Sunsets on Titan are teaching us about distant exoplanets
Sen—Saturn's smog-enshrouded moon Titan is helping scientists understand the atmospheres of exoplanets. A new technique shows the dramatic influence that hazy skies could have on our ability to learn about alien worlds orbiting distant stars.
A team of researchers led by Tyler Robinson, a NASA Postdoctoral Research Fellow at NASA's Ames Research Center, have published their findings in the Proceedings of the National Academy of Sciences.
"It turns out there's a lot you can learn from looking at a sunset," Robinson said.
Despite the staggering distances to other planetary systems, in recent years researchers have begun to develop techniques for collecting spectra of exoplanets.
When one of these worlds transits, or passes in front of its host star as seen from Earth, some of the star's light travels through the exoplanet's atmosphere, where it is changed in subtle, but measurable, ways.
This process imprints information about the planet that can be collected by telescopes. The resulting spectra enable scientists to tease out details about the temperature, composition and structure of exoplanets' atmospheres.
Robinson and his colleagues exploited a similarity between exoplanet transits and sunsets witnessed by the Cassini spacecraft at Titan. Called solar occultations, these observations allowed the scientists to observe Titan as a transiting exoplanet without having to leave the solar system.
Many worlds in our solar system, including Titan, are blanketed by clouds and high-altitude hazes. Scientists expect that many exoplanets would be similarly obscured.
Clouds and hazes create a variety of complicated effects that must be disentangled from the signature of these alien atmospheres, and present a major obstacle for understanding transit observations. Due to the complexity and computing power required to address hazes, models used to understand exoplanet spectra usually simplify their effects.
"Previously, it was unclear exactly how hazes were affecting observations of transiting exoplanets," said Robinson. "So we turned to Titan, a hazy world in our own solar system that has been extensively studied by Cassini."
An artistic impression of an exoplanet in transit across the face of its star. Image credit: NASA, ESA, and G. Bacon (STScI)
The team used four observations of Titan made between 2006 and 2011 by Cassini's visual and infrared mapping spectrometer instrument. Their results, including the complex effects due to hazes, can now be compared to exoplanet models and observations.
Robinson and colleagues found that hazes high above some transiting exoplanets might strictly limit what their spectra can reveal to planet transit observers. The observations might be able to glean information only from a planet's upper atmosphere. On Titan, that corresponds to about 150 to 300 km (90 to 190 miles) above the moon's surface, high above the bulk of its dense and complex atmosphere.
The study also found that Titan's hazes more strongly affect shorter wavelengths, or bluer light. Studies of exoplanet spectra have commonly assumed that hazes would affect all colours of light in similar ways. Studying sunsets through Titan's hazes has revealed that this is not the case.
"People had dreamed up rules for how planets would behave when seen in transit, but Titan didn't get the memo," said Mark Marley, a co-author of the study at NASA Ames. "It looks nothing like some of the previous suggestions, and it's because of the haze."
The technique applies equally well to similar observations taken from orbit around any world, not just Titan. This means that researchers could study the atmospheres of planets like Mars and Saturn in the context of exoplanet atmospheres as well.