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First Insight into Atmospheres of Potentially Habitable TRAPPIST-1 Planets

Friday, February 9, 2018 23:57
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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.

Astronomers can find traces of the composition of an exoplanets atmosphere in the light of its parent star, if the planet is in transit. Each chemical element in the atmosphere of the exoplanet blocks light of the star at specific wavelengths — creating absorption lines in the spectrum of the star. This way astronomers can identify the presence of specific atoms and molecules — like oxygen and water — on the exoplanet.

Credit: ESA/Hubble, M. Kornmesser

Seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth [1]. 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 [2].

This infographic displays some artist’s illustrations of how the seven planets orbiting TRAPPIST-1 might appear — including the possible presence of water oceans — alongside some images of the rocky planets in our Solar System. Information about the size and orbital periods of all the planets is also provided for comparison; the TRAPPIST-1 planets are all approximately Earth-sized.
Artist's illustrations of planets in TRAPPIST-1 system and Solar System rocky planets
Credit: NASA

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 [3].
 
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.

Directed by: Mathias Jäger

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.”

This animation shows how stellar light passing through the atmosphere of an exoplanet interacts with the atoms in the atmosphere. As each element block a specific wavelength of light, it leaves a distinguish pattern in the spectrum of the star.

Credit: ESA/Hubble, M. Kornmesser
 

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 [4].

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.

Credit: NASA

“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.”
Notes

[1] The planets were discovered using the ground-based TRAPPIST-South at ESO’s La Silla Observatory in Chile; TRAPPIST-North in Morocco; the orbiting NASA Spitzer Space Telescope; ESO’s HAWK-I instrument on the Very Large Telescope at the Paranal Observatory in Chile; the 3.8-metre UKIRT in Hawaii; the 2-metre Liverpool and 4-metre William Herschel telescopes on La Palma in the Canary Islands; and the 1-metre SAAO telescope in South Africa.

[2] The observations were performed in the infrared with the Wide Field Camera 3 (WFC3) between December 2016 and January 2017.

[3] An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near infrared. However, the spectra for TRAPPIST-1d, -e, and -f do not show significant features.

[4] This includes atmospheres dominated by water vapor, nitrogen, carbon dioxide or tenuous atmospheres composed of a variety of chemical species.

Contacts and sources:
Nicole Shearer,
ESA/Hubble Information Centre
 

 



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