What is an Exoplanet?
An exoplanet is any planet beyond our own solar system. It is shorthand for 'extra solar' planet. Most exoplanets orbit other stars, but free-floating planets, called rogue planets, orbit the galactic centre and are untethered by any star [1]!
How many exoplanets are there?
Think of the number of stars that are in the sky when you look up. These are mostly the stars form our own galaxy, the Milky Way. There are thought to be between 100 to 400 billion stars in our very own galaxy. Guillem Anglada-Escude, an astrophysicist at Queen Mary University of London, and part of the team that discovered Proxima b, Earth’s closest exoplanet says that “100 billion is a reasonable number” [2]. Now let's assume that with each star, there are about three planets in each system. That's around 300 billion planets in the milky way galaxy alone! This could also be thought of as a minimum, as our own solar system has 8 planets, so there could be many more than this, possibly even reaching the trillion mark!
With this many potential planets, which ones might be habitable?
What is the habitable zone?
The habitable zone (HZ), also known as the goldilocks zone, is where conditions for life are 'just right'. The HZ resides in the sweet spot of orbital distance from the star, not too hot, not too cold and so that its protective magnetic field is intact, radiation is low enough to allow an atmosphere to form and the surface is rocky and has liquid water. This HZ however, is a bit of a wavy term. For example, Venus does meet many of these criteria - it is just outside the HZ, has a solid surface and thick atmosphere but it has no liquid water [3]. It also depends on many factors, such as the star type and its solar activity, and is mostly thought of when considering rocky Earth-like planets, depending on the thickness and composition of its atmosphere. So although it's a good starting point, we need more information about the planets too! We also tend to favour rocky planets like Earth, and don't tend to consider subsurface liquid water, which can exist on very cold worlds, such as on some of the Moons of Jupiter and Saturn (Europa), and maybe we should! Europa may be the most promising place in our solar system to find present-day environments suitable for life [4,5]. So although the HZ is a good place to start, it doesn't necessarily mean that Earth's conditions are the optimum conditions for life.
Source : Astrobiology Magazine
It's interesting to think that Earth is the only sample that we have for life. Literally. We have no other evidence of life anywhere else in the universe. Earth is the only thing we have to go off when we think about life and so naturally, we are looking for 'Earth like" planets in our search for extraterrestrial life. So the Habitable zone is really only a guideline.
Looking for Life
By measuring the exoplanet diameter and mass, we can get some insights into their compositions and densities, inferring if they are rocky like Earth or Mars, or if they are gas rich like Jupiter and Saturn. We can also analyse the planet's atmospheres to find out more about their potential to host life. To analyse the atmospheric compositions of exoplanets, we need to measure their spectroscopic data!
Spectroscopy is a technique used by astronomers to see what's in the atmosphere. You can read a bit more about spectroscopy here. Each atom and molecule have their own unique spectral fingerprint, created during energy transitions. So when the light interacts with the molecule it creates an absorption or emission spectrum. If an atom or molecule is present in the exoplanets atmosphere, this molecular or atomic signature will appear on the spectrum of the starlight measured from the exoplanets atmosphere. We then see the signatures and can determine what the atmosphere is made of [6]!
Source: American Astronomical Society
Searching for life can also be inferred with spectroscopy. We can look for 'biosignature gases'. Biosignatures are defined as those produced by life and accumulate in the atmosphere at detectable levels and indicate whether or not life is present on that planet! These biosignatures could be Oxygen, methane, metal halides, sulphur compounds, carbon dioxide, Helium, Neon and Argon [7,8]. The methods for looking for biosignatures are again, going off what we know from Earth, so we know what Earth life produces, so let's look for that!
Remember, we are not looking for life, we are looking for evidence of it! So although Earth's atmosphere is predominantly N2-CO2-O2 atmosphere, we are also looking for biosignature gases that are different from Earth as many molecules are possible and we don't want to miss out on detecting signatures of life [6]!
Instrumentation and Future Searches
Source: NASA
NASA's Kepler and K2 Space Telescopes and NASA's Transiting Exoplanet Survey Satellite (TESS) has already done most of the research relating to exoplanets! She has confirmed over 4331 confirmed, 5708 NASA candidates and 3212 planetary systems already [9]! Gaia telescope is being used to very precisely measure where the exoplanets are, and then the James Webb Space telescope (JWST) which is to be launched THIS YEAR (2021) will be used to measure their molecular and atomic spectra better than ever before! Using infrared wavelengths we will be able to search for signs of water and biosignatures! This should give us more insights into which exoplanets might be habitable and have the right conditions to host life! Machine learning techniques will help with these methods due to large amounts of data and handling different parameters [10].
References
[1] NASA. "What is an Exoplanet?". [online]. Available at https://exoplanets.nasa.gov/what-is-an-exoplanet/overview/. Last Accessed 17th December 2020.
[2] Forbes Science. "Billions Of Exoplanets? Count On It, Say Space Scientists". [online]. Available at https://www.forbes.com/sites/billretherford/2017/12/31/billions-of-exoplanets-count-on-it-say-space-scientists/?sh=20ba70774146. Last Accessed 19th January 2021.
[3] Space Time, PBS Digital Studios. "Habitable Exoplanets Debunked!". [online]. Available at https://www.youtube.com/watch?v=9Q_0vOdzw4Y
[4] Space.com "More than half of all sunlike stars in the Milky Way may have a habitable planet". [online]. Available at https://www.space.com/habitable-planets-common-sunlike-stars-milky-way Last Accessed 19th January 2021.
[5] NASA. "Solar System Exploration: Europa". [online]. Available at https://solarsystem.nasa.gov/moons/jupiter-moons/europa/overview/ . Last Accessed `17th December 2020.
[6] MIT - Sarah Seager. "Exoplanet Biosignature Gases". [online]. Available at https://www.saraseager.com/exoplanet-biosignature-gases/ Last Accessed 19th January 2021.
[7] Space Time. "The Treasures of Trappist-1". [online]. Available at https://www.youtube.com/watch?v=h871oE5QkTU . Last Accessed `17th December 2020.
[8] NASA. "Planets & Origins of Life". [online]. Available at https://www.jwst.nasa.gov/content/science/origins.html Last Accessed 19th January 2021.
[9] NASA. "Exoplanet Exploration". [online]. Available at https://exoplanets.nasa.gov/ . Last Accessed 19th January 2021.
[10] Joshua Hayes. "Exoplanets". Jodrell Bank Observatory, The University of Manchester. 17th December 2020.