Demystifying the habitable zone
Because TRAPPIST-1 is so cool, the “habitable zone” where liquid water could condense on an Earth-like planet’s surface encompasses the close-in orbits of planets d, e, f, and g. This means that if these planets are similar to our own, they could have oceans of water.
Testing the properties of planets in the habitable zone can help astronomers understand how different conditions affect habitability. The strength of solar winds, the density of a planet, the prevalence of large moons, the orientation of the planet’s orbit, and the planet’s rotation (or apparent lack thereof) could all be key factors for habitability. A simple criterion like the habitable zone could prove to be a good rule of thumb or more trouble than it’s worth.
“This system provides an opportunity to test the concept of the habitable zone outside of the Solar System,” said Jacob Lustig-Yaeger, post-doctoral fellow at the Johns Hopkins University Applied Physics Laboratory and co-investigator of a JWST program that will observe TRAPPIST-1h, among other rocky planets.
“TRAPPIST-1 is so different from the Sun, and the planets orbit so close to it, that it’s likely that there will be many surprises in our study of this system, and our efforts to understand these surprises will push forward the boundaries of planetary science.”
Rocky planet atmospheres
A major theme of JWST’s upcoming TRAPPIST-1 observations is “atmospheric reconnaissance” — testing whether an atmosphere even exists around the TRAPPIST-1 planets. Having an atmosphere is a prerequisite for life as we know it, and atmospheres can carry molecules called biosignatures. These observational markers of molecules act as signposts toward life.
“In recent years astronomers spent significant telescope time to try and detect atmospheres on Earth-sized exoplanets,” said Daniel Koll, assistant professor at Peking University and co-investigator of a JWST program studying TRAPPIST-1c. “Unfortunately these efforts couldn’t tell whether the TRAPPIST-1 planets have very thick and cloudy atmospheres or no atmosphere whatsoever.”
Since its discovery in 2017, scientists have used the Hubble Space Telescope to attempt to detect atmospheres around all seven of the TRAPPIST-1 planets. They were able to say for certain that the small planets did not have large, puffy, hydrogen-rich atmospheres. But they were unable to convincingly detect smaller, thinner, atmospheres, like our own.
All that said, JWST can detect subtleties that Hubble can’t. For one thing, it has a much larger mirror than Hubble — three times the diameter and about six times the collecting area. It also operates in the mid-infrared, where cool stars like TRAPPIST-1 emit their light.
JWST is also on a more stable orbit: rather than whipping around the Earth once every 97 minutes like Hubble, JWST sits out at L2, a stable point in space where Earth’s gravitational influence cancels out the Sun’s tug. That allows JWST to continuously view its targets for hours at a time, important for transit events.
In JWST’s first year, astronomers will check each planet in the TRAPPIST-1 system for signs of an atmosphere.
“Our goal,” said Lim, “is to tell whether the planets TRAPPIST-1b, c, g, and h have an atmosphere or not, and to do that, we will try to detect features of molecules such as carbon dioxide, water , and ozone in the transit spectra of those planets.”
Detecting atmospheres around the TRAPPIST-1 planets would have major implications for the study of exoplanets. Some astronomers speculate that rocky planets so close to their host stars might have their atmospheres removed by solar flares. The properties of these atmospheres might be altered by tidal locking, a gravitational process affecting close-in planets that keeps one hemisphere of the planet always facing the star.
Geological and biological processes can cycle carbon, nitrogen, and oxygen through rocky planet atmospheres, leading to different compositions depending on the prevalence of oceans, plate tectonics, and even life. But studying all these fascinating topics requires detecting an atmosphere first.