Although we have yet to find an exoplanet, or planet outside our solar system, that is small and cool enough to resemble Earth and possibly host life, planetary scientists say it could happen within the next decade. They are preparing for that possibility by learning as much as they can about the atmospheres of progressively smaller exoplanets that can be thought of as stepping stones on the path to discovering — and making sense of — an Earth twin.
Recently, a team of researchers, including Sara Seager, the Ellen Swallow Richards Associate Professor of Planetary Science in the Department of Earth, Atmospheric and Planetary Sciences, and postdoctoral researcher Nikku Madhusudhan, studied the atmosphere of GJ 436b, an exoplanet located about 30 light years away that is similar in size to Neptune, which is less than four times the diameter of Earth. Although they are large planets with hydrogen-rich atmospheres, Seager describes these "exo-Neptunes" as important practice cases for preparing to make sense of Earthlike exoplanets.
In a paper published Thursday in Nature, Seager, Madhusudhan and colleagues from the University of Central Florida, Columbia University and NASA report that the atmosphere of GJ 436b does not contain the high levels of methane that are expected for a planet with a temperature of about 800 Kelvin (about 1000 degrees Fahrenheit). Instead of an atmosphere with plenty of methane and very little carbon monoxide, the researchers detected the opposite. While it's not yet clear why this is the case, the result of the first atmospheric analysis of an exo-Neptune suggests that scientists have to be more flexible in their theories about the molecular makeup of the atmospheres of smaller and cooler planets.
According to Madhusudhan, who led the interpretation of data about GJ 436b collected by NASA's Spitzer Space Telescope, the results "indicate that some form of extreme non-equilibrium chemistry must be taking place" in the planet's atmosphere.
Equilibrium chemistry refers to the formula that tells you which molecules should appear in the atmosphere of a planetary body if you know its temperature and pressure. Disequilibrium occurs when the type and ratios of these molecules deviate from this calculation. Based on its temperature and pressure, Earth is not supposed to have oxygen in its atmosphere, but because life and plants pump oxygen into the atmosphere, it exhibits disequilibrium chemistry.
Until now, scientists had assumed that the atmospheres of exo-Neptunes would exhibit equilibrium chemistry because detailed chemical modeling of exoplanetary atmospheres had not considered other possibilities. While it's not clear if disequilibrium chemistry can explain the chemical composition of GJ 436b's atmosphere, the authors believe that their work, which was funded by the National Science Foundation and NASA, will prompt a new generation of sophisticated computer models for analyzing smaller exoplanets that must now account for disequilibrium.
"Whatever the explanation ends up being, the MIT team's calculation demonstrates that it's not compatible with the standard approach usually used for exoplanets," says Joe Harrington, a planetary sciences professor at UCF who worked with lead author and graduate student Kevin Stevenson. "We're in new territory."
Scientists can figure out an exoplanet's radius and mass by measuring the changes in the light as an exoplanet crosses in front of, or transits, its parent star. They can also determine the temperature and atmospheric composition of an exoplanet by collecting data when it passes behind, or is eclipsed by, its star.
In 2008, Harrington and Stevenson used the Spitzer telescope to observe six eclipses of GJ 436b. The telescope measures different wavelengths of infrared light, including the flux, or amount of heat given off by a planet's atmosphere as it radiates light from its parent star. The flux values can then be used to figure out what molecules make up a planet's atmosphere.
After averaging the flux values measured by the telescope during the eclipses, Madhusudhan developed a sophisticated computer program to determine the molecular compositions of the planet's atmosphere based on these values. The program combines certain variables, such as the planet's temperature, with different amounts of the most stable and prominent molecules that exist in planetary atmospheres, which are methane, carbon dioxide, carbon monoxide, water vapor and ammonia, into one formula that produces flux values. The program was designed to analyze millions of combinations of these variables and to keep track of which combinations most closely match the flux values reported by Harrington's group. Statistical analysis of these values helped Madhusudhan determine the most likely composition of the atmosphere.
Based on equilibrium chemistry calculations, the researchers predicted that for a planet with a temperature of about 800K, methane would be the most dominant carbon-bearing molecule in its atmosphere. They also expected there would be a lot of water vapor and very little carbon monoxide. But their analysis indicates that the planet's atmosphere has a lot of carbon monoxide and very low levels of methane.
The researchers suggest that the low methane levels might result from vertical mixing, or the process of carbon monoxide being transferred to higher parts of the atmosphere at a rate that is faster than it can be converted into methane. Another possibility is that the temperature on GJ 436b causes methane to be converted into other compounds.
Adam Showman, a planetary scientist at the University of Arizona, says the "provocative result" raises questions about the evolution of this planet, as well as the possibility that its atmosphere might represent an entirely new class of atmospheres that has never been explored. This could mean that the planet is not necessarily a good analog for Neptune, although Showman says much more data is needed to make that conclusion.
Seager agrees that there's probably a lot of "unknown chemistry" operating on the planet, and that the challenge is to understand the range of that chemistry and figure out what it means for GJ 436b and other planets. "We have our work cut out for us," she says. Although the team plans to collect more data to help refine their theories, they are limited by current telescope technology and eagerly await the 2014 launch of the James Webb Space Telescope, which will be able to obtain much higher quality data of exoplanet atmospheres, including those of "super Earths," or planets that are as small as two times Earth's radius.