2.2 Life as a Planetary Phenomenon

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Planets have the necessary concentrations of elements to produce and sustain life

The universe had to reach a point in its development and chemical enrichment for living chemical systems to emerge.
Planets are the first point in the evolution of the universe where the dominance of hydrogen and helium is broken. Planets have high enough concentrations of elements heavier than helium to allow for complex chemical structures to form and sustain themselves.
Recent developments in astrophysics and the understanding of dark matter have emphasized the point that life is a planetary phenomenon.

Where are the planets that could contain life?

Searches for life should focus on detecting signs of life on the surfaces of planets as part of their geochemistry.
Very large Jupiter-like planets, called gas giants, are not thought to be suitable for life because their heavier elements tend to concentrate in the planetary core. Furthermore, they do not have true solid surfaces, and any heavier elements outside of the core get highly diluted in hydrogen and helium.
The search for life on distant planets is focused on a class of planets called super-Earths, which are planets with similar characteristics to our own, but larger in size and mass.
Super-Earths are generally rocky, have some amount of water (often much more than the earth), and are roughly between Earth and Neptune in size.
The only representatives of super-Earths in our Solar System are Earth and Venus, which are both on the small end of the scale compared to super-Earths observed around other stars.
Super-Earths are quite common throughout the galaxy and are in fact the most numerous type of planet observed orbiting other stars.

The “Goldilocks zone”

Kepler-62 is a star that was discovered by NASA’s Kepler Mission to have two super-Earths orbiting it. These planets, Kepler-62e and Kepler-62f, are particularly interesting because they both lie in their star’s habitable zone.
The habitable zone of a star is a set of orbits that are far enough from the star to prevent the total evaporation of liquid water, but close enough to prevent total freezing. These orbits are “just right” for the dominance of liquid water, hence the common term “Goldilocks zone”.
Water is so critical to this definition of habitability because life as we know it uses molecules that require water to function. Water plays many roles in biochemistry, particularly as a solvent and a source of protons.
Why not some other liquid, like methane or ethane? It turns out that water is the most common simple molecule in the universe because it consists of two abundant elements: oxygen and helium.

The current sample of dense exoplanets

The Kepler observatory uses transit photometry to detect planets, a method that involves measuring the decrease in light from a distant star when one of its planets passes in front of it. This type of observation provides a relatively accurate measurement of the exoplanet’s radius, as long as the plane of orbit is in our line of sight.
Exoplanetary mass is measured by observing small variations in the radial velocity of stars relative to Earth, caused by the gravity of orbiting exoplanets. This method works best on small stars that are close to the Earth, and mass measurements still include a significant amount of uncertainty.
Based on these measurements, five exoplanets have been found that match the density of Earth and Venus. These planets are likely to have similar bulk compositions to the Earth.
These planets with parameters close to the Earth’s can be broken into two major groups: rocky planets and water planets.
When planets form, their heavier constituent substances sink to the core, while lighter substances (including water) tend toward the surface. As a result, many super-Earths will be water planets. This is actually not a good thing for the development of life. The dominance of water at a planet’s surface will create a highly dilute environment without the necessary concentrations of other important molecules.
Earth is actually a relatively dry planet, and certainly not a water planet. We tend to conceive of water as abundant on the Earth, but it constitutes a tiny fraction of the planetary mass. The search for life on exoplanets is focused on dry Earth-like planets.