Planet-hunting NASA’s Space Craft Kepler

Ever since man stepped on moon, there has been tremendous interest and excitement among the scientific community to explore the vast space in pursuit of other worlds like our earth. Already the existence of three types of exoplanets- hot super Earths, gas giants and ice giants was established. Now there is renewed enthusiasm towards quest for terrestrial planets (those one half to twice the size of earth) those in habitable zone of their stars where liquid water and possibly life might exist. Kepler Mission was thus designed to survey our region of the Milky Way galaxy to discover Earth-like or other smaller planets in or near habitable zone and to determine the fraction of hundreds of billions of stars in our galaxy which might have such planets.

Mission Launch and Objectives

The Kepler Space craft was launched by the United Launch Delta II vehicle from the Cape Canaveral Air Force Station in Florida on March 6^th 2009. It was the first planet-hunting space craft sent into space on three and half year mission. The region in the Cygnus and Lyra constellations of our galaxy containing around 150,000 stars is watched by the Kepler space craft. The main objective of the mission is to monitor all stars continuously and measure their brightness at least once in every few hours. To ensure that Kepler telescope’s field of view (FOV) is never blocked at any time of the year by Sun it must be above the elliptic plane. The Spacecraft is placed in an Earth- trailing heliocentric orbit with a period of 372.5 days and it drifts away slowly from Earth in this orbit. By the end of four years, space craft is at distance of 0.5 astronomical units. This orbit is not subjected to earth torques due to gravity gradients, magnetic moments or atmospheric drag and helps in maintaining stable pointing altitude.

Instruments

The Space craft has a specially designed Kepler photometer with 0.95 meter aperture and 105 square degree FOV astronomical telescope. It is pointed at and records the data from just a single group of stars throughout the duration of its mission. Spacecraft provides power, pointing and telemetry to the photometer. Apart from the four small reaction wheels used for maintaining pointing and an ejectable cover no other movable parts were deployed. The data stored will be transmitted to the Earth once in a month.

 

Mode of detection of Extra Solar Planets:

When a planet passes in front of a star as viewed from earth it is called transit.  During the occasions of Mercury or Venus transit we observe a small dot creeping across the Sun and transiting planet would block the sunlight to the Earth. Similarly, Kepler finds new planets by looking for tiny dips in the brightness of the star when the planet crosses the star.  Subsequently the planet’s orbital size would be calculated based on the time taken by the planet to orbit once around the star and the mass by the Kepler’s third law of planetary motion. The size of the planet is found from the depth of the transit (amount of dip in the brightness of the star). From the Orbital size and temperature of the planet, its characteristics can be determined and its habitability will be estimated.

For the first four years after its commissioning, Kepler has successfully identified several planet candidates until it encountered a technical glitch in May 2013 where it lost two of the four reaction wheels that helped in fixing the star field it was staring. But the planet- hunting space craft received a new lease for life when a graduate student with his innovative idea of using pressure from the sunlight as virtual reaction wheel, stabilised the spacecraft. By June 2014, K2mission has become fully operational with expanded search for planets orbiting bright stars nearby. The spacecraft made a comeback with the discovery of a new exoplanet, HIP 116454b using its new mission K2. The newly confirmed planet announced on December 18^th is 2.5 times diameter of earth and follows 9-day orbit around the star which is smaller and cooler than Sun and is 180 light years away from Earth.

Till now the Kepler Space telescope has identified an overwhelming 4175 candidate planets for study and 1000^th planet has been recently verified. Scientists have now verified eight more new planets, six of which are near Earth size and orbit in habitable zone of stars similar to our Sun. This is the region where temperatures are just right for supporting liquid water on the planet’s surface or the Goldilocks Zone. Two of the newly validated planets Kepler-438b and Kepler-442b are less than 1.5 times the diameter of Earth and are supposed to be rocky like Earth. Kepler-438b is 475 light years away, 12% bigger than Earth and orbits its star once in 35.2 days. Kepler-442b is 1,100 light years away, 33% bigger than Earth and orbits its star in 112 days. Both of these planets orbit around stars which are smaller and cooler than our Sun, making the habitable zone to its parent star, in the direction of constellation Lyra. With the treasure trove of the planets discovered by Kepler, planets identified by other telescopes astronomers now have 1800 planets outside our solar system.

One major limitation with K2 mission is that it has to change its FOV after every 80 days. This will help the mission to observe not only on the Earth- sized planets but also at celestial bodies especially cluster of newly-formed stars. Accordingly it will turn its gaze to clusters Pleiades and Hyades followed by Beehive and M67 clusters in April. During the initial stages of the mission, it discovered that Neptune sized cold giants are most common in the Galaxy and that Jupiter-sized planets outnumber stars in the Galaxy. By April 2016 Kepler mission was planned to observe the centre of Milky Way in search of mysterious bodies termed as free-floating planets. By 2018, the mission would be taken over by NASA James Webbs Space Telescope, another exoplanet-hunting spacecraft.
 With this tantalizing new information, scientists are really excited about the possibility of finding another Earth in space and brimming with hope that we are not alone in this Universe.

 

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