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.
David Waltham’s central argument in Lucky Planet is that the geological evidence shows the Earth has had a "surprisingly stable climate."1 There are many reasons the Earth shouldn’t have one. He observes, "[O]ur beautiful, complex biosphere could never have occurred if Earth had not enjoyed billions of years of reasonably good weather."2
There are many processes that keep Earth’s environment habitable, "which [in] the Earth’s case may be special rather than universal."3 The evolution of an atmosphere through gas loss to outer space is one. This is primarily dependent on the planet’s gravity holding the gasses in the atmosphere, and the temperature at the outer edge, where gasses can escape if they have enough kinetic energy. The temperature at this upper level, called the exobase, is due to the energy from the host star, both in terms of electromagnetic radiation and solar wind. The exobase absorbs energy, and releases much of it back to space by infrared emission. The gasses in the exobase can vary in their ability to emit this radiation. An atmosphere is held by the planet’s gravity, but it must be strong enough to keep it from escaping when heated by the stellar energies. Mars and the moon have lost their atmospheres by being too small to keep much of their atmospheres from escaping in this way.4
The early Earth also must have been able to radiatively cool and confine the upper atmosphere to protect it from the extreme ultraviolet radiation and solar wind. Some researchers have proposed that the atmosphere must have contained about 100 times the amount of carbon dioxide to provide this protection.5 This fits well with the amount of greenhouse gas needed at that time to compensate for the lower luminosity of the Sun when it was younger. This is again the "faint young Sun paradox" which kept the Earth’s climate remarkably stable over its history. Yet the Sun only had about 85 percent of its present luminosity at the time of life’s appearance on the Earth.6
Such a history is one indicator among others that the Earth may in fact be very special.
Next up: Good luck or God?
(1) Waltham, Lucky Planet, 2.
(2) Ibid., 3.
(3) Forget, "On the Probability of Habitable Planets," 185.
(4) Forget, "On the Probability of Habitable Planets," 185.