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Hubble delivers first insight into atmospheres of potentially habitable planets orbiting TRAPPIST-1

Hubble delivers first insight into atmospheres of potentially habitable planets orbiting TRAPPIST-1

This is an artist’s impression of the TRAPPIST-1 system, showcasing all seven planets in various phases. When a planet transits across the disk of the red dwarf host star, as two of the planets here are shown to do, it creates a dip in the star’s light that can be detected from Earth. Also during such transits astronomers are able to study the potential atmospheres of these planets. Credit: NASA

Hubble delivers first insight into atmospheres of potentially habitable planets orbiting TRAPPIST-1

An international team of astronomers has used the NASA/ESA Hubble Space Telescope to look for atmospheres around four Earth-sized planets orbiting within or near TRAPPIST-1’s habitable zone. The new results further support the terrestrial and potentially habitable nature of three of the studied planets. The results are published in Nature Astronomy.

Seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth. This makes TRAPPIST-1 the planetary system with the largest number of Earth-sized planets discovered so far. These planets are also relatively temperate, making them a tantalizing place to search for signs of life beyond our Solar System. Now, an international team of astronomers has presented a study in which they used the NASA/ESA Hubble Space Telescope to screen four planets in the system — TRAPPIST-1d, e, f and g — to study their atmospheres.

Three of the planets orbit within the system’s habitable zone, the region at a distance from the star where liquid water — the key to life as we know it — could exist on the surface of a planet. The fourth planet orbits in a borderline region at the inner edge of the habitable zone. The data obtained rule out a cloud-free hydrogen-rich atmosphere for three of the planets — but for the fourth planet, TRAPPIST-1g, such an atmosphere could not be excluded.

The TRAPPIST-1 system contains a total of seven known Earth-sized planets. Three of them — TRAPPIST-1e, f and g — are located in the habitable zone of the star (shown in green in this artist’s impression), where temperatures are just right for liquid water to exist on the surface. While TRAPPIST-1b, c and d are too close to their parent star and TRAPPIST-1h is too far away, the remaining three planets could have the right conditions to harbour life. As a comparison to the TRAPPIST-1 system the inner part of the Solar System and its habitable zone is shown.
Credit: NASA/JPL-Caltech

Lead author Julien de Wit, from the Massachusetts Institute of Technology, USA, describes the positive implications of these measurements: “The presence of puffy, hydrogen-dominated atmospheres would have indicated that these planets are more likely gaseous worlds like Neptune. The lack of hydrogen in their atmospheres further supports theories about the planets being terrestrial in nature. This discovery is an important step towards determining if the planets might harbour liquid water on their surfaces, which could enable them to support living organisms.”

The observations were made while the planets were in transit in front of TRAPPIST-1. In this configuration a small section of the star’s light passes through the atmosphere of the exoplanet and interacts with the atoms and molecules in it. This leaves a weak fingerprint of the atmosphere in the spectrum of the star.

While the results rule out one type of atmosphere, many alternative atmospheric scenarios are still consistent with the data gathered by de Wit and his team. The exoplanets may possess a range of atmospheres, just like the terrestrial planets in our Solar System.

“Our results demonstrate Hubble’s ability to study the atmospheres of Earth-sized planets. But the telescope is really working at the limit of what it can do,” adds co-author Hannah Wakeford from the Space Telescope Science Institute, illustrating both the power and limitation of Hubble.

These latest findings complement the analysis of ultraviolet observations made with Hubble in 2017 (heic1713) and help us understand more about whether life might be possible in the TRAPPIST-1 system.

By ruling out the presence of a large abundance of hydrogen in the planets’ atmospheres, Hubble is helping to pave the way for the NASA/ESA/CSA James Webb Space Telescope.

“Spectroscopic observations of the TRAPPIST-1 planets with the next generation of telescopes — including the James Webb Space Telescope — will allow us to probe deeper into their atmospheres,” concludes Michael Gillon, from the University of Líege, Belgium. “This will allow us to search for heavier gases such as carbon, methane, water, and oxygen, which could offer biosignatures for life.”

The atmosphere of an exoplanet can reveal a wealth of information, such as the planet’s temperature, its air pressure, and whether it is suitable for life. However, studying exoplanet atmospheres is one of the most challenging tasks in modern astronomy — and a challenge for Hubble too. This new episode of the Hubblecast describes how exoplanet atmospheres are analysed and what makes it such a challenge.Credit: Directed by: Mathias Jäger
Visual design and editing: Martin Kornmesser
Written by: Lauren Fuge, Izumi Hansen, Mathias Jäger
Narration: Sara Mendes da Costa
Images: NASA, ESA/Hubble
Videos: NASA, ESA/Hubble
Animations: NASA, ESA/Hubble, M. Kornmesser
Music: Johan B. Monell (
Web and technical support: Mathias André and Raquel Yumi Shida
Executive producer: Lars Lindberg Christensen

Provided by: ESA/Hubble