Astronomers Propose a Novel Method of Finding Atmospheres on Rocky Worlds

December 2, 2019

When NASA’s James Webb Space Telescope launches in 2021, one of its most anticipated contributions to astronomy will be the study of exoplanets—planets orbiting distant stars. Among the most pressing questions in exoplanet science is: Can a small, rocky exoplanet orbiting close to a red dwarf star hold onto an atmosphere?

In a series of four papers in the Astrophysical Journal, a team of astronomers proposes a new method of using Webb to determine whether a rocky exoplanet has an atmosphere. The technique, which involves measuring the planet’s temperature as it passes behind its star and then comes back into view, is significantly faster than more traditional methods of atmospheric detection like transmission spectroscopy.

“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, a co-author on three of the papers.

Astronomers are particularly interested in exoplanets orbiting red dwarf stars for a number of reasons. These stars, which are smaller and cooler than the Sun, are the most common type of star in our galaxy. Also, because a red dwarf is small, a planet passing in front of it will appear to block a larger fraction of the star's light than if the star were larger, like our Sun. This makes the planet orbiting a red dwarf easier to detect through this "transit" technique.

Red dwarfs also produce a lot less heat than our Sun, so to enjoy habitable temperatures, a planet would need to orbit quite close to a red dwarf star. In fact, to be in the habitable zone — the area around the star where liquid water could exist on a planet's surface — the planet has to orbit much closer to the star than Mercury is to the Sun. As a result, it will transit the star more frequently, making repeated observations easier.

But a planet orbiting so close to a red dwarf is subjected to harsh conditions. Young red dwarfs are very active, blasting out huge flares and plasma eruptions. The star also emits a strong wind of charged particles. All of these effects could potentially scour away a planet’s atmosphere, leaving behind a bare rock.

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

Another key characteristic of exoplanets orbiting close to red dwarfs is central to the new technique: They are expected to be tidally locked, meaning they have a permanent dayside and nightside. As a result, we see different phases of the planet at different points in its orbit. When it crosses the face of the star, we see only the planet’s nightside. But when it is about to cross behind the star (an event known as a secondary eclipse), or is just emerging from behind the star, we can observe the dayside.

If a rocky exoplanet lacks an atmosphere, its dayside would be very hot, just as we see with the Moon or Mercury. However if a rocky exoplanet has an atmosphere, the presence of that atmosphere is expected to lower the dayside temperature that Webb would measure. It could do this in two ways. A thick atmosphere could transport heat from the dayside to the nightside through winds. A thinner atmosphere could still host clouds, which reflect a portion of the incoming starlight thereby lowering the temperature of the planet's dayside.

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