Our place in the universe

Photo by Castelski

Humans are born explorers. We are driven to understand our universe and why we are here. The search for exoplanets encompasses both of these themes. We hope that understanding how planets form and the kinds of planets will help us better understand Earth, our solar system and our role here.

When astronomers began searching for exoplanets using the traditional tools of astronomy, they looked for copies of our solar system. It's amusing to consider how this entire paradigm of science – how planets form – had to be built upon our understanding of our solar system.

No place like home?

Out of the thousands of stars with known planets, none resemble our solar system. Admittedly, our system is quite hard to find because of its unique configuration of planets. As it turns out, the diversity of planetary systems is incredible.

For instance, some planets are rare but easy to find. These giant balls of hydrogen and helium are known as hot Jupiters because they resemble the planet Jupiter. Unlike Jupiter, however, they are many times closer to their sun than even Mercury is.

We have also found planets we call mini-Neptunes. These exoplanets are small versions of Neptune. Neptune is four times Earth’s size. These mini-Neptunes, on the other hand, are about two to three times the size of Earth. Amazingly, these planets appear to be the most common type of planet in our galaxy. Yet, we have no solar system counterparts, so we don't understand how they formed. Indeed, there are many planets we've discovered that are very unusual.

The Kepler space telescope

The Kepler space telescope, which launched in 2009, changed everything. The telescope was many decades in the making. Its design was made possible by CCDs, a type of detector.

Moreover, it showed us that planets are incredibly common and that planets two to three times the size of Earth are the most common in our galaxy. This telescope also showed us that small rocky planets – which could host life – are also prevalent. Indeed, Kepler was a game-changer for the field of exoplanets.

Modern telescopes and exoplanets

The field of exoplanets will continue to evolve as more telescopes like Kepler are designed. Currently, astronomers are eagerly awaiting the next big telescope called the James Webb Space Telescope or JWST. NASA, ESA and the Canadians lead its development. Although the JWST is a successor of the Hubble Space Telescope, it will not find planets. Instead, it will work in infrared as opposed to visible light and will help characterise planets.

Photo by Vadim Sadovski

Furthermore, the JWST will not orbit the Earth as the Hubble does. Instead, it will travel a million miles away from Earth to be in a stable, dark and cold environment that's ideal for astronomy. It will study the atmospheres of transiting planets and help astronomers understand what those planets are made of and whether they might be suitable to host life.

Characterising planets

The characterisation of a planet is actually relatively crude. Astronomers would like to know what the planet is made of internally. We'd like to know if it's rocky or has liquid water – fundamental factors like that.

Characterising a planet also means trying to understand its atmosphere and the gases present. We'd also like to know the temperature of the planet's surface. For that, we need to understand the greenhouse gases present in the atmosphere.

Currently, when we find planets, we know very little about them. We might see the planet's size, its orbit and its distance from its star. We can also tell how much energy it absorbs. Yet, there is only so much information available.

Using the JWST

There are too many planets and too little time. Interestingly enough, it's a competitive endeavour to be an astronomer. There is a proposal system whereby astronomers can write a proposal to use the JWST.

Proposals are evaluated via a double-blind review. This means the proposer and reviewer are kept anonymous. A select group of peers serves on a panel which selects proposals. Those who are chosen will then use the telescope to observe a specific planet. Eventually, the data becomes public, and any capable individual can analyse it.

Moreover, if you have built an instrument for the JWST, you are guaranteed time. Naturally, these instrument builders have selected the most exciting planets. Unfortunately, planets can only be observed once. Although it’s a chaotic process, we hope as a community to choose the right planets to observe.

A needle in a haystack

Although we know of thousands of planets, only a tiny fraction of those are suitable for observation. This is because the planet and its star have to be bright enough for us to collect enough photons to measure the atmosphere. Fortunately, NASA has developed computer software tools to calculate how long a proposed planet needs to be observed. These factors are all considered in the proposal.

Unfortunately, only a few of these small rocky planets which potentially contain signs of life are suitable to observe with the JWST. As you might imagine, many were selected early on by the instrument builders with guaranteed time.

Making the right choice

Photo by Vadim Sadovski

There's a popular new planet that I've proposed to observe. Its name is K2-18B. That means it's a planet from the second version of the Kepler mission. Interestingly, this planet is one of the mini-Neptunes that are two to three times the size of Earth. Since we have no solar system counterpart, we don't know what these planets are made of.

Nevertheless, this planet is fascinating because we believe it’s a similar temperature to Earth. Although it likely doesn't have a solid surface, it is enveloped in hydrogen and helium. Furthermore, there is a small possibility that it has a liquid water ocean.

Sure enough, it’s exciting to observe a mini-Neptune. If these planets contain large amounts of water, their liquid water ocean layer may sit atop a layer of high-pressure ice. I propose to look for atmospheric gases that could indicate the presence of an ocean. Yet, I'm certain many are also interested in this planet because it's so compelling.

The right team

I have a core team of about six people who are familiar with data analysis. As a team, we have experience in observing planet atmospheres and looking for gas signatures.

However, my larger team that works on the problem is very interdisciplinary. We have a biologist and two biochemists. We also have an expert in atmospheric photochemistry. This chemistry involves the cascade of reactions caused by a star's ultraviolet radiation. Other members understand volcanoes and geologic formations that might contribute to the atmosphere.

I'm also connected to a much larger group of people who study the high-pressure interior of planets. If these mini-Neptunes have deep water oceans sitting atop water ice, we need to understand how energy and materials flow through these systems. All in all, it takes a broad array of people and disciplines to understand exoplanets.

Discover more about

How exoplanets are characterised

Pong, C., Lim, S., Smith, M. W., et al. (2010). Achieving high-precision pointing on ExoplanetSat: Initial feasibility analysis. Proceedings of The Society of Photo-Optical Instrumentation Engineers, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave, 77311V, 54–70.

Rogers, L. A., & Seager, S. (2010). A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions. The Astrophysical Journal, 712(2), 974–991.

Fentress, S. (2021). Kepler Space Telescope: The Original Exoplanet Hunter. Space.

Seager, S. & Deming, D. (2010). Exoplanet Atmospheres. Annual Review of Astronomy and Astrophysics, 48, 631–672.