Seven Earth-size planets with possible liquid water spotted


The TRAPPIST-1 star, an ultra-cool dwarf, has seven Earth-size planets orbiting it. Credit: NASA/JPL-Caltech

Scientists have spotted seven Earth-size planets with masses similar to Earth orbiting around a dwarf star (TRAPPIST-1) the size of Jupiter just 39 light-years from the Sun. The planets’ temperature is low enough to make possible the presence of liquid water on their surface.

In May last year, scientists found three planets passing in front of TRAPPIST-1. Based on further monitoring of the star from the ground and space, scientists have found four more exoplanets orbiting TRAPPIST-1. The results were published today (February 23) in Nature. Michaël Gillon from the Université de Liège in Liège, Wallonia, Belgium is the first author of the current and May 2016 papers.

“This is the first time we have so many Earth-like planets found around a star. The star is low mass and small,” Dr. Gillon said during a press briefing. “The seven stars could have some liquid water and maybe life. These planets are found in the habitable zone of the star. This is the first time we have found so many planets in the habitable zone of a star.”

The scientists have been able to make precise mass measurement for six of the seven planets. Though the mass measurements are preliminary, they do indicate that the planets are terrestrial with liquid water. “The seven planets are suitable for detailed atmospheric study,” said Dr. Gillon. “The architecture suggests that the seven planets formed farther from the star and migrated towards the star.”

“We can study the climate and chemical composition of the planets’ atmosphere,” Dr. Amaury H.M.J. Triaud from the Institute of Astronomy, Cambridge and one of the authors of the paper said during the briefing. “We are first trying to rule out the presence of large hydrogen envelop to make sure that the planets are indeed Earth-like. These will be followed by detailed study of climate and chemical composition to try and find out if there is life on these planets. If there is life on these planets we will know it in a decade.”

The four newly discovered planets orbit around the star every 4.04 days, 6.06 days, 8.1 days and 12.3 days respectively; the orbital period of two of the three planets discovered last year is 1.51 days and 2.42 days respectively. Whenever a planet passes or transits before a star, a small amount of light from the star gets blocked, thus exposing the planet. The time taken for a planet to form two consecutive passes before the star helps in knowing the orbital period of the planet.

Five planets have sizes similar to that of the Earth, while the remaining two are intermediate in size — between Mars (whose radius is half of Earth’s) and Earth. Based on the mass estimates of the six inner planets, the authors suggest that they would have a rocky composition. The sixth planet has low density suggesting a volatile rich composition. The volatile content could be either ice layer and/or atmosphere

The planets were spotted after intense ground-based observation using different telescopes in different countries, including a 20-day continuous monitoring using NASA’s Spitzer Space Telescope.

Published in The Hindu on February 22, 2017