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Introduction

The question that I wanted to investigate with this project is: how can atmospheric characterization using spectroscopy possibly be used to detect signs of life on another planet? I find this inherently interesting and it's a field in astronomy that has a lot of potential in the future. I think it could also tell us more about the conditions under which life can exist as well as the atmospheric compositions of exoplanets.

This project touches on many concepts that we have and will discuss in class like spectroscopy, planet transits, and exoplanets. For this project I researched by reading articles, watched videos, and interviews. I created a pop-up book to illustrate some of the concepts from my research.

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The Transit Method

For centuries astronomers have predicted that there may be planets orbiting other stars much like they do in our solar system. These are called exoplanets, meaning planets outside of our solar system. It wasn’t until the 1990s that astronomers actually confirmed that they exist.

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To date we have discovered thousands of exoplanets. There are multiple ways of detecting exoplanets, including the "transit method." Using the transit method astronomers can detect a small dimming of a star's light as a planet passes across it.

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The transit method is helpful in detecting planets which are large in comparison to their stars and which have short orbits, close to their stars. One drawback is that the planet and star must be perfectly aligned from the observer's perspective.

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With the transit method we can use spectroscopy to analyze the light that is filtered through the planet's atmosphere.

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As the starlight passes through the atmosphere it interacts with specific molecules. This process provides us with an absorption spectrum of the planet's upper atmosphere. From this spectrum we can get a better idea of the composition of the exoplanet's atmosphere, this is called "atmospheric characterization."

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Atmospheric Characterization Using Spectroscopy

The size, type, and density of a planet's atmosphere can affect our ability to sample the spectrum.

Top: In a low molecular weight atmosphere more spectral features are expected to be observed.

Middle: Dense and shallow atmospheres are more difficult to sample.

Bottom: Clouds can potentially block starlight and make sampling impossible.

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Types of atmospheric biosignatures

A biosignature is an element that provides scientific evidence of life. The following are four possible molecules or combinations of molecules which could be indicative of life. Even if a planet's atmosphere contains these elements doesn't necessarily mean it hosts life.

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1. Oxygen and ozone - Oxygen supports life and is also a byproduct of life, therefore is a very good indicator of the presence of life. A sustained high level of oxygen may be evidence of life. However, oxygen could be present without the presence of life.

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2. Phosphine and Ammonia - On Earth microorganisms and animals excrete phosphine and ammonia. Observing phosphine or ammonia in an environment where we wouldn’t anticipate them to exist could be a sign of life.

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3. Methane plus carbon dioxide - Observing both of these gases co-existing would be a good indicator that the methane is being continually replaced by a life form.

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4. Chemical imbalances - Combinations that we wouldn’t expect to see otherwise. Life gets very creative and with our earth-centric view, we don’t necessarily know what to anticipate.

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James Webb Space Telescope (JWST)

Using the JWST, which is particularly adept at detecting deep infrared light, allows us to see a greater variety of molecular signatures. The following is a comparison of two planets, in their star's habitable zone, which are being targeted by the JWST.

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K2-18b

You may have seen this planet in the news recently because there was a very faint detection of dimethyl sulfide, a potential biosignature. On Earth dimethyl sulfide is created by phytoplankton. Unfortunately, it is unlikely that this detection is accurate.

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Methane and carbon dioxide were recently detected in its atmosphere.

K2-18b has a hydrogen atmosphere and liquid water making it a (hydrogen + ocean) "hycean planet."

It is less dense than Earth but twice the diameter.

K2-18b may qualify as a super-Earth or mini-Neptune style planet.

1. If it’s a super-Earth it may be a rocky planet covered in a worldwide water ocean.

2. If it’s a mini-Neptune it may have smaller rocky core with a gaseous hydrogen atmosphere.

Whether it can harbor life is yet to be seen.

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TRAPPIST - 1e

Possibly an earth-like ocean planet, and could be a good candidate for habitability.

It has a mass, radius, density, gravity, and temperature similar to Earth.

It is in a system with 7 earth-size planets which orbit a cool red dwarf star. The TRAPPIST-1 system is currently of great interest to astronomers.

TRAPPIST- 1e only takes around 6 earth days to orbit its star.

Trappist planets may possibly have:

1. A rocky surface with no core.

2. A rocky surface with a smaller core than earth.

3. A deep ocean layer on their surface with a large core.

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The transit method is good for analyzing atmosphere's of specific types of planets that:

Orbit close to their stars and have short orbits.

Are large compared to their star.

Happen to transit their star in line with our perspective.

Have large gaseous atmospheres.

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The Direct Method

Another method which we use to find and observe exoplanets is called direct imaging. Direct imaging is what it sounds like: directly observing a planet. Direct imaging could be a very promising way to better observe exoplanets' atmospheres.

Because of how incredibly bright a star is compared to the planets orbiting it, planets not transiting their star appear very dim. In order to be able to see those planets with a telescope astronomers use physical barriers to block the light of the target star. This helps us to see light from the host star which is being reflected on the planets.

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Starshade

The direct method of observing planets is ideally conducted at some distance from our planet. One proposed future project would be to create a "Starshade" which could physically block the light of a targeted star in front of a telescope. The Starshade would formation fly tens of thousands of kilometers in front of a telescope.

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There are no immediate plans to launch a Starshade but the potential is very exciting!

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Sources:

https://www.science.org/doi/10.1126/sciadv.1500047

https://theconversation.com/four-ways-to-spot-hints-of-alien-life-using-the-james-webb-space-telescope-192445

https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/

https://www.youtube.com/watch?v=QWTtN3NQ1Tc&t=646s

https://www.youtube.com/watch?v=5LH1nKHIyjg

https://www.youtube.com/watch?v=STsI6IbPbGQ

https://www.spitzer.caltech.edu/image/ssc2021-02c-possible-interiors-of-the-trappist-1-exoplanets

https://www.youtube.com/watch?v=N8ImhB98FXo&t=2s