written by: jessica wang and sorina andrei
graphics by: irene zhao
From the movie Zathura to Dune, we imagine what alien life might be like. But, in order to interact with aliens, we have to find them first. So how exactly do we go about doing that?
Unfortunately, in our quest to find life-containing planets, we only have a tiny sample of one planet to start from: Earth. As a result, we can only try to find life from our terracentric or Earth-focused point of view.
Exoplanets: an overview
An exoplanet is a planet that orbits a star outside of our solar system. There are currently over 4000 confirmed exoplanets that have been discovered by the Keplar Telescope (2). In order for an exoplanet to warrant further analysis and exploration, it must meet certain criteria. Specifically, criteria corresponding to its ability to sustain life.
1. The planet should be rocky like the Earth. Gas planets, such as Jupiter, are not solid and would be unable to host any objects, living or not.
2. The exoplanet must be within a specific distance from the star it orbits, referred to as the habitable zone, also known as the Goldilocks’ zone (3). It’s important for the planet to be far enough away from the star so that radiation will not harm the organisms living on the planet. The temperature of the planet must also fall within a certain range so that water, if present, remains liquid (3). If it is too close to the star, the water will evaporate and stay in gas form. Conversely, if the planet exists too far, water will freeze and become ice.
3. The type of star that the planet orbits is also important (3). Brighter and larger stars tend to die within a few million years, which isn’t enough time for life to develop. The first evidence of life on Earth started about 1 billion years after the planet was created and it took over 3.5 billion years for life to evolve to what we see today. Instead, dwarf stars, especially K dwarfs, would be ideal for an exoplanet to orbit as they tend to live longer (3).
Generally speaking, an Earth-like, Earth-sized planet that orbits a star similar to our Sun at the same distance would be the ideal candidate to support life (3).
Techniques to find signs of life
Once an exoplanet fitting the necessary criteria is located, space agencies can begin to investigate for signs of life through transit observations (4). Transit means when something (in our case a planet) passes in front of its host star. From this, our telescopes, such as Hubble Space Telescope, Spitzer, James Webb Space Telescope can gather accurate information and data about the planet’s mass and composition (5,6).
In particular, these telescopes look at how the frequency and intensity of light changes when the planet passes the frame of observation through spectrometry. Observers can then gather this spectroscopic information twice per orbit; one when the planet passes in front of the star, called the primary eclipse, and once when it passes behind the star, the secondary eclipse (4,7). Think of the planet as casting a shadow. By seeing how much and how frequently the host star’s light is blocked, we can estimate the planet’s size and orbital period (4).
Most importantly, this data reveals what type of light is absorbed and emitted by the planet. Since different chemicals have different patterns of absorption and emission, known as the absorption and emission spectra, the composition of the planet and its atmosphere can be determined. Once the atmosphere is characterized, researchers can begin to look for the gaseous signs of life (4).
What exactly are the signs of life?
The pigmentation or colour of an exoplanet can often provide insight into whether or not vegetation, lichen, or algae can be supported on it in addition to its composition (8). Plants and some bacteria will make the planet appear green like the Earth and are crucial for producing oxygen (8).
Technosignatures
Signs of megastructures, laser pulses, and artificial sources of energy (9)
E.g. Solar panels
Use spectroscopy to look for pollutants produced by an industrialized civilization(9)
Methane, oxygen, or other artificial gases
Non-random electromagnetic emission patterns that can indicate the use of technology(9)
E.g. radios
In 1977, NASA sent out a golden record on the Voyager as a time-capsule depicting life on Earth in hopes of communicating with extraterrestrials(10)
Unfortunately, NASA has yet to receive a reply
Biosignatures
An indicator of simple life and primarily look at basic chemical components that are necessary for organisms to survive (8)
Spectroscopy is used to detect traces of life-baring elements and molecules such as oxygen, hydrogen, methane, nitrogen, phosphorus, carbon, and sulfur (11)
Water is widely regarded as the key molecular compound that's presence suggests whether life can or can not be supported (11)
To conclude, biosignatures reveal a lot about an exoplanet’s habitability and can help bolster humanity’s quest to discover extraterrestrial life. With such a diverse array of biosignatures to explore, it’s impossible to cover all of them within one article. Stay tuned for our next post on the signs of life where we delve deeper into Earth-like biosignatures and why they are important.
Glossary
Biosignatures: a substance or pattern that suggests the presence of past or present life.
Composition: what the object is made out of
Dwarf star: a smaller star with low luminosity
Habitable zone: the distance from a star at which liquid water could exist on a planet
Host star: the star the planet revolves around. For example, the Sun is Earth’s host star
K dwarf star: an orange dwarf star that is smaller and cooler than the Sun. It is relatively abundant and long-living
Orbital period: how much time it takes a planet to make a circle around another object. For example, Earth’s orbital period around the sun is 1 year
Pollutants: a substance that results in adverse effects to the environment
Spectrometry: measurement of light’s interaction with matter to produce spectroscopic data
Spectroscopy: study of how light interacts with matter
Techno signatures: signs of technological activity that may be generated by intelligent life
Transit: when an object passes across a larger object and covers part of the larger object (1)
References
Editors of Encyclopaedia Britannica. Transit [Internet]. Britannica. 2006. Available at: https://www.britannica.com/science/transit-astronomy
Can We Find Life? | The Search For Life [Internet]. Exoplanet Exploration Planets Beyond Our solar system. Available at: https://exoplanets.nasa.gov/search-for-life/can-we-find-life/
The Habitable Zone | The Search For Life [Internet]. Exoplanet Exploration Planets Beyond Our solar system. Available at: https://exoplanets.nasa.gov/search-for-life/habitable-zone
Seager S, Bains W. The search for signs of life on exoplanets at the interface of chemistry and planetary science. Sci Adv. Maart 2015;1(2):e1500047.
Wheatley PJ, West RG, Goad MR, Jenkins JS, Pollacco DL, Queloz D, et al. The Next Generation Transit Survey (NGTS). Mon Not R Astron Soc. 21 April 2018;475(4):4476–93.
Greene TP, Line MR, Montero C, Fortney JJ, Lustig-Yaeger J, Luther K. CHARACTERIZING TRANSITING EXOPLANET ATMOSPHERES WITH JWST. Astrophys J. 19 Januarie 2016;817(1):17.
Seager S. The future of spectroscopic life detection on exoplanets. Proc Natl Acad Sci. 02 September 2014;111(35):12634–40.
Extraterrestrial life probably exists. How do we search for aliens? [Internet]. National Geographic. Available at: https://www.nationalgeographic.com/magazine/2019/03/extraterrestrial-life-probably-exists-how-do-we-search-for-aliens/
Frank A. Do aliens exist? Technosignatures may hold new clues [Internet]. University of Rochester. 2020. Available at: http://www.rochester.edu/newscenter/technosignatures-hold-clues-to-advanced-extraterrestrial-life-441472/
Voyager - The Golden Record [Internet]. Jet propulsion laboratory California Institute of Technology. Available at: https://voyager.jpl.nasa.gov/golden-record/
NASA Rover Finds Conditions Once Suited for Ancient Life on Mars [Internet]. NASA. 2013. Available at: https://www.nasa.gov/mission_pages/msl/news/msl20130312.html
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