James Webb Telescope searches for life on Earth-like exoplanets

Forty light-years from Earth, seven small planets orbit a faint dwarf star. They resemble Earth in size and composition. On three of the seven, there might even be seas and oceans. Who knows, maybe something is alive; maybe one day you can find out by studying the atmosphere of such a planet. The new James Webb Space Telescope is doing just that this fall. This is the first time that astronomers have measured the atmosphere of Earth-like planets. “We don’t know what to expect,” says Yamila Miguel of the Leiden Observatory. ‘It’s very exciting.’

At the end of August, an international team of astronomers (including Miguel) announced that the Webb telescope could detect carbon dioxide (CO₂) in the atmosphere of the large hot gas planet Bocaprins. It orbits the solar star Wasp-39 once every four days, 700 light-years from Earth. “But measurements on small rocky planets are much more difficult,” explains Miguel. “We’re not even sure they still have an atmosphere.”

Since the end of the last century, astronomers have discovered more than five thousand planets around other stars. They come in all shapes and sizes. Most of the time you can’t see them – they’re overshadowed by their parent star. But you find them indirectly. For example, because their gravity tilts the star. Or because they move in front of the star with each orbit, intercepting a tiny bit of starlight.

Such a “transit planet” was first observed in 1999, with a small telephoto lens from the parking lot of an observatory in Colorado. The giant planet HD209458b passes in front of its star every 3.5 days, making it 1.5% dimmer than normal for a few hours. Since then, several thousand exoplanets have been discovered by the transition method, mainly by the Kepler space telescope.

Sara Seager of Harvard University already calculated in 2000 that one can sniff the atmosphere of the planet during such a transition. “It was an important paper, with great predictive value,” says Ignas Snellen, exoplanet researcher and recently scientific director of the Leiden Observatory. The idea is that a small amount of starlight seeps into the planet’s atmosphere, filtering it out somewhat. By analyzing starlight with extreme precision, you can find out what gases are present in the planet’s atmosphere.

Digital print

Snellen performs such measurements with the European Very Large Telescope in Chile. It’s not easy from the ground, he explains, because gases in Earth’s atmosphere also leave their fingerprints in starlight. “Thanks to a very sensitive spectroscope, we can still make this distinction. For example, we were the first to discover carbon monoxide in the atmosphere of a gaseous exoplanet and we were even able to study the wind patterns.

A telescope in space is less affected by disturbing effects, but the Hubble Space Telescope is actually too small for this kind of research, although, according to Snellen, Hubble has detected water vapor in the atmospheres of some gaseous planets. Also, most molecules can only be “seen” at infrared wavelengths. This is why the results of the Webb telescope are eagerly awaited: it is much larger and more sensitive than Hubble and only looks at this infrared “thermal radiation”.

Yamila Miguel became involved with the Webb Exoplanet Program in 2021, led by Natalie Batalha of the University of California, Santa Cruz. “We really didn’t know what to expect,” she says. “With exoplanets, you always know one thing: you will be surprised.” In March 2022, the team organized a sort of dress rehearsal based on simulated observational data, during a double workshop in Heidelberg and Baltimore. In mid-July, the actual observations on the planet Bocaprins (Wasp-39b) were made and the three hundred members of the team had access to the results.

Only the discovery of carbon dioxide has been made public so far. But according to Miguel, much more was found. There will soon be five articles in a special thematic issue of Nature‘ she says. “It’s actually quite special that nothing has leaked out so far.” Besides, the CO₂the measurements are also surprising: the planet’s atmosphere appears to contain far more carbon and oxygen than the giant planet Jupiter in our own solar system.

dwarf star

Things get really exciting with Webb’s observations of the seven small rocky planets of the dwarf star Trappist-1 (the name was coined by Michaël Gillon, the Belgian discoverer of the planetary system). One of these planets has already been examined by Webb to find out if it even has an atmosphere. If so, measurements can be made on the composition later. “With any luck, we might find carbon monoxide and water vapor there,” Snellen says.

Miguel dares not make predictions – never before have the atmospheres of Earth-like exoplanets been studied. “It’s a first attempt,” she said. “If this succeeds, Webb will certainly study many other minor planets.” That we will learn a lot about the atmospheres of distant giant planets in the years to come – at least there is no doubt. As for the smaller rock specimens, we hope Webb can determine which planets have the ideal conditions for life.

And find real evidence of the existence of extraterrestrial life? This will remain in the future for now. You then have to discover what are called “biomarkers” – molecules that can indicate biological activity on the surface of the planet, such as oxygen, ozone and methane, and even the 6.5 meter mirror of the space telescope is not big enough for that. Moreover, according to Miguel, scientists still disagree on which molecules are true, indisputable biomarkers.

We will therefore have to snoop around before we can answer the question of whether we are alone in the universe. “But,” said Miguel, “that’s where we’re ultimately going.”