Future space telescopes may probe Titan-like exoplanets5 min read

The haze rich around Saturn’s moon Titan could be visible around similar exoplanets.

Saturn’s huge moon Titan is one of the most intriguing worlds in the solar system.

NASA’s Cassini spacecraft captured this view of Saturn’s hazy moon Titan in January 2013, when it was about 895,000 miles (1.44 million kilometers) from the big moon.
(Image: © NASA/JPL-Caltech/Space Science Institute)

Titan boasts hydrocarbon rivers and seas that could potentially harbor “strange life,” as well as a subsurface ocean of liquid water where Earth-like organisms might be able to make a living.

Titan also has a thick, nitrogen-dominated atmosphere where complex chemistry — perhaps even the sort that leads to life — is known to occur. And now, researchers have determined that the smoggy haze of Titan-like exoplanets could be visible with the next generation of space telescopes.

While most exoplanet hunts focus on finding worlds similar to Earth, one team wanted to know if upcoming instruments could identify potentially habitable worlds dramatically different from our own. Using simulations, they modeled Titan-like worlds around a variety of star types. They considered worlds “Titan-like” if they were far enough from their star for methane to condense and had a high enough water-to-rock ratio to spew volatile compounds into the atmosphere that could create a haze.

One of the first stars the team modeled was Barnard’s Star, a nearby sun that hosts a planet candidate roughly three times as massive as Earth. That potential world, Barnard’s Star b, lies near the system’s snow line, the region where water and other gases can freeze solid, and where material like methane can solidify.

“We wanted to see if you could detect this planet through direct imaging with LUVOIR [the Large UV/Optical/IR Surveyor],” team member Ryan Felton told Space.com.

LUVOIR is a proposed multi-wavelength space telescope under consideration by NASA as a future observatory. Felton, a graduate student at the Catholic University of America in Washington, D.C., presented the results of the project in January at the semiannual winter meeting of the American Astronomical Society in Honolulu.

The team found that a Titan-like haze could be visible from Earth with a LUVOIR-like instrument.

“For Titan-like exoplanets, the atmospheres are characterizable with big space telescopes,” Felton said.

Puffy worlds

Titan’s air is dominated by nitrogen, along with some methane, a small amount of hydrogen and traces of various other gases.

As the nitrogen and methane gases rise into Titan’s upper atmosphere, sunlight and heat break them apart, leading to the creation of bigger molecules that generate a haze in the atmosphere similar to that found above large cities here on Earth. Because the size of haze particles changes based on temperature, Titan-like worlds around different stars should have different characteristics, the researchers reasoned.

First, Felton and his colleagues modeled worlds around sunlike stars like the sun. Not surprisingly, the worlds around these stars resemble Titan in terms of haze size and gas mixtures — with one important difference. While Titan is a moon, the researchers investigated planets.

The differences between the two were fairly minor, however. Titan is about 40% as wide as Earth and 2% as massive, but Felton said this wasn’t a serious issue. With the right combination of rock and water, an Earth-size world should be able to capture gas at farther distances from their stars, where temperatures are colder.

“I think it’s just the case for our solar system” that Titan wound up a moon rather than a planet, Felton said. Systems that lack gas giants in the same region would be more apt to host Titan-like worlds, he added.

Timing is also important. In the past, Earth once boasted a thick haze, though it was likely not as thick as Titan’s is today. “That’s further evidence of an actual terrestrial planet having a haze,” Felton said.

Felton and his colleagues also modeled haze-rich worlds around cooler stars. The cooler temperatures lowered how high in the atmosphere the haze reached, although Felton said the difference shouldn’t be significant enough to impact attempts to image the worlds.  “[They] should still be pretty puffy,” he said.

Puffy worlds

Titan’s air is dominated by nitrogen, along with some methane, a small amount of hydrogen and traces of various other gases.

As the nitrogen and methane gases rise into Titan’s upper atmosphere, sunlight and heat break them apart, leading to the creation of bigger molecules that generate a haze in the atmosphere similar to that found above large cities here on Earth. Because the size of haze particles changes based on temperature, Titan-like worlds around different stars should have different characteristics, the researchers reasoned.

First, Felton and his colleagues modeled worlds around sunlike stars like the sun. Not surprisingly, the worlds around these stars resemble Titan in terms of haze size and gas mixtures — with one important difference. While Titan is a moon, the researchers investigated planets.

The differences between the two were fairly minor, however. Titan is about 40% as wide as Earth and 2% as massive, but Felton said this wasn’t a serious issue. With the right combination of rock and water, an Earth-size world should be able to capture gas at farther distances from their stars, where temperatures are colder.

“I think it’s just the case for our solar system” that Titan wound up a moon rather than a planet, Felton said. Systems that lack gas giants in the same region would be more apt to host Titan-like worlds, he added.

Timing is also important. In the past, Earth once boasted a thick haze, though it was likely not as thick as Titan’s is today. “That’s further evidence of an actual terrestrial planet having a haze,” Felton said.

Felton and his colleagues also modeled haze-rich worlds around cooler stars. The cooler temperatures lowered how high in the atmosphere the haze reached, although Felton said the difference shouldn’t be significant enough to impact attempts to image the worlds.  “[They] should still be pretty puffy,” he said.


SOURCESpace.com
Featured Image: NASA

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Sebastien Clarke
Sebastien Clarke

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