Saturday 25 March 2017 News Updated at 01:03 PM IST
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Looking beyond our solar system - Deccan Herald
Looking beyond our solar system
Kenneth Chang, March 7, 2017, The New York Times
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habitable ecosystem? An artist's rendering of the seven planets that orbit the star named Trappist-1; (below) the planets' distance from the star, compared to Earth's solar system. ILLUSTRATIONS BY JPL-Caltech/NASA VIA NYT
Not just one, but seven Earth-size planets that could potentially harbour life have been identified orbiting a tiny star not too far away, offering the first realistic opportunity to search for signs of alien life outside the solar system. The planets orbit a dwarf star named Trappist-1, about 40 light years, or 235 trillion miles, from Earth. That is quite close in cosmic terms, and by happy accident, the orientation of the orbits of the seven planets allows them to be studied in great detail.


One or more of the exoplanets in this new system could be at the right temperature to be awash in oceans of water, astronomers said, based on the distance of the planets from the dwarf star. "This is the first time so many planets of this kind are found around the same star,” Michael Gillon, an astronomer at the University of Liege in Belgium and the leader of an international team that has been observing Trappist-1, said during a telephone news conference organised by the journal Nature, which published the findings last month.

Evidence of life

Scientists could even discover compelling evidence of aliens. "I think that we have made a crucial step toward finding if there is life out there,” said Amaury HMJ Triaud, an astronomer at the University of Cambridge in England and another member of the research team. "Here, if life managed to thrive and releases gases similar to that we have on Earth, then we will know.”

Cool red dwarfs are the most common type of star, so astronomers are likely to find more planetary systems like that around Trappist-1 in the coming years. "You can just imagine how many worlds are out there that have a shot to becoming a habitable ecosystem,” Thomas Zurbuchen, associate administrator of NASA’s science mission directorate, said during a recent NASA news conference. "Are we alone out there? We’re making a step forward with this - a leap forward, in fact - towards answering that question.”

Telescopes on the ground now and the Hubble Space Telescope in orbit will be able to discern some of the molecules in the planetary atmospheres. The James Webb Space Telescope, scheduled to launch next year, will peer at the infrared wavelengths of light, ideal for studying Trappist-1. Comparisons among the different conditions of the seven will also be revealing. "The Trappist-1 planets make the search for life in the galaxy imminent,” said Sara Seager, an astronomer at the Massachusetts Institute of Technology, USA who was not a member of the research team. "For the first time ever, we don’t have to speculate. We just have to wait and then make very careful observations and see what is in the atmospheres of the Trappist planets.”

Even if all the planets turn out to be lifeless, scientists will have learned more about what keeps life from flourishing. Astronomers always knew other stars must have planets, but until a couple of decades ago, they had not been able to spot them.

Now they have confirmed more than 3,400, according to the Open Exoplanet Catalogue. The authors of the Nature paper include Didier Queloz, one of the astronomers who discovered in 1995 the first known exoplanet around a sunlike star.

While the Trappist planets are about the size of Earth - give or take 25% in diameter - the star is very different from our Sun. Trappist-1, named after a robotic telescope in the Atacama Desert of Chile that astronomers initially used to study the star, is what astronomers call an "ultracool dwarf,” with only one-twelfth the mass of the Sun and a surface temperature of 4,150 degrees Fahrenheit, much cooler than the 10,000 degrees radiating from the Sun. Trappist is a shortening of Transiting Planets and Planetesimals Small Telescope. During the NASA news conference, Michael gave a simple analogy: If our Sun were the size of a basketball, Trappist-1 would be a golf ball.

Until the last few years, scientists looking for life elsewhere in the galaxy have focused on finding Earth-size planets around Sun-like stars. But it is difficult to pick out the light of a planet from the glare of a bright star. Small dim dwarfs are much easier to study. Last year, astronomers announced the discovery of an Earth-size planet around Proxima Centauri, the closest star at 4.24 light years away. That discovery was made using a different technique that does not allow for study of the atmosphere. Trappist-1 periodically dimmed noticeably, indicating that a planet might be passing in front of the star, blocking part of the light. From the shape of the dips, the astronomers calculate the size of the planet.

Trappist-1’s light dipped so many times that the astronomers concluded, in research reported last year, that there were at least three planets around the star. Telescopes from around the world then also observed Trappist-1, as did the Spitzer Space Telescope of NASA. Spitzer observed Trappist-1 nearly around the clock for 20 days, capturing 34 transits.

Crucial observations
Together with the ground observations, it let scientists calculate not three planets, but seven. The planets are too small and too close to the star to be photographed directly. All seven are very close to the dwarf star, circling more quickly than the planets in our solar system. The innermost completes an orbit in just 1.5 days. The farthest one completes an orbit in about 20 days. That makes the planetary system more like the moons of Jupiter than a larger planetary system like our solar system.

Because the planets are so close to a cool star, their surfaces could be at the right temperatures to have water flow, considered one of the essential ingredients for life.

The fourth, fifth and sixth planets orbit in the star’s "habitable zone,” where the planets could sport oceans. So far that is just speculation, but by measuring which wavelengths of light are blocked by the planet, scientists will be able to figure out what gases float in the atmospheres of the seven planets. So far, they have confirmed for the two innermost planets that they are not enveloped in hydrogen.

That means they are rocky like Earth, ruling out the possibility that they were mini-Neptune gas planets that are prevalent around many other stars. If observations reveal oxygen in a planet’s atmosphere, that could point to photosynthesis of plants - although not conclusively. But oxygen together with methane, ozone and carbon dioxide, particularly in certain proportions, "would tell us that there is life with 99% confidence,” Michael said. Astronomers expect that a few decades of technological advances are needed before similar observations can be made of Earthlike planets around larger, brighter sunlike stars.

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