Editor’s Note: As a series at ENV, we are pleased to present "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he critically examines recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos. For previous articles in the series, see here.
The galactic environment contributes to habitability in several ways. The two major relevant factors are the ability to form Earth-like bodies, and protection from harm to a habited body. These two parameters define what has become known as the Galactic Habitable Zone.1 We need, first, a place where Class I planets can and do form. Second, to permit intelligent life and civilization to appear, a planet’s environment needs to be protected from disturbances over billions of years.
The first factor requires an environment where the elements are available to make terrestrial planets along with an appropriate star in a protoplanetary system.2 Planets can only form where previous generations of stars have lived and given up the heavier elements produced during their lives and deaths. Stars are primarily made of hydrogen and helium, the only elements available in the universe before stars fuse them into the heavier elements. The process is called stellar nucleosynthesis, which over the entire history of the Milky Way is called galactic chemical evolution.
These heavy elements, such as oxygen, carbon, nitrogen, silicon, iron, etc., build up at different levels depending on the density of stars and types of stars in the different parts of the galaxy. The availability of these elements at a sufficient level is a requirement for a terrestrial planet, and for advanced life. Astronomers refer to the heavier elements beyond hydrogen and helium as "metals." Metal concentration over time increases with stellar density, and where stars have been living and dying the longest. This roughly means that metallicity is highest near the core of a galaxy, and decreases with distance from the core. It is estimated that the presence of Earth-like planets increases with increasing metallicity, dropping to zero when the value is one-tenth the metallicity of our solar system.3 As new data is gathered from the discovery of exoplanets, this exact level may be quantified.
It could be that for an Earth-like planet to form, the metallicity of its solar and galactic neighborhood needs to be close to that of our early solar system. Too far from the galactic core, and the metal concentrations fall below what is likely needed for a terrestrial planet. As the galaxy ages, the availability of metals increases from the core of the galaxy outwards. Yet it appears that our Sun must have formed 4.6 billion years ago in a more metal-rich environment than its present distance from the core would indicate. Current research indicates that the Sun formed significantly closer to the core of the galaxy, and migrated to its current position of about 26,000 light years from the core. It is estimated that it formed 15,000 to 18,000 light years closer to the galaxy’s core.4
To account for the complement of heavy elements, it has also been theorized that the Sun was born in a large cluster of at least a thousand stars, and was exposed to the right kind of high mass stars early in its formation to seed our solar system with a technologically fortuitous complement of metals. Then the Sun was somehow thrown out of the star cluster with a trajectory that placed it further from the core of the galaxy by a very significant amount. There it assumed a relatively circular orbit, within the plane of the galaxy so as to not expose the solar system to the radiation from the core of the galaxy.5 Dynamical simulations show that about 1 percent of G-type stars may have had this kind of history, including being a single star, with an enrichment of elements from a nearby supernova.6 G-type stars already account for less than 2 percent of the stars in the Milky Way galaxy.7
This has some implications for the habitability of the Earth. The Earth’s chemical makeup may require that it originated much closer to the core of the galaxy, yet, at the same time, Earth must be located at or near its current distance from the core for a significant fraction of its existence in order to protect life on its surface from the high radiation levels closer to the core. Our civilization may require a planet with a history similar to our own. It is unlikely that many stars meet these requirements, and this could severely limit the number of technological civilizations in the galaxy.
As metallicity increases in a galaxy, so does the number of giant planets. The size of planets, then, roughly increases toward the core of the galaxy, and become smaller on average toward the edge. These large planets also have an impact on habitable planets. If there are too many large planets with very elliptical orbits, habitability on any planet in that solar system decreases, with precipitous drops.8
In the largest scale, the universe itself has a habitable age. Life was impossible in the early universe, since all that was available was hydrogen and helium. Too late in cosmic history, and we approach the heat death of the universe, when all the available materials for stars are exhausted.9
There are far more restrictive limits than these, as we have been discovering in this series of articles. Although we have been discussing the Milky Way galaxy, there are factors that apply to galaxies in general, and these habitability factors of course are met in our own. There is a temporal habitability zone when star formation is occurring in regions that are sufficiently supplied with heavy elements that can then give birth to planetary systems capable of producing Class I bodies.10 These factors are not present in every galaxy. There are numerous dwarf galaxies in the universe, but they may never develop the necessary planet-building materials because they do not have enough gas to support the multiple generations of stars required.11
Another class of galaxies, the elliptical, also may not support the formation of later generations of stars enriched with the needed elements because they seem to invest almost all their complement of gas in stars very early in their history. This means there may not be later solar type stars, and very little gas within their system to shield potential planets from the radiation from their cores that contain many very energetic stars and supermassive black holes.12 The orbits of the stars within elliptical galaxies, the most numerous type in the universe, are more chaotic and, as the name implies, elliptical, bringing them into close proximity to the dangerous radiation in their galactic cores with every orbit. The habitability of these types of galaxies and galactic environments merits further research, yet answering these questions about our own galaxy is difficult enough.
Next up: What about the actual data we have been acquiring about exoplanets?
(1) Gonzalez, "Setting the Stage for Habitable Planets," 42-43.
(2) Ibid., 42.
(3) Ibid., 43.
(4) Maria-Fernanda Nieva and Norbert Przybilla, "Present-day Cosmic Abundances, A Comprehensive Study of Nearby Early B-type Stars and Implications for stellar and Galactic Evolution and interstellar dust models," Astronomy & Astrophysics 539 A143 (March 2013):20, accessed March 12, 2014, http://dx.doi.org/10.1051/0004-6361/201118158.
(5) Guillermo Gonzalez, Donald Brownlee and Peter Ward, "Refuges For Life In A Hostile Universe," Scientific American (October 2001): 67.
(6) Richard J. Parker, et. al., "Supernova enrichment and dynamical histories of solar-type stars in clusters," Monthly Notices of the Royal Astronomical Society (January 1, 2014): 958, accessed March 15, 2014, doi: 10.1093/mnras/stt1957.
(7) Gribbin, Alone in the Universe, 87.
(8) Gonzalez, "Setting the Stage for Habitable Planets," 50.
(9) Gonzalez, "Setting the Stage for Habitable Planets," 44.
(10) Guillermo Gonzalez, "Habitable Zones in the Universe," Origins of Life and Evolution of Biospheres 35 No. 6 (December 2005): 601-602.
(11) Ward and Brownlee, Rare Earth, 31.
(12) Gonzalez, Habitable Zones 34.
Image: By ESO/Y.Beletsky [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons.