Free Astronomy Magazine March-April 2018

9 MARCH-APRIL 2018 EXOPLANETS A stronomers used Hubble to ana- lyze light from the nearby star TRAP- PIST-1 that passed through the atmo- spheres of four Earth-sized planets in the star’s habit- able zone. This zone is a region at a dis- tance from the star where liquid water, the key to life as we know it, could exist on the planet's sur- faces. The astrono- mers were looking for the signatures of certain gases, includ- ing hydrogen, in the atmospheres that were imprinted on the starlight. The graphic at the top shows a model spec- trum containing the signatures of gases the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation. The Hubble Space Telescope observations, however, revealed that the planets do not have hydrogen-dominated atmospheres. The flat spectrum shown in the illustration at the bottom indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmospheres. The researchers concluded that the atmospheres are composed of heavier ele- ments residing at much lower altitudes than could be measured by the Hubble observations. [NASA, ESA, and Z. Levy (STScI)] and TRAPPIST-1g, about 3 to 7 million km away from the star. The enthusiasm generated by these poten- tially Earth-like planets, however, was somewhat lack- ing in the following months. Indeed, in June and July, two teams of researchers from Harvard-Smithsonian Center for Astrophysics (CfA) announced the results of two independent studies on the effects of red dwarf’s stellar activity in the surrounding environment and therefore on the planets. One of the two teams of researchers, led by Manasvi Lingam, focused on the consequences of TRAPPIST-1’s ultraviolet radiation on the planetary system. As that radiation is much more intense than that from the Sun which hits the Earth, researchers came to the conclu- sion that the atmospheres of those worlds could be destroyed in a short time, astronomically speaking. This scenario, described in the International Journal of Astrobiology , obviously clashes with the possibility that the dwarf star’s planetary system can accommo- date life. Indeed, CfA researchers estimate that under those conditions, life has less than a 1% chance of ap- pearing compared to the Earth. The second CfA team, led by Cecilia Garraffo, and also attended by the University of Massachusetts, high- lighted another deadly threat to those planets. Like the Sun, even TRAPPIST-1 pours copious amounts of high-energy particles into the surrounding space, but as the dwarf is much closer to its worlds, the stellar wind pressure that invests them is 1000 to 100,000 times greater than that exerted on the Earth by the solar wind. As the stellar wind carries with it the mag- netic field associated with the particles, researchers claim (in an article in The Astrophysical Journal Let- ters ) that this specific magnetic field can be directly connected to the magnetic fields of the individual planets. If this is the case, the flux of particles emitted all around by TRAPPIST-1 would continuously affect the atmospheres, which would gradually be eroded