A New Technique for “Seeing” Exoplanet Surfaces Based on the Content of their Atmospheres

In November of 2021, the James Webb Space Telescope (JWST) will make its long-awaited journey to space. This next-generation observatory will observe the cosmos using its advanced infrared suite and reveal many never-before-seen things. By 2024, it will be joined the Nancy Grace Roman Space Telescope (RST), the successor to the Hubble mission that will have 100 times Hubble’s field of view and faster observing time.

These instruments will make huge contributions to many fields of research, not the least of which is the discovery and characterization of extrasolar planets. But even with their advanced optics and capabilities, these missions will not be able to examine the surfaces of exoplanets in any detail. However, a team of the UC Santa Cruz (UCSC) and the Space Science Institute (SSI) have developed the next best thing: a tool for detecting an exoplanet surface without directly seeing it.

The paper that describes their research, titled “How to Identify Exoplanet Surfaces Using Atmospheric Trace Species in Hydrogen-dominated Atmospheres,” recently appeared in The Astrophysical Journal. As they indicated, the team sought to develop ways to study the surfaces of exoplanets based on their atmospheric composition. This is necessary since none of the upcoming space telescopes have the capacity to study surface features of an exoplanet indirectly.

However, these same telescopes will be excellent tools for determining the composition of exoplanet atmospheres. Beyond the James Webb and Roman Space Telescopes, a number of next-generation ground-based observatories will become operational in the coming years that will have similar capabilities. These include the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT).

With their combination of high-sensitivity, coronographs, and adaptive optics, these observatories will be able to conduct Direct Imaging studies of exoplanets, where light reflected directly from an exoplanet’s atmosphere will be studied to determine atmospheric composition. This will help astronomers and astrobiologists place tighter constraints on which exoplanets are “potentially habitable” and which are not.

However, the conditions we consider prerequisites for life also include geological processes like volcanic activity and plate tectonics, which are discernible from their associated surface features. While we may not be able to discern these in the near future, Xinting Yu (an Earth and Planetary Sciences Postdoctoral Fellow at UCSC) and her colleagues have proposed a novel way to determine surface features based on the abundances of atmospheric gases.

As Dr. Yu explained to Universe Today via email, the inspiration for this method came from two bodies in our Solar System – Jupiter and Titan (Saturn’s largest moon). Both bodies have dense gaseous atmospheres with two chemical species – ammonia (NH³) and methane (CH⁴) – that play a major part in atmospheric processes. Said Yu:

“Titan has a cold and shallow surface with almost no (or not supposed to be any) ammonia and methane, while Jupiter’s atmosphere has lots of ammonia and methane. Why is this happening? In the upper atmosphere of both Jupiter and Titan, ammonia and methane are destroyed by UV photons constantly, forming nitrogen (for ammonia) and more complex hydrocarbons (for methane). On Titan, the photochemistry-formed nitrogen and complex hydrocarbons keep forming and piling up.”

In short, methane and ammonia are destroyed in Titan’s atmosphere and then consumed to form nitrogen and hydrocarbons. This is what led to nitrogen becoming the dominant gas in Titan’s atmosphere (98% by volume) and the large deposition of hydrocarbons on its surface, leading to the formation of an organic-rich environment. Due to the extreme cold of Titan’s surface, this conversion process is irreversible.

Jupiter, on the other hand, also has ammonia and methane in its dense atmosphere but has no surface to speak of. As Yu explained, this results in a rather different process where the chemical species involved:

“Because there is no surface on Jupiter, the atmosphere just extends all the way to thousands of Earth surface pressures and thousands of kelvins. The photochemistry-formed nitrogen and complex hydrocarbons in the upper atmosphere can transport to this deep, hot part of the atmosphere. There, they could combine hydrogen to reform methane and ammonia. The reformed methane and ammonia are then “recycled” back into the upper atmosphere. This cycle continues to replenish the destroyed methane and ammonia.”

Another key point addressed by Yu and her team has to do with the current exoplanet census. To date, the majority of exoplanets discovered have been mini-Neptunes – i.e., planets that are less massive than Neptune but have a thick atmosphere dominated by hydrogen and helium. In fact, of the 4,401 confirmed exoplanets to date, 1,488 have been identified as “Neptune-like,” with masses ranging from 9 times that of Earth to slightly less than Jupiter.

Because of their gaseous envelopes and the distances involved, it is impossible to determine if these planets have surfaced and where they are located. Because of their statistical significance, Yu and her team decided to use one in particular to test their novel approach. This was K2-18b, a mini-Neptune with about 8 times the mass of Earth that orbits within the habitable zone (HZ) of a red dwarf star (K2-18) located 124 light-years from Earth.

