Products You May Like
Hardly a week goes by anymore that we don’t hear about a new and potentially habitable exoplanet. Invariably, the study authors suggest that the James Webb Space Telescope is uniquely capable of observing this world to determine if it could host life—or even if there’s something alive there now. While it’s true that Webb is more capable than other telescopes in this way, it’s unfortunately not as simple as pointing it at a star and snapping a picture. Two NASA astrophysicists working on Webb explain in a new interview why even this powerful instrument will find it difficult to detect alien life.
With more than 5,000 known exoplanets, only a few dozen are potentially habitable. These worlds are small enough that they are most likely rocky and close enough to their stars theoretically to have liquid water. However, not even all of these worlds are possible targets for Webb. According to Christopher Stark and Knicole Colón of NASA’s Goddard Space Flight Center, only exoplanets with transiting orbits are going to get any attention from Webb, and plenty of conditions can negatively impact Webb’s ability to gather data.
A transiting exoplanet has an orbit that takes it in front of its host star from our perspective here on Earth. When it passes in front of the star, some of its light filters through the exoplanet’s atmosphere (if it has one). To detect that light and differentiate it from un-altered starlight, you need a very sensitive telescope that can observe infrared wavelengths—that’s Webb. From this data, it is possible to detect molecules that are important for life, like water vapor, methane, and carbon dioxide. Then, biosignature molecules like ammonia, phosphine, and chloromethane could suggest current life.
Even when planets have the proper orbit, numerous factors make it difficult for Webb to characterize the atmosphere. Stark and Colón use an exoplanet called LHS 1140 b as an example. Recent analysis on the threshold for detecting gasses in the atmosphere of this super-Earth suggests it could take 10-50 transits to gather enough data. That means 40-200 hours of observation time before the telescope would detect biosignature molecules. Because LHS 1140 is not visible year-round, it could take multiple years—even a decade—to collect enough data.
Credit: NASA
There’s also the problem of inclement weather, which can affect any planet with an atmosphere. Heavy cloud cover can obscure the transmission spectrum of the atmosphere. Likewise, starspots on the host star can have high levels of water vapor, making it hard to know if there is water on the planet. Webb could find success studying so-called “Hycean planets,” like the super-Earth K2-18 b. These exoplanets are believed to have thin hydrogen-rich atmospheres and liquid water oceans. Webb is soon scheduled to peek at K2-18 b with its NIRSpec and MIRI instruments.
While Webb can study exoplanets in more detail than ever, it will take perfect conditions for the telescope to collect convincing evidence of life or habitability. With countless exoplanets and almost 20 years of life ahead of it, Webb could eventually find the right combination of a cloud-free atmosphere, ideal orbit, and a well-behaved star. al