The search for extraterrestrial life has led scientists to find more than 4,000 planets as they try to locate one similar to Earth.
However, discovering hospitable worlds is not easy. The planet would need to have all the right ingredients for life and be in a habitable zone – a safe distance from its star, so liquid water could exist on the planet’s surface.
Nasa projects such as the Kepler Space Telescope and Transiting Exoplanet Survey Satellite have helped in many discoveries.
“The years and decades ahead will bring us ever closer to the ultimate in self-reflection: a mirror image of our own planet Earth. A small, rocky world with clouds, oceans and an atmosphere bearing signs of possible life,” Nasa said on its website.
“Such a world might be hundreds of light-years away, perhaps forever out of reach. But the molecular evidence we read in its atmosphere, using ever more acute technology, could give us the answer we've awaited since the dawn of humanity: no, we are not alone.”
The National highlights some of the fascinating discoveries of Earthlike planets made beyond the solar system
Trappist-1: a gold mine for space scientists
The discovery of Trappist-1 set off a wave of excitement across the space community.
It was the first time multiple Earth-sized planets were found in a habitable zone and were orbiting the same star – Trappist-1.
All seven planets are believed to be rocky and have potential for water on the surface.
Trappist-1, a cool dwarf star, was discovered in 1999. Scientists found three planets orbiting it in 2016, then four more planets were identified two years later.
"We now know more about Trappist-1 than any other planetary system apart from our own," Sean Carey, manager of the Spitzer Science Centre, said.
The planets are so close to each other that a person on the surface would be able to see neighbouring planets in the sky – appearing larger than how the Moon looks from Earth.
Kepler-22b: Earth’s twin
Located in a habitable zone, this exoplanet was labelled by Nasa as ‘super Earth’.
Researchers believe the planet, which is larger than Earth, could be another ocean world.
Discovered in 2011, the exoplanet is at a safe distance from its star and could have a surface temperature of 15.5°C.
However, because of the planet’s extreme tilt, the north and south poles would experience sunlight and darkness for half a year each.
Scientists said the discovery was a major milestone in finding Earth’s twin.
"The tremendous growth in the number of Earth-size candidates tells us that we're honing in on the planets Kepler was designed to detect – those that are not only Earth-size, but also are potentially habitable," Natalie Batalha, Kepler deputy science team lead at San Jose State University, said.
"The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods."
Kepler-22b has a mass 36 times of Earth and takes 289.9 days to complete an orbit around its star.
TOI 700 d: 100 million light years from Earth
Discovered in 2020, TOI 700 d is only 20 per cent larger than Earth and orbits an M Dwarf star – a cool and stable type of star that does not release killer stellar flares, or radiation.
However, the planet is tidally locked to its star, like how the Moon is to the Earth, which means only one side gets daylight and its cloud formations and wind patterns could be much different.
One year on the planet is 37.4 days.
Discovered by Nasa’s Transiting Exoplanet Survey Satellite, TOI 700 d is 100 million light years away from Earth.
Kepler-20e/Kepler-20f: orbiting a star like the Sun
Discovered in 2011, Kepler-20e and Kepler-20f are Earth-size planets that orbit a sun-like star outside the solar system.
They are too close to their star to be in the habitable zone and cannot hold liquid water.
However, their discovery was still groundbreaking because it was the first time small exoplanets were found to be orbiting around a star that was similar to the Sun.
Kepler-20e is 0.87 times the radius of Earth and the other planet is 1.03 times its radius – slightly larger than Earth.
Both have very short orbital periods around their stars – Kepler-20e completes one full circle in 6.1 days, while Kepler-20f takes 19.6 days.
“This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them,” Francois Fressin, lead author of the study that published the findings, said when the planets were first discovered.
The rocky planets are too hot to be habitable. Kepler-20f is as hot as Mercury at 426°C and the surface temperature on Kepler-20e is 760°C – hot enough to melt glass.