs. distance from the star but it is also a function of the movement of colder disk materials toward the sun and of the heat generated by friction in the disk. The snow line moves inward because materials migrate inward from the cold outer disk and bring their cool temperatures with them, and because the friction in the disk decreases as the disk thins. This is why computer models without the Jupiter dust barrier or a similar hypothesis place the snowline at 0.6 AU.
Martin and Livio investigated whether a dead zone in the circumsolar disk might have prevented inward migration of materials in the disk between Jupiter and Mars.[9] However, they concluded that if there was a dead zone, it was not a regular occurrence in other planetary systems.
Martin and Livio found that only 4% of giant planets were outside the snow line in other stellar systems.[10] This means that almost all of the observed giant exoplanets migrated into the inner part of exoplanet systems. They stated, “The observational results, therefore, agree with the theoretical models of Armitage et al. (2002) and suggest that the solar system may be rather special.” In a 2015 paper, Martin and Livio revisited the question of how special our solar system is by comparing the characteristics of planets in the solar system (upper left circles) to observed exoplanet systems in Figure 4-24.[11] There was a preponderance of large exoplanets (large circles) near the host star; however, they stated that the uniqueness of the solar system with its four inner small planets at a relatively large distance from the sun and outer gas giants might be a selection effect.
Scientists are looking for ways to compare the solar system to exoplanet systems that do not have the bias of current planet detection methods toward large planets close to the star. One method is the direct measurement of lithium in stars. The fraction of lithium in stars depletes over time because high temperature breaks down lithium nuclei. The sun’s lithium concentration is two standard deviations lower than expected for a star of its age. Stars with low lithium generally have low concentrations of refractory metals (tungsten and other metals). Because the association with lower metals, Louis et al. (2019) proposed that low lithium in the sun was due to the “presence of rocky planets and the unique architecture of the solar system.” [12]
Because our solar system might be somewhat unique among other observed planetary systems, an increasing number of scientists are beginning to believe that we are essentially alone in the universe. For example, Howard Smith, senior astrophysicist at Harvard, made the following statement.
“We are alone in the universe, at least for all practical purposes. This is the most probable conclusion to be drawn from a host of fundamental physical constraints and new astrophysical observations, and in particular the discovery (at this writing) of 4,696 exoplanet candidates, including some Earth-sized planets in their habitable zones (Quintana et al. 2014). The implications of these discoveries, and their modern context, are radical.” [13]
When Smith stated that we are “for all practical purposes” alone in the universe, he did not mean that there are no other civilizations in the entire universe. Astronomers have only confirmed several thousand exoplanets in our local part of the Milky Way, which only represents a tiny fraction of the planets out there. Even if Earth is the only planet with life in our part of the galaxy, there are billions of other galaxies in the universe, and most of the Milky Way is not observable.
[1] Raymond, Sean. How common are Solar Systems. The MOJO Project. https://youtu.be/dtwyb6eQJ9Q
[2] Raymond, Sean N., Andre Izidoro, and Alessandro Morbidelli. "Solar System Formation in the Context of Extra-Solar Planets." arXiv preprint arXiv:1812.01033 (2018).
[3] NASA/Kepler Mission Mystery --"We Have No Idea Why Our Solar System is So Unusual" Daily Galaxy. Accessed on August 14, 2016 at <http://www.dailygalaxy.com/my_weblog/2016/08/nasakepler-mission-we-have-no-idea-why-our-solar-system-is-so-unusual.html>
[4] Batygin, Konstantin, Gregory Laughlin, and Alessandro Morbidelli. "Born of Chaos." Scientific American 314, no. 5 (2016): 28-37.
[5] P. Garaud and D. N. C. Lin, “The Effect of Internal Dissipation and Surface Irradiation on the Structure of Disks and the Location of the Snow Line around Sun-Like Stars,” ApJ 654 (2007): 606–624.
[6] Akinori Oka, Taishi Nakamoto, and Ida Shigeru, “Evolution of Snow Line in Optically Thick Protoplanetary Disks: Effects of Water Ice Opacity and Dust Grain Size,” The Astrophysical Journal 738(2) (2011): article id. 141.
[7] Martin, Snow line.
[8] R. Machida and Y. Abe, “Terrestrial Planet Formation through Accretion of Sublimating Icy Planetesimals in a Cold Nebula,” Astrophysical Journal. 716 (2010): 1252, http://isotope.colorado.edu/~geol5700/Machida_2010.pdf. (This is not the same Dr. Machida as in ch. 3 and 4).
[9] Rebecca Martin and Mario Livio, “On the Evolution of the Snow Line in Protoplanetary Disks.”
[10] Rebecca Martin and Mario Livio, “On the Formation and Evolution of Asteroid Belts and Their Significance for Life,” Mon. Not. R. Astron. Soc. 000 (November 2012).
[11] Martin, Rebecca G., and Mario Livio. "The Solar System as an Exoplanetary System." The Astrophysical Journal 810, no. 2 (2015): 105.
[12] Carlos, M., Jorge Meléndez, Lorenzo Spina, L. A. Dos Santos, Megan Bedell, Iván Ramírez, Martin Asplund et al. "The Li–age correlation: the Sun is unusually Li deficient for its age." Monthly Notices of the Royal Astronomical Society 485, no. 3 (2019): 4052-4059.
[13] Smith, Howard. "Alone in the Universe." Zygon 51, no. 2 (2016): 497-519.
Banner. Solar system planets. Credit: WP. Used here per CC BY-SA 3.0