The astronomer Rafael Bachiller reveals in this series the most spectacular phenomena of the Cosmos. Topics of pulsating research, astronomical adventures and scientific news about the Universe analyzed in depth.

The TRAPPIST-1 planetary system is located just 40 light-years away and has several rocky planets similar to those in the solar system. However, recent observations indicate that these planets have lost their atmospheres.

Mini-solar system

In 2016, astronomers announced the detection of three planets around a small red dwarf star in the constellation Aquarius. As the discovery was made with the Small Transiting Planets and Planetesimal Telescope (TRAPPIST), located in Chile, the star was named TRAPPIST-1. The short distance to this star, only 40 light years, made this planetary system one of the most interesting to be studied from Earth.

But the biggest surprise about TRAPPIST-1 came on February 22, 2017, when the discovery of four additional planets around this same star was announced. And what is more surprising, it was deduced that the seven planets are small and rocky (similar to Earth), and that three of them (the so-called TRAPPIST-1e, f and g) are in the "habitable zone", that is, in a zone in which the temperature is adequate to allow the presence of liquid water. From that point on, these exoplanets were considered to be the best for studying the atmospheres of potentially habitable Earth-like worlds.

Trappist-1 system compared to NASA/JPL-Caltech solar system

However, the differences between the TRAPPIST-1 system and our solar system are very large. TRAPPIST-1 is a much smaller and cooler star than our Sun. TRAPPIST-1's seven planetary orbits are closer to their host star than Mercury is to the Sun and the planets are also very close to each other. It's like a miniature solar system. A person standing on the surface of one of these exoplanets could look up and make out the clouds or the surface of their neighboring worlds, as some of them would appear larger than the Moon seen in Earth's sky.

1b and 1c: no trace of atmospheres

Naturally, the Webb Space Telescope immediately set about studying the atmospheres of TRAPPIST-1's exoplanets. Last March, a first study led by Thomas Green (astronomer at NASA's Ames Institute) and published in Nature [http://bit.ly/3PFbd9H] showed that the temperature of the planet TRAPPIST-1b (the innermost of the seven) is about 200-250 degrees Celsius, which indicated that, if there was water on the planet, it would be in the form of vapor. But, in addition, this temperature is expected for a dark planet without an atmosphere. If there was air, pressure differences would be created between the light and dark areas, generating winds and the temperature would be lower.

Now another study has just been published suggesting that the next planet, in order from the star, TRAPPIST-1c, quite possibly, like planet 1b, does not have an atmosphere either. And, if it had an atmosphere, it would be very thin and composed of carbon dioxide.

Trappist-1c brightness compared to three NASA/ESA/CSA/J. Olmsted models

The study has been carried out by a team led by Sebastian Zieba (from the Max-Planck Institute in Germany) who observed the exoplanet in the infrared with Webb (at 15 microns wavelength). At first glance TRAPPIST-1c looked similar to Venus: it has the same size

and is exposed to a level of radiation similar to that which Venus receives from the Sun. But the calculated daytime temperature for this planet is about 100 degrees Celsius, suggesting that if the planet has an atmosphere based on carbon dioxide, it must be thinner than that of Mars.

The results of the study are illustrated in the attached figure showing the measurement at 15 microns (red dot) compared to three different models: planet with an atmosphere like that of Venus (yellow), with a thin atmosphere of carbon dioxide (blue) and planet without atmosphere (spectrum in green). As we can see, from this data (red point), it is not possible to distinguish between these last two options (blue and green curves).

Lost atmospheres

Although the seven planets of the TRAPPIST-1 system are the size of Earth and some are in a habitable zone, these observations cast many doubts on the possible presence of atmospheres that could favor the emergence of life. As we have said, these planets orbit around a red dwarf star, a stellar type very prone to the emission of high-energy radiation, such as X-rays. Also, in the past, these red dwarfs were much more active.

It does not seem impossible that, if one day there were atmospheres on these exoplanets, the emissions of TRAPPIST-1 ended with them, blowing them and dispersing them in space. The timescale required for this process of escape from atmospheres is similar to that of stellar evolution. All attempts to detect hints of the atmospheres on each of the seven exoplanets have so far failed. But, despite this, and despite the fact that the absence of atmospheres seems today a real possibility, the planets of the system will continue to be observed, giving preference to those of the habitable zone, to continue studying the viability of atmospheres in this promising system.

The article by S. Zieba et al., entitled No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c has been published in Nature and can be consulted at this link.

Rafael Bachiller is director of the National Astronomical Observatory (Instituto Geográfico Nacional) and academician of the Royal Academy of Doctors of Spain.

  • Astronomy

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