​Tokyo, Nov 29 (IANS) In an important step towards the search for extra-terrestrial life, astronomers in Japan have observed the transit of a potentially habitable Earth-like extra-solar planet known as K2-3d.

A transit is a phenomenon in which a planet passes in front of its parent star, blocking a small amount of light from the star, like a shadow of the planet.

While transits have previously been observed for thousands of other extra-solar planets, K2-3d is important because there is a possibility that it might harbour extra-terrestrial life, the researchers said.

The group of researchers from the National Astronomical Observatory of Japan (NAOJ) and University of Tokyo, among others, observed the transit using the MuSCAT instrument on the Okayama Astrophysical Observatory 188-cm telescope. 

K2-3d is an extra-solar planet about 150 light-years away that was discovered by the NASA K2 mission (the Kepler telescope's "second light").

About 30 potentially habitable planets that also have transiting orbits were discovered by the NASA Kepler mission, but most of these planets orbit fainter, more distant stars. 

Because it is closer to Earth and its host star is brighter, K2-3d is a more interesting candidate for detailed follow-up studies, the researchers said.

By observing its transit precisely using the next generation of telescopes, such as Thirty Meter Telescope (TMT), the scientists expect to be able to search the atmosphere of the planet for molecules related to life, such as oxygen.

The study, published in The Astronomical Journal, also succeeded in measuring the orbital period of the planet with a high precision of about 18 seconds. 

This greatly improved the forecast accuracy for future transit times. So now researchers will know exactly when to watch for the transits using the next generation of telescopes. 

K2-3d's size is 1.5 times the size of the Earth. The planet orbits its host star, which is half the size of the Sun, with a period of about 45 days. 

Compared to the Earth, the planet orbits close to its host star (about one-fifth of the Earth-Sun distance). 

But, because the temperature of the host star is lower than that of the Sun, calculations showed that this is the right distance for the planet to have a relatively warm climate like that of the Earth's.