Detection of Exoplanets: A New Technique

The first so-called exoplanet was discovered by researchers at the Pennsylvania State University in 1993. They found planets around a pulsar. Since then 330 exoplanets have been found (March 2009) outside our Solar System. The first exoplanet to orbit around an ordinary Sun-like star has been discovered. This was followed by first detection of an exoplanet with an atmosphere (1995); the first Earth-like exoplanet, which is made up of carbon or silicon-based rocks rather than acting as gas giants. Ground-based as well as space telescopes have discovered them rather quickly.

One of the most striking advances in the use of telescopes has occurred in interferometry, especially optical interferometry. It produces spatial resolutions, unimaginable until recently in single aperture telescopes. For instance, the twin Keck telescopes on Hawaii’s Mauna Kea, now function as the Keck Interferometer. Seven of the Mauna Kea telescopes are joined by fibre optics in the OHANA (Optical Hawaiian Array for Nanoradian Astronomy) network. Baselines of up to 800 metres separated the telescopes. The challenge here is to keep the distance between telescopes constant to within a faction of a wavelength of visible light.

In a related technique, light from a star can be ‘cancelled’ out only to see the light from an exoplanet, believed to orbit it. The technique was developed since the light from the star would be so bright that the planet would be invisible, just like a torchlight near a powerful car headlight. The technique, called nulling interferometry, is needed to observe a single star, using several satellites spread out in time. The light arriving at different telescopes is combined in an interferometer.

Ground-based telescopes measure the wobble of stars caused by the pull of orbiting planets. Another technique, based on photometry, which shows the size of the planet, seeks to detect distant planets by measuring the minute dimming of the star as an orbiting planet passes between it and the Earth. Such a passage of a planet is called a transit. The background light from a star, scattered by the telescope’s mirror is suppressed to obtain a clean exoplanet signal. The dimming of the starlight directly reflects the size ratio between the star and the planet.

An European satellite, COROT, launched in December 2006, is designed to detect Earth-like planets by registering the slight changes in the luminous intensity of the star due to the passage of a planet in front of it, using a highly precise photometric telescope. Photometry is a complement to spectroscopy which provides an estimate of a planet’s mass and not its size.

COROT was put into a polar orbit of about 850 km and is oriented in such a way the Sun will always be behind the entrance pupil of the telescope. The spacecraft has a line of sight in the same direction for 150 days at a time, and it would thus observe some 120,000 faint stars for extrasolar planets. Most recently, it has found a planet orbiting a star similar to the Sun, only about 1500 light years from the Earth. At least 40 terrestrial planets and thousands of gaseous ones may be located. The data would enable ground-based telescopes determine the mass and density. It will reject ‘stray’ light not directly coming from the target CORT has paved the way for NASA’s Kepler mission (launched in March 2009) and Darwin mission, expected after 2015.

A telescope based on the Moon has also been suggested. It can provide up to 14 days of continuous observing time, as against no more than 12 hours a night by a telescope on the ground. The far side of the Moon offers electro-magnetically quite sites that could host a sensitive radio telescope. However, opinion is divided on the advantages of a Moon-based telescope. Some astronomers are of the view that the logistics of operating a telescope on the Moon and the maneuverability of the dish would be difficult, whereas radio interferometry using a few spacecraft could provide a better alternative.

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