Is There Potential for Life Outside Our Solar System?
The Big Picture
Chuanfei Dong is looking to the stars to try to answer one of astronomers’ most vexing questions: Are there other habitable planets in our universe?
Dong, an assistant professor of astronomy, specializes in the physics of plasma, the fourth state of matter. Our sun and other stars are composed of plasma, and Dong’s knowledge of how these hot, electrically charged gasses work helps him understand the impact of stars on exoplanets—planets outside our solar system—and on their potential habitability.
Dong says surface habitability comes down to one thing: atmosphere. A planet’s atmosphere needs the right balance of gasses to support the conditions for most life to thrive. “For example, on the moon and Mercury, there’s practically no atmosphere. Mars’ atmosphere is very thin,” he says.
Scientists can learn a lot about the evolution of an exoplanet’s atmosphere by studying the activity of nearby stars. Stellar magnetic activity, like coronal mass ejections—the eruption of plasma from the stellar surface—can impact the atmospheres of exoplanets over time. To predict whether an exoplanet could be habitable, Dong calculates the loss of its atmospheric gasses to space, due to stellar activity. By running numerical simulations, he’s able to estimate whether any atmosphere remains. “If there’s no atmosphere, then intelligent life like us cannot survive on the surface of a planet.”
Arts x Sciences spoke with Dong about his research and his involvement as an institutional principal investigator on the Mauve mission, which will begin in 2025 and could provide answers to the mystery of exoplanet habitability.
AxS: Your work straddles the fields of astronomy and physics. What inspired you to pursue this research on the habitability of exoplanets?
Dong: [Researching] the habitability of exoplanets is highly interdisciplinary. We need knowledge from different fields. When I did my PhD at the University of Michigan, I mainly focused on plasma physics and the solar wind interaction with Mars.
The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft was launched by NASA to study how the Martian atmosphere escaped from an early warmer wet Mars. We know Mars had a thick atmosphere in the past, but today’s Mars has almost no atmosphere. One of the primary objectives of this mission is to study the solar wind stripping, which needs plasma. So, we need to use the knowledge of plasma to study how those charged particles streaming away from the sun interact with the planetary atmosphere.
Most of our knowledge used to study exoplanets is based on what we learn from our solar system, because observation is highly limited for those exoplanets. Traditionally, astronomers were the ones to detect those exoplanets and characterize them, but about 10 years ago, planetary science, earth science, space physics, all the people from these disciplines, including me, jumped in together to study and characterize exoplanet habitability. I use my knowledge in space plasma physics, and I found it’s very suitable to study how stellar wind and activity interact with those exoplanets.
How does your work studying plasma relate to the search for habitable exoplanets?
For my Sloan fellowship proposal last year, that’s what I was thinking about—how to combine my research of plasma physics and exoplanetary science.
A star, or the sun, is a plasma ball basically. In order to understand how stellar activity impacts exoplanets, we need to understand the origin of the stellar activity—and that requires knowledge of plasma physics.
You may have heard about solar flares, or solar storms. During those events, the radiation becomes much higher, and the star releases a huge amount of mass that impacts the planets. Those flare events, or stellar storm events, can be explained by the breaking and rejoining of magnetic field lines—an ubiquitous process in plasmas. With my knowledge in plasma physics, I can understand the origin of stellar activity, including flares and coronal mass ejections.
What does understanding the origin of the stellar activity tell you about the habitability of a planet? Are there key factors that you consider in order to make a prediction as to whether an exoplanet is habitable or not?
After we understand the stellar activity, we can study how it impacts those planetary systems. I published one paper in 2018, in which I predicted that those close-in exoplanets orbiting around the Trappist-1 star [a red dwarf star with seven Earth-size exoplanets] have no atmospheres. Recently, observations from the James Webb Space Telescope (JWST) on the inner two planets verified what I predicted several years ago.
Stellar wind, stellar activity, is very important in removing heavy elements like oxygen [from the atmosphere]—heavy compared to hydrogen or helium. When we define habitability, especially for intelligent surface-based life, the atmosphere is one of the most important factors. For example, on the moon and Mercury, there’s practically no atmosphere. Mars’ atmosphere is very thin.
The reason I study atmospheric escape is, if I have the escape rate, I can calculate how much atmosphere will be stripped away during the process. And in most cases, we know how old a system is. If I do an integrated calculation of the escape rate over time, that is the total amount of the escaped atmosphere, then I can estimate if there’s anything left or not. If there’s nothing left, then there’s no atmosphere. If there’s no atmosphere, intelligent life like us cannot survive on a planet. The main point is, I want to link habitability to the existence of an exoplanet’s atmosphere.
Do you have a hypothesis about where we are most likely to find habitable exoplanets?
You know the movie Avatar? In it, they are living on a moon, not on a planet. So, if there is a giant planet like Jupiter, and if there is an Earth-sized moon orbiting around this planet, then the planet has a magnetic field that can protect the moon from stellar wind erosion. The magnetic field acts like a shield.
If we focus on exoplanet habitability, we should look at K-type stars and G-type stars. M-type stars are smaller and dimmer, G-type stars are like our sun, and in the middle are K-type stars. Basically, the larger the star, the shorter its lifetime is, which means its activity period will not be as long as those of M-type stars. Our sun was very active mainly in the first 1 billion years, and later it became less active. But M-type stars can be active for several billion years. Their stellar flares and coronal mass ejections are much more frequent and extreme [which contributes to a higher rate of atmospheric escape]. Our sun is already in its middle age. But for M-type stars, their lifetime can be as long as the universe. So we don’t know how long they can survive. Many of them formed at the beginning of the universe, but our sun formed only 4.6 billion years ago, and the universe is around 14 billion years old.
Tell me about your involvement with the Mauve mission and how it relates to the search for habitable exoplanets.
With the Mauve mission, we mainly observe stars. If we know the stars better, it helps a lot with understanding planetary habitability. I’m the institutional principal investigator of the Mauve telescope. It’s an ultraviolet (UV) telescope. UV is in short wavelengths, and the shorter the wavelengths, the more energetic the phenomena—like flares—we can observe.
We want to observe stellar activity, like stellar flares or stellar coronal mass ejections. In principle, this can also be done by NASA’s Hubble telescope, which also covers UV wavelengths. But the Hubble telescope is not dedicated to studying stellar flares because a lot of scientists from other fields also want to use it to study their topics. With Mauve, we can allocate a lot of time to focusing on the observation of stellar activity.
If we observe that a star is highly magnetically active, with a lot of flares and coronal mass ejection events, then it’s implied the close-in rocky exoplanets probably won’t have an atmosphere, because of stellar wind erosion or stellar impact.
But we can also do studies to see if there are stars with low stellar activity. If we find such a candidate, then we can actually focus on that system and we can leverage observational time from the JWST to further characterize those exoplanets. Although it’s challenging, with the JWST we should be able to observe the transmission spectra of those exoplanetary atmospheres, which can tell us about the composition and the abundance of those atmospheres. So the Mauve mission can basically use stellar observations to infer if those exoplanets have atmospheres or not.