Originally detected by the Kepler Space Telescope in 2015, K2-18b is the first HZ exoplanet found to have significant amounts of water vapor in its atmosphere. Using a photochemical model, Yu and her team simulated how the presence of a surface on this exoplanet would affect the atmospheric evolution of K2-18b. They also accounted for different atmospheric pressure and temperature levels, factors that are linked to surface elevation.

“We are wondering if we can use the abundance of species like ammonia and methane to tell if an exoplanet has a surface or not,” said Yu. “A cold and shallow surface would cut all the “recycling” reactions that require high temperatures and pressures in deep planetary atmospheres to reform methane and ammonia. Thus, we expect to see little methane and ammonia on an exoplanet with a cold and shallow surface, and lots of methane and ammonia on an exoplanet with no surface or a deep and hot surface.”

What they found was that ammonia and methane, as predicted, were both sensitive to both the presence and elevation of a surface. This is consistent with what has been observed with exoplanets that have cold and shallow surfaces, where chemical species like water, hydrogen cyanide, and heavier hydrocarbons are broken down by UV exposure. Meanwhile, species like carbon monoxide and carbon dioxide (which are less prone to UV destruction) are retained.

What was unexpected, however, was the way that different chemicals are sensitive in different ways to different levels of elevation. According to Yu, this is due to the fact that carbon and nitrogen species have a “sweet spot” where they can be fully recycled. Whereas ammonia and hydrogen cyanide (HCN) are sensitive to atmospheres with densities of 100 bar at the surface (100 times that of Earth, similar to Venus), methane, carbon monoxide, and carbon dioxide are sensitive to pressures below 10 bar at the surface (ten times that of Earth).

These findings present multiple implications for the study of exoplanets, foremost of which are the fact that planetary surfaces matter. Said Yu:

“Previously, scientists were predicting the atmospheric compositions of exoplanets using thermochemical equilibrium models. The atmospheric compositions are solely determined by the pressure and temperature of the atmosphere. But our study shows, even if pressure and temperature are the same, adding a surface can drastically change the atmospheric composition of an exoplanet!”

Another implication of this study is that it is possible for astronomers to learn about exoplanet surfaces based on their atmospheric composition. “For example, when the observers see depleted amounts of ammonia and HCN, we can tell this exoplanet has a surface of less than 100 bar,” added Yu. “Then if we also see depleted quantities of methane, hydrocarbons, and an increased amount of carbon monoxide, that indicates a surface less than 10 bar. That is pretty promising for identifying habitable exoplanets!”

Beyond the characterization of mini-Neptunes, this research also has implications for all other types of exoplanets – including rocky, “Earth-like” ones. In fact, as long as the planet in question has an atmosphere and is subject to UV radiation in its upper atmosphere, the size of the exoplanet is irrelevant. In all cases, astronomers will see the same differences in chemical abundances depending on whether or not there is a surface.

According to Yu, it is the smaller colder exoplanets that are more promising testing targets for this method since they are more likely to have shallow and cold surfaces. However, smaller planets are also more likely to have interior or surface processes that will affect the abundance of certain chemicals in their atmospheres – such as volcanic activity and plate tectonics. The smaller they are, the more significant these processes could be.

These and other concerns are things that Yu and her team look forward to studying in greater detail in the future to determine the robustness of their results and how it might be affected by different perturbations from the surface/interior of the exoplanets. Their efforts, and those of astrobiologists in general, will benefit greatly from the launch of the JWST, which is currently scheduled to take place sometime in November of 2021. Said Yu:

“Our study points out an exciting science angle for JWST. It is fine to have solely atmospheric characterization data. Without direct surface observations, we can still tell if an exoplanet has a surface, and even roughly where the surface is located. Knowing whether an exoplanet has a surface is also undoubtedly important for astrobiology. A liquid or a solid surface is likely necessary for sustaining complex lifeforms. Thus, the existence of a surface would be an essential thing to look for when assessing an exoplanet’s habitability.”

The ability to study exoplanets directly, combined with the ability to constrain their surface conditions, will advance the study of astrobiology considerably. The field will also benefit from innovative methods that could allow scientists to search for life (aka. biosignatures) based on different levels of entropy in an environment or different levels of complexity with organic particles. Little by little, we are narrowing the focus and tightening the constraints!

If there is life out there to be found, we find it sooner or later!

Further Reading: The Astrophysical Journal