Lesson 1: Much Ado About Aliens
"Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying"
Arthur C. Clarke
Key Learning Objectives:
Define the key aspects of an exoplanet
Identify the features of an exoplanet that make it suitable for the search for life
Define the major principle of spectroscopy and examples where it is useful in society
Describe some upcoming missions that aim to further understanding of exoplanets and their atmospheres
In this lesson you'll find...
Searching for Habitable Exoplanets
Types of Exoplanets
Looking for Molecules on Exoplanets: Transmission Spectroscopy
Future Telescopes
Searching for Habitable Exoplanets
An exoplanet is a planet outside our solar system that orbits another star. We have currently discovered more than 5500 exoplanets with over 10,000 more planetary candidates awaiting confirmation!
As humans, it is natural to wonder if there are other Earth-like planets that have developed complex life, or even if our descendants might one day live on another world. While we consider that Earth is the only habitable planet in our Solar system, we discovered that other stars in our galaxy also have planets and solar systems around them.
Class Discussion
There are countless exoplanets within the range of our telescopes but we do not have the time or equipment to survey them all. What are some criteria we could lay down to make sure we are investigating planets that might host life as we know it here on Earth? Consider things like planet size, distance from star etc.
Humans have limited technological resources to search for exoplanets so scientists characterise exoplanets in order to narrow the list of exoplanets they need to identify. They categorise the star the planet orbits, and the distance of the planet to its host star to determine whether it would be possible for liquid water to exist on the surface of the exoplanet. It is important to note that at this current moment, scientists are focussing on water-based life, even though we acknowledge that it may be possible for other forms of life to exist. Essentially, we can only start looking for life that is similar to our own since we understand the specific set of conditions that must be met for our type of life to thrive. Consider, if we were looking everywhere for everything, would we find anything important if we cannot identify what the results mean?
Scientists usually refer to the Habitable Zone, also known as the Goldilocks zone, which refers to the range of distances from a star within which a celestial body, such as a planet, could have the right conditions to support liquid water on its surface. This is considered a key factor in determining the potential habitability of a planet.
The habitable zone is typically defined based on the star's characteristics, such as its luminosity and temperature. If a planet is too close to its star, the heat may be too intense, causing water to evaporate. On the other hand, if it's too far away, the planet might be too cold, and water would exist only in frozen form. The habitable zone represents the "just right" range where the conditions could be suitable for the existence of liquid water—a crucial ingredient for life as we know it. The figure below presents the TRAPPIST-1 exoplanetary system, situated ~40 light-years away from Earth, highlighting its habitable zone (green circle).
Types of Exoplanets
With over 5500 exoplanets confirmed to date, it is only reasonable to expect a huge diversity of demographics showcasing the planet's physical and chemical characteristics. We can categorise exoplanets based on their similarities to the planets that we know in our own solar system. Here are some common types:
Hot Jupiters: These are gas giant planets similar to Jupiter but orbit very close to their host stars (their orbital period can be of less than 10 days!). They often have high temperatures due to their proximity to the star.
Super-Earths: Super-Earths are rocky planets with masses larger than Earth but smaller than Neptune. They can have diverse compositions, including rocky surfaces or water-rich structures.
Neptune-like Planets: These planets are larger than Earth but smaller than Neptune. They are thought to have thick atmospheres, possibly composed of hydrogen and helium, surrounding rocky or icy cores.
Terrestrial Planets: Similar to Earth, terrestrial planets are rocky and have solid surfaces. They may have atmospheres and are potential candidates for habitability.
Ice Giants: Ice giants are planets similar to Uranus and Neptune in our solar system. They are often composed of heavier elements like water, ammonia, and methane and have thick atmospheres.
Exomoons: Moons orbiting exoplanets are also of interest. While not yet confirmed, scientists are actively searching for them.
Note that these categories are not mutually exclusive, and many exoplanets exhibit characteristics that make them fall into more than one type. The study of exoplanets is a rapidly evolving field, and new discoveries continue to expand our understanding of the diversity of planets beyond our solar system.
You can learn more about exoplanets using the Eyes on Exoplanets website by NASA. Explore the link below and discuss with you peers all the amazing information given! Note that in the image below, every "star" corresponds to a different exoplanet.
Looking for Molecules on Exoplanets: Transmission Spectroscopy
Finding exoplanets largely relies on physics. Understanding what is on the exoplanet, however, relies on chemistry! Upcoming space telescopes will be able to identify the molecular composition in exoplanet atmosphere's by looking for the fingerprints of molecules in the light. This is accomplished by using spectroscopy: a fascinating discipline situated at the interface between chemistry and physics!
Spectroscopy: The study of the interaction of electromagnetic radiation with atoms and molecules. This interaction happens to be particular for each systems, which makes it work as a fingerprint for the identification of atoms and molecules.
There are two main chemistry concepts involved in exoplanet atmospheres that are studied in the Year 12 HSC Chemistry Syllabus:
Equilibrium (Module 5)
Spectroscopy (Module 8)
This Depth Study assumes you have completed the basics on Module 5 but not started Module 8. Lessons 7 and 8 will tell you everything you need to know about spectroscopy for this Depth Study and provide excellent pre-work for Module 8.
Activity
Read the following article and summarise how transmission spectroscopy is used by telescopes to characterise the chemical composition of exoplanet atmospheres.
It's all in the atmosphere: Exploring planets orbiting distant stars
The red hue of the moon during a total lunar eclipse gives astronomers a cue on how to find out more about the planets being discovered around other stars.
Future Telescopes
Current and upcoming telescopes are designed to exponentially increase the number of molecules we can discover on exoplanets, paving the way towards even more profound insights into the exoplanets atmospheric chemical diversity. The videos below present a short introduction to some of these current and new telescopes that will help us search for chemical activity that may potentially point to life on another planet!
James Webb Space Telescope (JWST)
JWST promises to revolutionise our understanding of the universe by probing the infrared spectrum with unprecedented sensitivity, enabling the study of celestial objects and events obscured by dust clouds.
ARIEL
ARIEL will employ a spectrometer to analise the starlight filtered through the atmosphere of exoplanets, unveiling crucial details about their chemical composition and physical properties.
The Extremely Large Telescope (ELT)
ELT's colossal mirror will offer unprecedented resolution, revolutionising our exploration of the cosmos and providing invaluable insights into the most distant and enigmatic celestial phenomena.