What comes to mind when you think of water? Personally, water reminds me of some of my favorite activities: canoeing down the spring-fed rivers of southern Missouri, bass fishing in Ozark lakes, watching the torrential downpours of thunderstorms, and deep-sea fishing in the Gulf of Mexico. Beyond the fun and enjoyment water provides, it also plays a critical role in Earth’s capacity to host life (as well as the biochemical processes required by life). Consequently, astronomers ardently search for planets capable of hosting water—and those searches have paid dividends.
Using the Hubble Space Telescope (HST), astronomers made detailed observations of a Neptune-sized planet, HAT-P-26b, orbiting a star 400 light-years away from Earth. HAT-P-26b makes a revolution around its host star every 4.2 days and it transits across the face of the star once per revolution. As the exoplanet starts to transit, light from the host star passes through its atmosphere. The HST’s sensitivity allows astronomers to analyze this light and determine what gases exist there. The measurements reveal the presence of water vapor in quantities that exceed those found in the solar system by a factor of 5.1
Another team of astronomers detected an atmosphere around a low-mass exoplanet. The exoplanet, named GJ 1132 b, orbits an M-dwarf star about 40 light-years away and has a mass of 1.6 times the mass of the Earth, making the exoplanet a super-Earth. Using an instrument called GROND, the team observed GJ 1132 b during 9 transits to look for transmission features indicative of water in the exoplanet’s atmosphere. Along with finding unusually large radii for both the exoplanet and its host star, the observations showed a transmission band consistent with atmospheric water. This was one of the first low-mass exoplanets with a temperature below 1000K to show any spectral features. Although an exciting discovery, additional studies “found that the presence of H2O implied either an H2 envelope or low UV flux from the host star early in the lifetime of the system, and the ongoing presence of a magma ocean on the planet’s surface.”2 Consequently, this exoplanet has no hope of hosting life.
Closer to home, the Cassini spacecraft orbiting Saturn found evidence of water/rock interactions on Enceladus, one of Saturn’s moons. Past observations of the moon revealed a large liquid ocean below a thick layer of ice. More recently, astronomers detected plumes of material escaping from the surface of Enceladus. The Cassini probe flew directly through one of these plumes and detected molecular hydrogen, H2. Although not definite, the most probable source of the hydrogen in the plumes is chemical reactions of water with rocks bearing minerals and organic material.3
Life Requires More Than Liquid Water
It may seem like finding water everywhere we look is a sign that life pervades the universe. That may be true, but one should remember that water ranks as the third most abundant molecule in the universe (behind two forms of molecular hydrogen), in part because hydrogen and oxygen are two of the most abundant elements in the universe. Additionally, water on an exoplanet (or a moon) does not automatically make the exoplanet habitable. It seems like life requires far more than just liquid water. Even early Genesis describes an early Earth covered in water, yet hostile to life.
From a scientific perspective, if we ever want to assess what makes a planet truly habitable, astronomers must find a wealth of planets with varying degrees of similarity to Earth and then determine if life actually exists on any of those planets. As I said nearly a decade ago,
The commonly assumed model . . . is that life arises easily in environments that meet a rather small set of criteria. I will refer to this as the “minimalist” model. In contrast, RTB’s creation model argues that life requires a planet exhibiting numerous parameters fine-tuned to exacting specifications. Planets that meet some, but not all, of these criteria serve as test-beds to distinguish which model best describes reality. The more planets astronomers find, the more powerful tests may be conducted.4
Let the testing begin.