One of the aims of the James Webb Space Telescope is a profound one: To search for habitable exoplanets, where one day we could look for evidence of life beyond our own planet. But habitability is no simple matter, and finding another “Earth-like” planet might be more complicated than you imagine.
Around 40 light years away lies a remarkable star system. The TRAPPIST-1 system became world-famous five years ago when NASA announced it and several partners had discovered a whole bevy of Earth-sized planets orbiting within the habitable zone of a single star. Since then, astronomers have been desperate to learn more about these nearby worlds which could be like our planet.
Enter the James Webb Space Telescope, which will investigate the TRAPPIST-1 system in its first cycle of research programs. The research will aim to see if the planets in the TRAPPIST-1 system have atmospheres and, in the long term, to set up the process of looking for biosignatures that could indicate if life were present on any of the planets.
But there are many challenges when it comes to understanding worlds that are different from ours. Recent modeling work shows that it isn’t just a planet’s distance from its star or its composition that affects its climate: The distribution of land across a planet’s surface also significantly impacts its potential habitability.
What’s new — In a presentation at the National Astronomy Meeting on Monday, July 11, graduate student Evelyn Macdonald of the University of Toronto, Canada, shared the findings of recent research into exoplanets. The researchers used the nearby Earth-sized exoplanet Proxima Centauri b for their models. Like TRAPPIST-1, Proxima Centauri is a red dwarf star. These stars are much smaller and cooler than our sun, so the planets can orbit very close to them while still residing in the habitable zone.
Like the TRAPPIST planets, Proxima Centauri b is tidally locked. This means that one side of the planet always faces the star, and the other side always faces out to space. This phenomenon is not unusual to find in exoplanets, though we don’t have anything similar in planets in our Solar System. (People used to think Mercury was tidally locked, but the situation turned out to be rather different.) We see something similar with our planet’s moon, though, where one side is always facing us and the “dark side” is always facing away.
Tidal locking of exoplanets can have implications for habitability because astronomers expect one side of the planet to be much hotter than the other. However, this could be mitigated by an atmosphere with winds that spread heat around the planet and make it more hospitable to life. The side of the planet facing the star is called the dayside because it is in perpetual daylight, and the other side is called the nightside.
How they did it — MacDonald’s group looked at how the amount of land on a planet’s dayside affects its climate. They compared a situation where land was in a circular continent in the middle of a liquid ocean to one with a circle of land with ice in the middle. “We varied the amount of land on the planet, keeping it in these configurations, making the circle of land larger or smaller, to understand the climate effects of this,” Macdonald explained in her presentation.
When there was more land on a planet, the model showed there was less rain, higher temperatures, and more of a difference in temperature between the day and night sides. When a planet had more ocean, there was more rain and clouds. Sometimes ocean water might be frozen into ice, so the amount of liquid ocean affected the amount of water in the climate and the planet’s temperature.
Even when the total amount of land stayed the same, the researchers found that the average surface temperature of a tidally locked planet could vary by as much as 20 degrees Celsius depending on the distribution of that land. “This is a pretty substantial climate difference, just caused by changing how much land there is and where it’s located,” Macdonald said.
What’s next — The challenge comes from applying this theoretical research to real data on exoplanets, as it’s hard to tell how much of a planet is land versus ocean by looking at spectra like those James Webb will take.
“Land diversity has a large effect on climates of tidally locked planets. This effect is very difficult to detect on real planets, and depends on several factors,” Macdonald said. To truly understand the habitability of such a planet, we’ll need to know factors like its surface pressure, its clouds, and the amount of radiation it receives from its star, as well as the amount and distribution of land.
The upshot is that James Webb will begin investigating the TRAPPIST-1 system in its first year of science, and it will be incredibly exciting to get more data on these exoplanets. But don’t start planning a move to “Earth 2” just yet, as finding a habitable planet from billions of miles away won’t be an easy task.
