Earth-like planets beyond this solar system, also known as exoplanets, are a popular target of research in the search for extraterrestrial life forms. However, the evolutionary processes that give rise to life depend heavily on the physical and chemical characteristics of habitable environments. 

The role of climate in shaping the evolution of life on Earth, and potentially on exoplanets with similar environments, was the theme of a live panel discussion hosted by McGill’s Space Institute (MSI) and the Department of Earth and Planetary Sciences on Oct. 15. The event, titled “Climate and Habitability of Terrestrial Exoplanets,” included panelists such as Raymond Pierrehumbert, Professor of Physics at Oxford University; Sarah Hörst, Professor of Earth and Planetary Sciences at Johns Hopkins University; Nikole Lewis, Professor of Astronomy at Cornell University; and Michael Way, a researcher at the NASA Goddard Institute for Space Science.

A planet’s climate is defined by how the components of its atmosphere––an envelope of gaseous and other small molecules that surround a planet––interact to regulate the light, temperature, and air on the surface.

“Light interactions, temperature, humidity, pressure, and the conversion of gases into different types of particles are all variables that affect how a [planet’s] atmosphere functions,” said Hörst. 

Pierrehumbert added that fundamental physics plays a substantial role in shaping the interactions of particulate matter as they transition from solids to liquid and gases. 

“Climate physics comes from putting together the basic building blocks [of physics, including] light interactions, thermodynamics, and the Newtonian motion for atmospheric fluid,” Pierrehumbert said.

The atmospheres of exoplanets are also known for hosting biosignature gases at detectable levels, implying the possibility of life. These are gases whose origins require biological agents, such as oxygen gas, which is a byproduct of photosynthesis, a quintessential indication of plant life.

 Most of what is known about climate was determined using simulation models based on Earth’s climate.

“We have a good handle on how climate changes over time and the broad range of climates we can expect to see in other places,” said Lewis. 

However, while Earth can act as a standard measure against which scientists can compare atmospheric observations from other planets, Lewis warned that relying on Earth-based data can create biases. Such inherent assumptions can be limiting for astro-climatologists when attempting to distinguish whether an exoplanetary climate condition is conducive to life.

“We have to use this world that we have around us as a kind of benchmark to decide what to look for in our early stage of astrobiological object hunting,” Way said. 

Liquid water, which is considered a requirement for life, is also presumed to be a strong indicator of extraterrestrial life. Way explained that the presence of water on other planets is likely, but the limitation to life could be a matter of whether the amount of water is too little or too much. The latter case poses a potential constraint on evolution.

“Some people think life exists near the sea-floors on Earth [….] Maybe life gets there, but it would be hard for it to evolve into more complex forms later on,” Way said.

The challenges of finding life on exoplanets are multifaceted, even if they have the right atmosphere, moderate climate conditions, and other characteristics of habitability. It is possible that some exoplanets are at different stages of development, eventually evolving to host life. Tracking climate trajectories and other properties that affect the habitability of exoplanets can not only predict the existence of extraterrestrial life, but also give researchers a chance to preview the past and potential future of Earth.

“There is a lot we can learn from Earth’s sweeping climate history,” Pierrehumbert said. “We can [also] look at exoplanet climates and think about something that could be happening on earth in the next century.”

Way echoed his excitement toward the potential of other planets to answer questions about the early history of Earth.

“We’d really like to know how this planet became habitable, and it’s not clear that we really fully understand that [yet],” Way said.