How will NASA’s upcoming Habitable Worlds Observatory (HWO) mission differentiate Earth-sized exoplanets from other exoplanets, specifically Earth-sized exoplanets within the habitable zone, also called exoEarths? This is what a recent study accepted for publication in The Astronomical Journal hopes to address as an international team of researchers investigated the potential future capabilities of HWO and what shortcomings need to be addressed for it to conduct groundbreaking science, specifically with discovering exoEarths.
For the study, the researchers used a series of computer models called Bioverse to simulate how HWO will identify exoEarths regarding albedo, which is a planetary object’s ability to reflect solar radiation measured between 0 to 1. The goal of the study was to ascertain how HWO could observe and identify weaker albedos due to the small size of exoEarths compared ot much larger gas giants, whose albedos are typically easier to identify. In the end, the researchers concluded that a future telescope tasked with identifying exoEarths would have to exhibit capabilities and power that exceed the requirements outlined by the Astro2020 Decadal Survey.
“In this study, we demonstrated the ability of the Bioverse framework to simulate the ability of future direct imaging missions to test hypotheses such as this albedo-instellation relation,” the study concluded. “In future research we intend to apply the methodology developed in this study to investigate the detectability of other population-level trends using future telescopes such as HWO. Using a performance metric of the number of exoEarths required to test these hypotheses, future trade studies will be able to determine whether specific HWO designs will be able to meet this target sample size, assessing whether or not a given science case is feasible.”
As noted, albedo measures a planetary object’s ability to reflect solar radiation. For exoplanets, albedo can identify clouds, liquid water, and atmospheric composition, which are all used to determine the potential of habitability. Albedo and temperature are similar in that they both can fluctuate based on the other’s measurements. For example, clouds are cooler in temperature, thus emitting a brighter albedo, and the atmospheric composition can help determine if an exoplanet is rocky or gaseous.
An example of an exoplanet with an extremely high albedo is the Neptune-sized LTT 9779 b, which is located approximately 264 light-years from Earth and has a measured albedo of approximately 80 percent, or 0.8 based on the 0 to 1 measurement tool mentioned above. Astronomers have hypothesized that titanium-laced silicates are responsible for LTT 9779 b’s extreme brightness, which includes temperatures of approximately 2,000 degrees Celsius (3,632 degrees Fahrenheit) due to its close orbit to its host star at 0.8 days. Examples of albedos within our solar system include Earth at 0.3, Jupiter at 0.5, Venus at 0.76 and Saturn’s moon Enceladus at 0.81. The reason for the bright albedos of Venus and Jupiter is due to their highly reflective clouds while Enceladus is entirely covered in ice.
HWO is currently scheduled to launch sometime in the 2040s with the primary goal of using direct imaging to identify 25 exoEarths. Once identified, HWO will examine their atmospheres using spectroscopy by analyzing the light that passes through each exoplanet’s atmosphere. Spectroscopy is a longstanding method of analyzing exoplanet atmospheres, with NASA’s powerful James Webb Space Telescope using this method to identify water molecules, carbon dioxide, and sulfur dioxide. HWO will potentially be able to directly image these exoEarth by using a coronagraph to block the starlight being emitted by a host star, revealing the exoplanets that were hidden within the glare, and has become a common method for discovering new exoplanets.
How well will HWO identify exoEarths in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!