Finding Life: Methods of Determining the Habitability of Exoplanets
Introduction to Exoplanets
For hundreds of years, man has looked up at the stars and wondered if there was any forms of life on other planets. In our small sliver of existence mankind has learned a tremendous amount about how fragile and unique life on Earth really is. The question then must be asked. Could other planets in the universe support the development of life? Since the 1960’s, scientists have collected and analyzed data showing that the development of even the most basic life form requires an almost perfect mixture of conditions and elements. It wasn’t until the official confirmation and naming of the first exoplanet orbiting a main sequence star, 51 Pegasi b, in 19951, that scientists officially begin the counting and documenting exoplanets, as well as verifying and validating data involved. With this, science has expanded our search for life outside of our atmosphere and solar system to far off exoplanets. In the following paragraphs, our group will try and help offer a few answers to some of those individuals that look up and stare at the sky. We will start by defining what an exoplanet is, and what makes an exoplanet potentially habitable. We will also explore, in more depth, the boundaries of the building blocks of life in our universe in terms of what is precisely needed to sustain the development of life. Also, we will look at how scientists can determine if an exoplanet has any chance developing life or had life in the past. By the conclusion of this essay, the reader should have a basic understanding of how life needs nearly perfect conditions to have the slightest chance at long term survival.
What Exactly Are Exoplanets?
To begin with, we must start by gaining an understanding of what an exoplanet is, and what separates it from other heavenly bodies travelling through space. Just as the Earth and other planets in our solar system travel orbiting our sun, other planets do the same in their own systems. An exoplanet is any planet that orbits a star in a system other than ours2. Over the last 100 years, thousands of stars and dwarf stars have been discovered, all with varying gravitational pulls. This force acts on the planets and keeps them in a consistent orbital path.
Scientists can determine if something is an exoplanet from Earth in a very interesting way. They study the rotation of what they believe to be the central star of the system, just as our sun is to us. During observations of these stars they carefully watch the rotation on its axis for any imperfections or star wobble. This is called finding the barycenter of a star. It essentially means that the gravitational force of an orbiting exoplanet pulls its host star slightly off its axis.
The methods used by scientists to make these determinations about the status of possible also varies. Since 1988, two main detection methods have been used successfully. The transit method, which basically measures how a star dims when a planet passes in front of it from the perspective of the Earth, and the radical velocity method that measures the wobble of a star caused by the orbiting of exoplanets. There are other methods used as well, on a smaller scale, such as pulsar timing, astrometry, direct imaging, and microlensing.3 All of these methods have contributed to the discovery of 4099 exoplanets as of November 24, 2019.
Can Exoplanets Be Habitable?
Is it possible for an exoplanet to be habitable? The short answer would be yes. The same rules that apply to Earth’s ability to support life would also apply to a different planet from a different solar system. So where does a planet need to be located and what basic features does it need? The first thing that would be needed for an exoplanet to be habitable is that it would need to be located close enough to its sun, this would be called the habitable zone. The habitable zone is a range that is located not too close to its sun so the planet does not overheat and is not far enough that the planet is too cool. A habitable zone can be different in other solar systems depending on the amount of heat that is released from the sun as well as the size of the solar system’s sun. If the solar system’s star has a larger energy output than our sun, the habitable zone would be further from its sun compared to our own habitable zone. A planet also needs to have a molten core. A molten core would be a source of geothermal energy, as well as let the planet recycle its raw materials4. The core would also set up a magnetic field that would create an atmosphere around the planet that would help protect the planet. Having a habitable zone that is just right for a planet, as well as having a molten core would help sustain life and help the planet grow.
What Is Needed For An Exoplanet To Be Habitable?
We know what an exoplanet is and we also know the basics for how an exoplanet can be habitable, but do we know the specifics which are needed for an exoplanet to be habitable and to be able to sustain life? We know that in order for a planet to be habitable it needs key features. A key feature would include the temperature on the planet’s surface that is not hostile to living life forms. Life on Earth is shown to be limited to a temperature range of -15°C to 115°C. However, if looking at the human species, the limits for humans have been shown to be able to survive 4°C to 35°C5, this of course is without shelter and proper gear. So, when looking for a planet that is habitable for humans without proper gear and proper shelter, we can survive between 4°C to 35°C. Earth having an average temperature of 14.9°C (2017)6, we would look for a planet close to this temperature.
