The 'Dr. Jekyll and Mr. Hyde' of Planets
The following post was written by Professor Stephen Kane, who has been researching planets around other stars for almost 20 years and has discovered and characterized hundreds of exoplanets, including Kepler-186f - which is the smallest planet yet to have been found in the Habitable Zone of a star. After spending many years working at the NASA Exoplanet Science Institute, Kane is now a Professor of Astrophysics at San Francisco State University.
I have shown in previous blog posts that we have reached a significant threshold in human civilization in that we now know beyond mere speculation that terrestrial (rocky) planets are indeed common. This is incredibly good news to those of us who dare to dream that humanity may one day become a space-faring race and expand among the stars. The opening scenes of Civilization: Beyond Earth depict many spacecraft which leave Earth to find new homes on other worlds. However, knowing that your destination is a rocky planet is not enough. After all, we have numerous examples of rocky planets within our own solar system, only one of which is habitable. Clearly we need to be more careful than simply pointing our navigation system at the nearest star system with a rocky planet.
The clearest example of the potential danger can be seen by comparing Earth with Venus. Venus is often referred to as Earth's "sister planet". This is because it is relatively close and is of similar size and mass as the Earth. It also has a substantial cloudy atmosphere which is effective at reflecting sunlight, giving it a bright appearance in our morning and evening skies. Any similarities to Earth end there. Venus has a mean surface temperature of 462°C (863°F), the result of a runaway greenhouse in its primarily carbon dioxide atmosphere. With a surface atmospheric pressure 92 times that on Earth and clouds of sulfuric acid, Venus can be aptly described as the epitome of uninhabitable. Below is a picture taken of the Venusian surface by the Venera 13 spacecraft, which lasted only 2 hours before it was both crushed and melted by the extreme conditions.
Venus and Earth formed under very similar conditions and probably had water delivered to their surfaces in the same way. However, at some point in their histories, the evolution of their surfaces and atmospheres diverged dramatically! I liken Earth to be Dr. Jekyll and Venus to be Mr. Hyde; two sides of the same coin but with almost opposite natures. To understand the history of the Earth, we must also understand the tenuous gap that separates the Earth from a runway greenhouse, such as that which exists on Venus. That understanding will come from determining the frequency of Venus-like planets as well as possible habitable planets like the Earth.
Fortunately, we are starting to develop techniques which will allow us to distinguish habitable planets from completely barren locations such as Venus. My research group recently developed the Venus Zone using calculations based on climate models but increasing the amount of stellar energy received. In the same way that the "Habitable Zone is the region around a star where a planet similar to Earth could have liquid water on the surface, the Venus Zone is the region around a star where the atmosphere of a planet like Earth would likely be pushed into a runaway greenhouse producing surface conditions similar to those found on Venus. The below figure shows the Venus Zone (red) and Habitable Zone (blue) for stars of different temperatures. The outer boundary of the Venus Zone is the "Runaway Greenhouse" line which is calculated using climate models of Earth's atmosphere. The inner boundary (red dashed line) is estimated based on where the stellar radiation from the star would cause the atmosphere of the planet to erode and disappear relatively quickly. The pictures of Venus shown in this region represent planet candidates detected by NASA's Kepler space telescope. For reference, some of the Solar System terrestrial planets are also shown.
Using all of the available Kepler data and our knowledge of how many stars have planets, we estimate that approximately 32% of small low-mass stars have terrestrial planets that are potentially like Venus. For stars like our Sun, this number rises to 45%. This is the first estimate for how common our sister-planet is in the universe. This means that we need to exercise extremely caution when choosing a potential new home, for it appears as though planets like Venus could indeed be quite common indeed!
Unfortunately it is presently beyond our reach to know for sure if indeed these planets have a runaway greenhouse type of atmosphere. That will require a detailed spectroscopic analysis of the atmospheres to determine the molecular abundances present, such as the dominance of the carbon dioxide spectral lines that we see for Venus's atmosphere. The transit detection method is far more sensitive to short period-planets than long-period planets, meaning that we can detect a Venus-analog much easier than an Earth-analog. Upcoming space telescopes, such as the Transiting Exoplanet Survey Satellite (TESS) will be efficient Venus detectors in the same way that Kepler is. However, the host stars of planets detected via TESS will be significantly brighter than those found with Kepler, enabling detailed follow-up observations with the James Webb Space Telescope (JWST) due to be launched in 2018. Thus there is a strong chance that we will have our first confirmation of a runaway greenhouse atmosphere on an exoplanet within the next 10 years.
This figure shows how the spectrum of Earth's atmosphere differs dramatically from both Mars and Venus (source: ESA 2001). The complexity of Earth's spectrum is being largely caused by biosignatures, gases produced by biological activity on the surface of the planet. Molecular oxygen in the presence of water vapor and methane are strong indicators of habitability or, at least, that Dr. Jekyll has not transformed into the more sinister Mr. Hyde. Current advances such as these are paving a path so that not only can we leave this world behind, but that we'll have a place to go forward to. As you watch the opening scenes of Civilization: Beyond Earth, remember that the exodus will have been preceded by several centuries of painstaking research in which scientists have carefully studied potential homes and excluded the Mr. Hydes lurking in dark. The new beginning for mankind has already begun.