A rocky, Earth-size planet located in our Milky Way galaxy may have an atmosphere around it, according to new research, raising the possibility that it could also have liquid water on its surface and could therefore support life.
In two separate papers published Monday in the Astrophysical Journal Letters, astronomers zeroed in on the TRAPPIST-1 system, which consists of seven rocky planets that orbit a single star. Both studies outlined initial results from observations by NASA’s James Webb Space Telescope, suggesting that one planet in particular, known as TRAPPIST-1e, may have a nitrogen-rich atmosphere like Earth’s, though follow-up studies are needed to confirm the discovery.
The results are an important step in the ongoing search for extraterrestrial life in the solar system and beyond.
This week, NASA announced that a rock sample collected on Mars may contain evidence of ancient microbial life. Present-day Mars has a thin atmosphere mostly made up of carbon dioxide, nitrogen, and argon gases, but the Red Planet is thought to have had a thicker atmosphere billions of years ago, when liquid water flowed on its surface.
Scientists have long held that water is an essential ingredient for life.
To keep water in liquid form, rather than it evaporating instantly into space, a planet or moon needs to have an atmosphere. That makes the search for exoplanet atmospheres one of the most tantalizing in the field of astronomy.
“Ultimately, we want to get to the point where we find a planet, and huzzah, we see a molecule that can only be produced by life,” said Ryan MacDonald, an exoplanet astronomer at the University of St Andrews in Scotland and a co-author of both studies. “But you need to have an atmosphere, so what we’ve been working on first is to find planets that have atmospheres.”
The TRAPPIST-1 system, which is 40 light-years away from Earth, has been widely studied since it was discovered in 2016 because several of the planets could have conditions suitable to support extraterrestrial life.
Each light year is approximately 6 trillion miles.
TRAPPIST-1e, in particular, is thought to orbit its star in the theoretical “habitable zone,” a distance that is not too close for it to be hellishly hot, and not too far to be icy cold, but rather just right for liquid water to exist on the surface.
For the new studies, astronomers used NASA’s James Webb Space Telescope to observe four “transits” of TRAPPIST-1e, or times when the planet passed in front of its star. The telescope did not directly see an atmosphere around TRAPPIST-1e, but rather it measured how the exoplanet absorbed light to figure out what, if anything, surrounds the planet.
Much like a prism, light can be separated into different bands of color on a rainbow spectrum, and how certain colors are blocked or filtered can be telltale signatures of specific atoms or molecules of gas.
If certain colors are absorbed, for instance, it can suggest high concentrations of carbon dioxide. Other changes in color can hint at different chemical properties, including the presence of hydrogen, oxygen, methane or nitrogen.
“If we see no variation in color, then the planet is probably just a bare rock,” MacDonald said. “A bare rock doesn’t care if you shine red light or blue light on it. It will just block them all equally.”
In four transits, the researchers did not find evidence of a hydrogen-rich atmosphere around TRAPPIST-1e. Nor did they see signs that its potential atmosphere is dominated by carbon dioxide. The Webb telescope’s observations, however, do hint that its atmosphere could be rich in nitrogen.
“This is an exciting step and it really helps us narrow down the possibilities of an atmosphere that is perhaps more Earth-like,” said Caroline Piaulet-Ghorayeb, a postdoctoral fellow at the University of Chicago who was not involved with the new studies.
Earth’s own atmosphere contains high concentrations of nitrogen gas. Titan, one of the moons around Saturn, also has an atmosphere that is mostly nitrogen. NASA has said that Titan likely harbors a vast underground ocean, which could make it habitable, but the moon’s methane-filled environment would mean that any life that does exist there would be very different from life on Earth.
Piaulet-Ghorayeb separately was the lead author of a study published last month in The Astrophysical Journal that focused on a different planet in the TRAPPIST-1 system: TRAPPIST-1d, the third planet from the star that also orbits within the habitable zone. That study found no evidence of molecules that are common in Earth’s atmosphere, such as water, carbon dioxide or methane.
Studying these celestial bodies come with significant challenges.
The TRAPPIST-1 star is small but extremely active, which creates a lot of background noise for researchers to sift through. MacDonald and his colleagues, for instance, spent more than a year analyzing data from the Webb telescope and trying to differentiate chemical signatures coming from TRAPPIST-1e and its star.
To confirm the presence of an atmosphere, MacDonald and his colleagues are planning to study TRAPPIST-1e during 15 more transits in the coming years.
Studies are also forthcoming for three other planets that are farther out in the system, TRAPPIST-1f, TRAPPIST-1g and TRAPPIST-1h, he said.
The research should help scientists inch closer to answering some of the most enduring questions about exoplanets and the search for life.
“We haven’t yet convincingly found an atmosphere on any rocky planet outside of the solar system, which makes studying and searching for atmospheres on temperate planets extremely exciting,” Piaulet-Ghorayeb said. “But there’s a lot of work to do.”