An exoplanet would also need an atmosphere that is capable of trapping heat, which is able to shield the surface from harmful radiation, and provide chemicals and nutrients needed for life, such as nitrogen and carbon dioxide. Using Earth, Venus and Mars as examples we know that having an atmosphere keeps the surface warm, Earth and Venus were similar planets, the difference being that Venus is no longer habitable because it is closer to the Sun which caused the planet to overheat and trap harmful gases because of the greenhouse gas effect7. We know that Mars as well was a similar planet, only that the planet’s internal core cooled too quickly which is causing its atmosphere to slowly fade away7. These events that happened on Venus and Mars should be kept in mind when searching for a habitable exoplanet.
In order to maintain life a planet needs to have nutrients including a form of water. Nutrients like proteins & carbohydrates are needed for organisms to grow. Planets also need systems that deliver nutrients to its organisms like the water cycle. Without a system of delivering nutrients, a planet cannot support life. To summarize, we know that in order for a planet to be habitable it must contain key features, including a non-hostile environment due to temperature, an atmosphere to contain what is needed on the planet and to protect, and a planet needs nutrients so that life can grow and maintain habitability on a planet.
How Do We Know If An Exoplanet Is Habitable?
Currently there is only one planet known to man that supports life, and that is our own Earth. To begin with, one must examine the composition of the planet’s atmosphere. For example, oxygen (O2), an incredibly common element on Earth, exists in greater amounts on Earth than any other planet8. This is due to photosynthetic life which produces great amounts of oxygen from carbon dioxide (CO2), forming compounds with nearly every other element on the periodic table8. Current researchers are also looking for water that could support unicellular life, which existed long before oxygen saturated the ocean and filled the atmosphere. These extremophiles, microorganisms that thrive in extreme environments9, have led scientists to look outside our understanding of the Goldilocks zone to other environments, most notably Europa. One of Jupiter’s moons with a surface frozen over by a layer of ice, it is hypothesized that an ocean lies beneath that could possibly hold life in its waters, since life would not affect the visible biosphere, it would be difficult to detect its presence10. Learning that complex life exists in areas of our ocean once thought to be uninhabitable, such as the Hadalpelagic Zone (or deep trenches) support life through chemicals released from ocean vents10. In effect, looking for surface water and an oxygen-rich atmosphere are good filters for pinpointing extraterrestrial life, especially water with tides which some believe lead to the complex molecular fragments that created life10.
When looking habitable worlds, scientists use a variety of methods to determine the basic characteristics of an exoplanet including its mass, density, temperature and the composition of its atmosphere. When an exoplanet passes in front of its home star, it blocks light. The time it takes to move out of the way and the amount of light blocked lets us calculate its size and orbit3. Observing the pull of the planet’s star on its orbit lets us calculate its mass and dividing the mass by the radius of the planet lets us determine the density, to see if it is composed of rock like Earth, or if it is gaseous like Jupiter or Saturn, which have a lower density than a rocky planet11.
In order to determine what the exoplanet’s atmosphere is made of, scientist’s measure the wavelengths of light that reach us. Based on what chemicals make up the atmosphere some wavelengths are absorbed, and some continue to travel through the atmosphere and eventually reach the Earth11. We can use comparative research between the wavelengths gathered from the exoplanet with information from our planet’s atmosphere, since chemicals are the same no matter what part of the universe you are in (the oxygen we breath on Earth is the same oxygen found throughout the universe). Of course, care must be taken to look places other than what we believe supports life, as no one believed before the discovery of fish in the Hadalpelagic Zone that those conditions could support life, alien life could possibly exist on planets with extreme atmospheres such as Venus.
In conclusion, the idea of what makes a planet capable of supporting life is skewed by the earthly idea of life i.e; the slim set of circumstances most life requires on our planet. But we now know that even on Earth there are outliers, life such as the extremophiles that live inside and around exothermal vents deep under the ocean, or the discovery that tardigrades are living on the outside of the ISS without oxygen and are bombarded by stellar radiation. But, of course, the “easiest” way to find a habitable exoplanet would be to look for an Earth-like planet with a molten core producing a protective magnetic field within its local star’s Goldilocks Zone, an oxygen-rich atmosphere, and standing water to act as climate control12 by absorbing solar rays and capturing CO213. These give us our best chance at finding Earth-like life in the stars.
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12University of Delaware. Oceans act as a ‘heat sink’: No global warming ‘hiatus’. https://www.sciencedaily.com/releases/2016/11/161122182458.htm (Accessed November 23, 2019)
13American Museum of Natural History. The Ocean: Climate Control. https://www.amnh.org/exhibitions/climate-change/changing-ocean/the-ocean-climate-control(Accessed November 23, 2019)