Exoplanets are planets that orbit a star other than our Sun. The prefix ‘exo’ means ‘outside’. Astronomers have confirmed more than 4,000 exoplanets orbiting distant stars. They found first two exoplanets orbiting a pulsar in 1992.they can be hot enough to boil metal or locked in deep freeze. They can orbit their stars so tightly that a ‘year’ lasts only a ‘few days’. They can orbit orbit two suns at once and some of them are sunless rogues, wandering through the galaxy in permanent darkness.

There are four types of exoplanets exist:

1. Gas giant

A gas giant is a large planet mostly composed of helium and/or hydrogen. These planets, like Jupiter and Saturn in our solar system, don’t have hard surfaces and instead have swirling gases above a solid core. Gas giant exoplanets can be much larger than Jupiter, and much closer to their stars than anything found in our solar system. The known Gas giant is‘51 Pegasi b’. More variety is hidden within these broad categories. Hot Jupiter, for instance, were among the first planet types found – gas giants orbiting so closely to their stars that their temperatures soar into the thousands of degrees

• Super earth

A gas giant is a large planet mostly composed of helium and/or hydrogen. These planets, like Jupiter and Saturn in our solar system, don’t have hard surfaces and instead have swirling gases above a solid core. Gas giant exoplanets can be much larger than Jupiter, and much closer to their stars than anything found in our solar system. The 2^nd closest exoplanet which is from super earth ‘Barnard’s star b’.

• Neptunian

Neptunian exoplanets are similar in size to Neptune or Uranus in our solar system. Neptunian planets typically have hydrogen and helium-dominated atmospheres with cores or rock and heavier metals. The known Neptunian exoplanet is ‘Kepler-1655 b’. They likely have a mixture of interior compositions, but all will have hydrogen and helium-dominated outer atmospheres and rocky cores. We’re also discovering mini-Neptune, planets smaller than Neptune and bigger than Earth. No planets of this size or type exist in our solar system.

• Terrestrial

In our solar system, Earth, Mars, Mercury and Venus are terrestrial, or rocky, planets. For planets outside our solar system, those between half of Earth’s size to twice its radius is considered terrestrial and others may be even smaller. Exoplanets twice the size of Earth and larger may be rocky as well, but those are considered super-Earths. They are Earth sized and smaller, composed of rock, silicate, water or carbon. Further investigation will determine whether some of them possess atmospheres, oceans or other signs of habitability.

How do we detect Exoplanets?

There are five significant methods to detect exoplanets:

1. Direct imaging:

The exoplanet is imaged directly using large telescopes fitted with adaptive optics and coronagraphs. The technique is most sensitive to the warmer, bright (young) and massive exoplanets on wide and/or eccentric orbits (large sky projected separations). The separation from the host star allows for spectra to be obtained directly and allows for the direct measurement of the luminosity.

    II.          Radial velocity:

The exoplanet is detected by measuring the Doppler shift in the host star light, a consequence of the gravitational affects between the two bodies. The technique is most sensitive to exoplanets with a large mass orbiting close to their host star perpendicular to the plane of the sky. The radial velocity technique allows for a minimum mass (dependant on orbital inclination) to be calculated.

 III.          Transits:

The exoplanet is detected by measuring a periodic decrease in the flux received from the host star, as a consequence of the exoplanet transiting in front of the host star. The transiting technique is most sensitive to large exoplanets orbiting close to their host star stars and provides an accurate determination of the planetary radius relative to the host star.

 IV.          Microlensing:

The exoplanet is detected by measuring characteristic light curve changes caused by changes in the lensing effect observed when a star with a planet passes in front of a distant star. The technique is limited to distant one time events and by the lack of accurate determinations of the planet and orbit parameters. It is however a very valuable technique due to the lack of strong radii or mass biasses making it ideal for statistical population studies.

   V.          Transit timing variations:

The exoplanet is detected by observing a change in periodic phenomena due to the presence of an exoplanet. Examples include a change in transit time (known as TTV) of one planet, due to the presence of others in multiple planet systems and pulsar timing, where anomalous movement (measured at radio wavelengths) can be used to infer the presence of a planet.

Latest reports about Exoplanets:

• A newly discovered, Neptune-like planet some 90 light-years away might possess a robust atmosphere – and perhaps even a ‘tail.’
• NASA’s Hubble Space Telescope is giving astronomers a rare look at a Jupiter-sized, still-forming exoplanet that is feeding off material surrounding a young star.
• Our nearest neighbouring star, Proxima Centauri, has a bad habit: frequently erupting in potentially damaging flares.