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  TRAPPIST-1 Habitable-zone Planets

        Seven Earth-sized planets circle a star 39 light years from the Earth. Three orbit in the ultra-cool red star’s habitable zone, where liquid water could exist and life might evolve. The star, TRAPPIST-1 lies near the ecliptic, within the constellation of Aquarius. It has a mass 12 times smaller than the sun’s, radiates mostly in the infra red, and is slightly larger than Jupiter. The planets orbit about 50  times closer to their star than the Earth does to the Sun. However, the star is about 50 times cooler than the sun, so the planets receive about the amount of light received in the Solar System at Mercury to beyond Mars.

       The initial discovery of this planetary system was made with the 0.6m TRAPPIST telescope installed at the ESO La Silla Observatory, high in the Chilean Atacama desert, by the University of Liege Belgium and the Geneva Observatory, Switzerland. This remarkable achievement has opened the way for astronomers around the world to participate in the search for Earth-like planets. There are already many such telescopes operating, including the global La Cumbre network, automated like Trappist to allow remote operation.

    The initial discovery was made by a team led by Michaël Gillon, of the Department of Astrophysics, Geophysics and Oceanography at Liège. In selecting target stars that might have planets, Gillon and his colleagues had focused on stars of extremely low mass and effective temperatures of less than 2,700 K. Planet creation theory predicts that these ultracool dwarfs should have a large but hitherto undetected population of terrestrial planets orbiting them. These might range from planets like Mercury to others more like Earth and be hospitable to life. 15 per cent of nearby stars are this type of star.

     In fact, by means of transit photometry the Liege University team found three Earth-sized planets around the star now called Trappist-1. 15 per cent of nearby stars are this type of star. The innermost two were found to be tidally locked to their host star while the outermost appears to lie either within the system's habitable zone or just outside of it.  In 2016 the team published its findings in the English journal Nature.

       Four additional planets were subsequently identified (Nature 2017) using TRAPPIST and the 0.8 m the NASA Spitzer space telescope, the 2.0 m Liverpool Telescope, the 3.8 m United Kingdom Infrared telescope, the 4.2 m William Herschel Telescope, and the European Southern Observatory 8.2 m Very Large Telescope. Additional planets may await detection.

       In the fall of 2016, Spitzer observed TRAPPIST-1 nearly continuously for 500 hours. Spitzer is uniquely positioned to observe enough crossing–transits–of the planets in front of the host star to reveal the complex architecture of the system. Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated. Five planets are similar in size to the Earth, Two are closer to half the size of the Earth. The density measurements show the planets are probably largely composed of rocks.

       Following up on the Spitzer discovery, NASA's Hubble Space Telescope searched for the presence of puffy, hydrogen-dominated atmospheres around these planets, typical of gaseous worlds. In May 2016, the Hubble team observed the two innermost planets, and found no evidence for such atmospheres. This strengthened the case that the planets closest to the star are rocky in nature.

       Circling their star every 1.5, 2.5, 4, 6, 9 and 12 days, the red dwarf’s six inner planets are close enough to each other for their gravitational interactions to show they are 0.4 to 1.4 times the mass of the Earth. Such a collection of planets is of great interest for understanding planetary system formation and stability, and consequently for estimating the number of such system in the Galaxy that might support civilizations. The mass of the planets and their closeness to the star indicates they were unlikely to be formed as close to the star as they now are. There would not be enough mass in the accretion disk at this location. The planets were probably formed much further out, and migrated inwards. Consequently, they brought with them large quantities of ice, which may be present in some locations as surface water. The closeness of the planets to each other means that some will appear much larger than our Moon when viewed from an adjacent planet.

       The TRAPPIST telescope system that made the initial discover consists of two 0.6 m robotic telescopes. One is located in the Southern hemisphere at the ESO La Silla Observatory in Chile; the other in the Northern hemisphere at Oukaïmden Observatory in Morocco.  The Ritchey-Chretien 0.6 m telescope  has a Peltier-cooled commercial camera equipped with a Fairchild 3041 back-illuminated 2k × 2k chip. The pixel scale is 0.64”/pixel. The camera field of view is 22’ × 22’. Housed in a 5 m diameter dome, the observatory is fully robotic and can be operated from Belgium.

    The possibility of life arising on red dwarf planets and giving rise to a civilization continues to be actively debated. The fact that the planets are tidally locked with one hemisphere always facing the star leads to very large temperature differences between the star-side and night-side surfaces. The possibility remains that there may be a temperate zone favourable to life at the junction of the two hemispheres, and that this zone may be widened by strong winds in the atmosphere producing cool cases gases from winds at different temperatures. There is also the possibility that life might exist underground where temperatures are less extreme.

    The fact that the planets are tidally locked might be an advantage for the evolution of a civilization. There is an argument that civilization on Earth has arisen largely as a result of our large moon stabilizing Earth’s spin axis, so that wildly fluctuating climate regimes do not disrupt the evolutionary process. Tidal locking  could provide a similar stability.

    TRAPPIST-1  is relatively quiescent and appears to be well past its initial phase of high bursts of radiation. This suggests that it is at least 500 million years old. It could be,older, as such stars burn their fuel so slowly that they may last 10 trillion years or more.

    The closeness of the planets to their star still implies high radiation levels at their surfaces. Using the XMM–Newton X-ray space telescope, a team led by Peter Wheatley, University of Warwick, England, has found that TRAPPIST-1 is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower overall luminosity. Because the planets are so much closer to the star, the XUV irradiation they receive is many times stronger than experienced by the present-day Earth. There is probably sufficient X-ray and EUV irradiation to significantly alter their atmospheres. Whether this high-energy irradiation makes the planets more habitable or less so the team considers an open question.

    Radiation can remove water from the atmosphere, but it could remain at the poles of a planet or on the dark side. Receiving higher radiation levels, the two inner planets may have lost much of their water but those further out may retain appreciable quanatities.