Article Posted on : - May 21, 2013

Mysteries of the universe have always attracted humans, with scientists venturing beyond what is just visible. Exploring the universe in search of far-off worlds humans have struggled with the question of whether we are alone in the universe. Today, scientists are armed with evidence to prove that planets do indeed orbit other stars.

With the discovery of the first exoplanet in the mid-1990s, there has been an explosion of discoveries, revealing hundreds of strange worlds orbiting faraway stars.

Scientists, till date, have discovered about 406 exoplanets. The vast majority have been detected through radial velocity observations and other indirect methods.

Exoplanets

An exoplanet, or extrasolar planet, is a planet beyond the solar system, orbiting a star other than the Sun (the "exo" prefix means "outside" in Greek).

Exoplanets are very small compared to other stars. As a result the planets are lit almost entirely by their star's reflected light.

The first true exoplanet discovery came in 1992, when Dr. Alexander Wolszczan, a radio astronomer at Pennsylvania State University, USA reported what he called "unambiguous proof" of extrasolar planetary systems. Wolszczan indicated that exoplanets existed around PSR B1257+12 pulsar which is located 980 light-years from the Sun. He made the discovery by observing regular variations in the pulsar's rapidly pulsed radio signal, indicating the planets' complex gravitational effects on the dead star.

The first discovery of a planet orbiting a star similar to the sun came in 1995. The Swiss team of Michel Mayor and Didier Queloz of Geneva announced that they had found a rapidly orbiting world located blisteringly close to the star 51 Pegasi.

Methods for detecting exoplanets

The search for exoplanets is one of the most exciting fields in astronomy. Astronomers have therefore developed multiple techniques that enable them to find planets around other stars.

1) The Radial Velocity method

The radial velocity method has been the most successful so far in finding extrasolar planets. Radial velocity of a star or other luminous but distant object can be measured accurately by taking a high-resolution spectrum and comparing the measured wavelengths of known spectral lines to wavelengths from laboratory measurements.

2) The Transit Photometry method

This method uses the fact that when a smaller and less bright object passes in front of a bright object, such as a star, the star appears to fade in luminosity, even if the reduction is very small astronomers can detect it.

3) Microlensing

Microlensing is a phenomenon that occurs when an object with enough mass passes between Earth and a background star. A passage that lasts for long would indicate that the planet is orbiting at a greater distance than it would have, if the passage only lasted for hours.

4) The Astrometry method

With the help of astrometry, astronomers study the precise, periodic wobble that a planet induces in the position in the sky of its parent star. This method is old and can be used to find unseen companions, such as planets, brown dwarfs or even faint stars.

5) Direct Imaging

There are exceptional circumstances in which it is possible to observe an exoplanet with a telescope, and several such planets have been directly imaged to date.

The space missions for finding exoplanets include NASA's Space Interferometry Mission, Terrestrial Planet Finder and Kepler , ESA's Darwin, Gaia and (Eddington) and France's COROT.

Recently, on 19th October, 2009, astronomers have found 32 new planets outside the solar system with the High Accuracy Radial Velocity Planet Searcher, better known as HARPS, the spectrograph for the European Southern Observatory's (ESO) 3.6-metre telescope.

With this discovery, the search for small planets, those with a mass of a few times that of the Earth — known as super-Earths and Neptune-like planets — has received a dramatic boost.

A large number of the exoplanets found so far are known as 'Hot Jupiters' - gas giant planets similar in size to Jupiter. Unfortunately, they are unsuitable for hosting life. Therefore, the most likely location for life to exist is on small 'Earth-like' rocky planets within a region of the stellar system known as the 'Habitable zone' where temperatures are suitable for liquid water to form.

Finally, human destiny lies in exploring the universe and finding the roots of humankind, biologically and chemically, out among the stars. New space missions will definitely play a vital role in this regard.


- Courtesy:

NASA/JPL

The Extrasolar Planets Encyclopaedia

The Planetary Societyexoplanets