Shadow that brightens
(appeared in Apr 2016)

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The search for planets outside the Solar System is set to follow a new path, says S.Ananthanarayanan.

The methods used to detect planets around distant stars were effective mainly with planets that were fairly close to the parent stars. This was not a disadvantage because it was felt that it was the ‘earth-like’ planets, which would be in closer orbits, which we were interested in. But being confined to inner orbits left us ‘in the dark’ about a swarm of planets at greater distances, and hence a great part of the population of the near cosmos.

The instrumentation on board Kepler, the exoplanet detecting satellite that has been following the earth in orbit around the sun since 2009, and also earth based stations, have notched up a score of more than 5,000 candidate planets , using a technique of measuring a slight drop in intensity when a near-orbit planet moves across the face of a star. The same facility has now been readied to do the reverse – detect a spurt in the intensity when a distant-orbit, or free-floating planet moves past!

The difficulty in detecting planets, especially outside the Solar System, is that planets have no light of their own, they only reflect the light of their sun. We can detect this reflected light from nearby planets when it is night-time and the sun is behind our backs. But with planets of distant stars, the dim reflected light is lost in the glare of the star and distant planets cannot be seen in telescopes.

The way distant planets, known as exoplanets, were first detected is by slight movement of the parent star, which is caused by the orbiting of the planet. This movement of the star, for the same reason that the whistle of a train engine that ruses past is shrill at first and then falls to lower pitch, causes a slight rise and fall of the frequency of light emitted by the star. The change in frequency can be detected on the earth and this helps work out what kind of planet it is that has caused the motion of the star. One can imagine that the movement and hence the change in frequency would be greater with planets that are in near orbits and moved fast and it is planets of this kind that were mostly detected by this method.

But with advances in sensitivity of detection of the intensity of light coming from stars, another method was developed, based on the very small but definite blocking of light when a planet crossed the path of light coming to us from a star. This kind of detection is many times more sensitive when carried out from outside the scattering effects of the earth’s atmosphere, and this is what the Kepler space telescope was sent up to do.

Kepler

The Kepler Space Observatory, which was launched in 2009, is a 15 ft by 9 ft craft, consisting essentially of photometer telescope, which can continuously monitor the brightness of 1,45,000 stars in a fixed direction. And along with the telescope, there is an arrangement to send images back to earth by wireless, gyro-wheels to make small adjustments of orientation and solar panels to provide power. The craft is launched to orbit the sun, like the earth, and is positioned just behind the earth, with the distance increasing gradually, as Kepler moves slightly slower that the earth. And the craft is oriented so that sunlight does not enter the telescope, but only starlight from the most suitable direction. The huge mass of data that is transmitted to earth is then analysed by computer algorithms to find the patterns that indicate a planet, and also by human volunteer researchers, to pick up instances the computers may miss.

But in 2012, Kepler lost one of its four position adjusting devices and another one in 2013. It did look like many planned mission objectives may need to be dropped. But with great ingenuity, making use of the pressure of light on the solar panels, it has now become possible to change Kepler’s orientation, nevertheless, as it goes around the sun. NASA has hence pressed Kepler‘s brightness sensing capacity, in a new mission known as K2, or ‘second life’, and Kepler continued on its exoplanet detecting spree. It is now set to join with other, earth based observatories in a 3-month intense hunt for exoplanets that are in outer orbits of host stars and also a category of ‘free floating’ planets wandering in the central region of the Milky Way. These are also important components of the universe and there is some evidence to suggest that there are more of such bodies than stars in our galaxy!

Gravitational lensing

Planets that are at a distance from a parent star block less of the light emitted by the star than planets that are in near-orbits. Distant planets thus cause very little ‘dimming’ of starlight, which cannot be detected by the Kepler photometers. But stars at a distance bring about another effect which makes them detectable, not because of ‘dimming’, but because they act like a magnifying glass and result in ‘brightening’ of starlight.

Objects, like planets, which have mass, exert the force of gravity, which has been interpreted as being in fact an effect on the structure of space itself. It is this distortion of space by an object that other objects with mass sense as the force of gravity of the first object, and so on. Distortion of space also leads to bending of the path of light. This effect was a major prediction of the General Theory of Relativity and was verified during a total solar eclipse, when stars that should have been hidden by the sun’s disc came into view as a result of light being bent while passing the sun.

In the same way, a massive planet in the path of light from a star would have the effect of bending the rays of light, so that light could be concentrated, and hence more intense, at a point further away. The effect would be more discernable in the case of planets at greater distances from stars, as the bending is slight, in fact it is known as ‘micro-lensing, and the effect is more marked if the cone of light passing the planet is narrow.

As Kepler has sensitive photometers that continuously scan stars in a fixed field of view it would be able to detect a possible exoplanet as soon as there is a rise in the intensity, and alert an array of earth based observatories. Simultaneous observation from a number of places would help confirm that is a case of an exoplanet and as Kepler is 160 million km away, it would be possible to fix an exoplanet’s distance and position.

Starting this month, K2 and some fifty earth based observatories in five continents are carrying out a survey called Campaign 9, to monitory a region of space near the centre of the Milky Way, to look for earth sized planets in orbit at long distances from their parent stars, or, in some cases, not even orbiting a star. Over the 80-odd days of the campaign, over a hundred lensing events are expected, with about ten percent showing the characteristics of exoplanets in an unexplored part of space.

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