The exoplanet TRAPPIST-1b has no atmosphere


New measurements with the James Webb Space Telescope have discovered that the rocky exoplanet TRAPPIST-1b orbiting a star known as TRAPPIST-1 most likely does not have an atmosphere. The preliminary results were published in a study (ref.) in Nature.

The discovery crushes the hope that this distant world could host life. Fortunately, there are six other Earth-like exoplanets in the TRAPPIST-1 star system. Moreover, compared to the past, we have James Webb, which has demonstrated its ability to study them.

The lack of an atmosphere

Astronomers used James Webb’s Mid-Infrared Instrument (MIRI) to measure the temperature of the planet TRAPPIST-1b. Of the seven planets in the star system, this planet orbits closest to its star and is about 1.4 times larger than Earth.

The measurement, which according to the European Space Agency (ESA) represents Webb’s first detection of “any form of light” emitted from a rocky exoplanet, reveals that the planet’s daytime temperature is 230 degrees Celsius. For this reason, astronomers believe that it is too high for the planet to have an atmosphere.

Thomas Greene, an astrophysicist in the Space Sciences and Astrobiology Division at NASA’s Ames Research Center in California, who led the observations, said, “Some theoretical groups predicted that the planet would have a dense atmosphere, while others thought the opposite. I was more disappointed than surprised to see that the TRAPPIST-1b exoplanet does not have an atmosphere”. The distance between the planet and its star is only about one-hundredth of the distance between the Sun and Earth. It is 40 times closer than the distance between the Sun and the innermost planet of the Solar System, Mercury.

The TRAPPIST-1 planetary system

Although the star at the center of the TRAPPIST-1 system is much weaker than the Sun, the planet still receives about four times the light that Earth receives every day. Astronomers, therefore, did not expect this planet to be habitable without ruling out the presence of an atmosphere. However, the observation is a breakthrough because it shows that Webb can directly collect information from such distant exoplanets.

The TRAPPIST-1 system is an extremely popular target in exoplanet research. According to a NASA estimate, it is the most explored planetary system after the Solar System. In fact, there are at least three TRAPPIST-1e, 1f, and 1g planets that show conditions for the existence of liquid water on their surface and, therefore, could host life.

Located about 40 light-years away, the star at the center of the system is a so-called M dwarf or red dwarf. These stars are the smallest type of stars known to burn hydrogen in their cores. Their sizes range from 0.08 to 0.6 times the size of the Sun and are the most numerous type of star in our galaxy.

M dwarf stars

“There are about ten times more M stars like TRAPPIST-1 than there are G stars like the Sun” Greene said. “M stars also have about twice the chances of having Earth-sized rocky planets. So, about 95% of Earth-sized rocky planets in the Milky Way will have stars like TRAPPIST-1 and not like the Sun”.

Previous observations with Hubble did not find any traces of atmospheres on any of the TRAPPIST-1 planets. However, Greene said, there is still a possibility that a very thin atmosphere surrounds TRAPPIST-1b, an atmosphere that could be completely different from the atmospheres that envelop planets in the solar system.

“We have some follow-up observations scheduled in June at another wavelength. We have proposed to observe a wider portion of the planet’s orbit to examine and perhaps rule out other types of atmospheres,” concluded Greene.

Stefano Gallotta
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