Seeing Atmospheres Around Distant Worlds in a New Light

The Cosmic Companion
Dec 2 · 5 min read

A group of astronomers have developed a revolutionary new way to detect atmospheres around distant exoplanets using the James Webb Space Telescope. Here’s how it works.

When the James Webb Space Telescope (JWST) launches in 2021, the observatory will (more than likely) represent a keystone change in the way we observe the Universe, much as Hubble did three decades ago.

Now, astronomers have developed a new way of using this revolutionary instrument to answer one of the great questions in astronomy — the search for planets around other stars possessing substantial atmospheres.

A large hexagonal mirror hangs from above, as lights fill a room in gold and blue.
A large hexagonal mirror hangs from above, as lights fill a room in gold and blue.
A segment of the multi-faceted primary mirror of the James Webb Space Telescope. Image credit: James Webb Space Telescope

Currently, astronomers look for atmospheres around planets orbiting other stars by searching for light from that star to pass through any gases surrounding a target exoplanet. An international group of astronomers now suggest that the insulating effects of atmospheres may, instead, may betray their presence around distant exoplanets.

Red dwarf stars — smaller and cooler than our own sun — are the most common class of star in our galaxy. Around these stars, we are likely to find small, rocky worlds similar to those found in our inner solar system.

The discovery of such worlds would lead to another series of questions, including the presence — or not — of an atmosphere. New techniques allowing astronomers to find atmospheres around exoplanets could also aid in the search for life around other stars.

I’ve Got Good News, and Bad News. And Good News, and Bad News…

As exoplanets revolve around their parent stars, many can be seen, from Earth, to pass “behind” their stellar companion. Just before they disappear from our sight, and just after they reappear, Earth-bound astronomers can see nearly the entire planet lit up by light from its local star, like a full Moon.

Measuring the coolest and hottest temperatures of an exoplanet could reveal the presence of an atmosphere, this new series of studies contend. Planets with atmospheres are expected to have more moderate temperatures than worlds lacking the insulating effects of a gaseous blanket.

“We find that Webb could easily infer the presence or absence of an atmosphere around a dozen known rocky exoplanets with less than 10 hours of observing time per planet,” said Jacob Bean of the University of Chicago.

Because red dwarf stars are relatively small, planets passing in “front” of them (as seen from Earth) are easier to detect than they would be in a system with a larger star.

This technique is ideally suited for finding atmospheres around exoplanets orbiting in close quarters with a star. Despite the diminutive size of red dwarf stars, their presence can rip an atmosphere off a planet, sending it into space. Compounding this problem is the fact that red dwarf stars are highly active, often exploding in tremendous flares and eruptions of plasma.

“Atmospheric loss is the number one existential threat to the habitability of planets,” Bean explained.

This artist’s impression shows a rocky exoplanet with a wispy, cloudy atmosphere orbiting a red dwarf star. Astronomers have identified a new method that could allow Webb to detect an exoplanet’s atmosphere in just a few hours of observing time. Image credit: L. Hustak and J. Olmsted (STScI)

However, the close orbit means that such a world will orbit its parent star quickly, making repeated observations possible in a relatively short time frame.

Another theorized quality of these systems is that such planets would likely be tidally locked to their star. This means that one face of the planet would continually face its stellar parent, just as our own Moon always presents one face toward the Earth. Therefore, when an exoplanet in such a system is seen just before or after spilling behind its local star, astronomers on Earth see its “daylight side.” When the world passes in front of the star, we see its “night side.”

Such a climate would result in significant winds, bringing heat from the sunny side to the dark hemisphere of the planet. Thermal energy leaving the sunlit side of the world would lower temperatures on that half of the exoplanet. Clouds which form in the atmosphere would also serve to moderate temperatures on the hotter half of the world, researchers describe.

“Whenever you add an atmosphere, you’re going to lower the temperature of the dayside. So if we see something cooler than bare rock, we would infer it’s likely a sign of an atmosphere,” explained Daniel Koll of the Massachusetts Institute of Technology (MIT).

What Makes the Webb Wonderful

A look techniques used to search for planets around other stars. Video credit: NASA/James Webb Space Telescope/STSci

The James Webb Space Telescope is readying for launch in the spring of 2021, providing astronomers with an unprecedented view of the Universe.

Sporting a primary mirror 6.5 meters (260 inches) in diameter, this observatory will gather roughly six times as much light as the 2.4 meter (94.5 inch) mirror built into the Hubble Space Telescope. The JWST is also designed to study targets in infrared light, allowing measurements of such planetary temperatures.

Using these detection methods, Webb should be able to detect atmospheres around small, rocky worlds in just two orbits of the planet — a matter of hours — compared to the eight or more orbits needed using current methods, investigators concluded.

Using the current method to find exoplanet atmospheres holds another problem as well — clouds. These features on exoplanets can return the same signal as it would if the world were simply an airless rock.

“In transmission spectroscopy, if you get a flat line, it doesn’t tell you anything. The flat line could mean the universe is full of dead planets that don’t have an atmosphere, or that the universe is full of planets that have a whole range of diverse, interesting atmospheres, but they all look the same to us because they’re cloudy,” said Eliza Kempton of the University of Maryland.

Astronomers have yet to find an exoplanet possessing a dense atmosphere without clouds.

I Was Thinking a Place with Atmosphere…

Roughly one dozen candidate worlds targeted by Webb will be analyzed using the technique championed in these studies. Almost certainly, the JWST will discover scores more world that could be home to atmospheres, and perhaps even some worlds teeming with life.

As currently envisioned, this technique would be more likely to find atmospheres around planets too hot to support life, But, knowing which of these worlds can possess an atmosphere will allow scientists to extrapolate about planets further from their sun.

“That there were other worlds, invisible, unknown, beyond imagination even, was a revelation to him.”
― Kim Edwards, The Memory Keeper’s Daughter

Should astronomers find atmospheres around other worlds using this method, observations using transmission spectroscopy can be undertaken to confirm or refute the finding, researchers state.

The discovery of a wide range of exoplanets with atmospheres would provide astronomers with a better idea of how many of these world may have their own atmospheric cocoon.

Researchers released their findings in four papers published in The Astrophysical Journal.

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The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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