Astronomers Question the Habitable Zone for TRAPPIST-1A

Astronomers Question the Habitable Zone for TRAPPIST-1A

Astronomers Question the Habitable Zone for TRAPPIST-1A
Image Credits: Physics World

Researchers from the University of Arizona (UA) have found evidence that could force scientists to redefine the habitable zone for TRAPPIST-1A exoplanets.

TRAPPIST-1 system has always attracted the attention of humanity since its discovery in 2016. The existence of 7 Earth-sized planets around a cool star is the reason for its popularity. A number of observations have revealed that 3 of these planets are located in the habitable zone (the region where water could exist in liquid state) of the TRAPPIST-1A. Having said that, a couple of recent studies suggest that we may have to redefine the habitable zone for this system for different reasons.

High Energy Particles from TRAPPIST-1A

Federico Fraschetti, alongside his team of researchers, found that the star of the system is emitting bursts of high-energy particles that are smashing into these exoplanets. In addition to identifying these dangerous particles, the team also managed to determine their power as they strike the surfaces of these planets. They simulated the journeys of these particles through the magnetic field of TRAPPIST-1A and found that the innermost exoplanet inside the habitable zone (4th from the star) is experiencing a powerful bombardment of protons. This was a huge surprise for the researchers because the magnetic field of TRAPPIST-1A is tightly wound around the star and it was believed that the particles would get trapped inside it. Fraschetti referred to this shock by saying,

“The flux of these particles in the TRAPPIST-1 system can be up to 1 million times more than the particles flux on Earth. You expect that the particles would get trapped in these tightly wrapped magnetic field lines, but if you introduce turbulence, they can escape, moving perpendicularly to the average stellar field.”


TRAPPIST-1A is considerably smaller and cooler than our sun. However, it is extremely active and emits a massive amount of high-energy particles (protons) in its surroundings. The turbulence in the magnetic field, caused by the flares on the surface of the star, allows these protons to travel great distances. The most likely alignment of this field guides energetic particles towards the face of the 4th planet. The intensity of these protons could literally destroy the complex molecules that are essential to support life on this exoplanet.

The idea that TRAPPIST-1 planets are tidally locked means that the night side of the planets still pose an opportunity for the life to grow. Benjamin Rackham, a Research Associate with UA Department of Astronomy who wasn’t involved in the research, acknowledged that the night side could be warm enough and it doesn’t get bombarded by the powerful radiation.

Raising Tides

Hamish Hay, a Graduate Student in the Lunar and Planetary Laboratory who led the second study, suggested that the planets of TRAPPIST-1 are playing gravitational tug-of-war with each other. Consequently, tides (similar to what the moon raises on the Earth) are raised on their surfaces. In addition to these tides, tidal forces tend to deform the spherical shape of the Earth’s mantle and crust. Hay mentioned that tidal deformation does produce heat and TRAPPIST-1 is the only known system whose planets can raise significant tides. He said,

“When a planet or moon deforms from tides, friction inside it will create heating. It’s such a unique process that no one’s thought about in detail before, and it’s kind of amazing that it’s actually a thing that happens.”

Hay estimated the cumulative amount of heat these tides bring to the system by examining the gravitational effect of all the planets. He found that the two inner planets come so close that they can raise powerful tides on each other. In extreme cases, this tidal heating can fuel volcanic activity, which can sustain atmospheres. Such an atmosphere might transfer some of the heat to the otherwise-too-cold side of the planet. This might warm that part of the exoplanet to a point where it could become suitable for sustaining life. TRAPPIST-1g is experiencing some serious amount of tidal tugging as both the star and the neighboring planets are heating it. Rackham summed up the matter in the following words:

“We need to really understand the suitability of these systems for life, and energetic particle fluxes and tidal heating are important factors to constrain our ability to do that.”

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