Two planets orbiting a star 950 light-years from Earth are the smallest, most Earth-size alien worlds known, astronomers announced Tuesday, Dec. 20. One of the planets is actually smaller than Earth. In the distant past they may have been able to support life and one of them may have had conditions similar to our own planet - a so-called Earth-twin - according to the research team.
They have described their findings as the most important planets ever discovered outside our Solar System. Dr Francois Fressin, of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US, who led the research, said that the discovery was the beginning of a "new era" of discovery of many more planets similar to our own. Both planets are now thought to be too hot to be capable of supporting life.
The larger of the two might have been an Earth twin in the past. It has the same size as Earth and in the past it could have had the same temperature.
The researchers say that these planets are rocky and similar in composition to our own planet. The planets' composition may be similar to Earth's with a third of it consisting of iron core. The remainder probably consists of a silicate mantle. He also believes that the outer planet (Kepler 20f) may have developed a thick, water vapour atmosphere.
The discovery is important because it is the first confirmation that planets the size of Earth and smaller exist outside our Solar System. It also shows that the Kepler Space Telescope is capable of detecting relatively small planets around stars that are thousands of light-years away. The telescope has discovered 35 planets so far. Apart from 20e and f, they have all been larger than the Earth.
The most significant discovery, also by a group including Dr Fressin, was of a planet nearly two-and-a-half times the size of Earth that lay in the so-called "Goldilocks zone". This is the region around a star where it is neither too hot, nor too cold, but just right for liquid water and therefore life to exist on the planet.
But Dr Fressin believes that the two new planets are a much more important discovery. The telescope is scanning 150,000 stars and it is believed that soon it will find a planet the size of Earth in the Goldilocks Zone.
These planets, while roughly the size of our planet Earth, are circling very close to their star, giving them fiery temperatures that are most likely too hot to support life, researchers said. The discovery, however, brings scientists one step closer to finding a true twin of Earth that may be habitable.
Two of the star system's planets, Kepler-20e and Kepler-20f, are 0.87 times and 1.03 times the width of Earth, respectively, making them the smallest exoplanets yet known. They have masses less than 1.7 and 3 times Earth's mass, respectively.
Kepler-20e makes a circle around its star once every 6.1 days at a distance of 4.7 million miles (7.6 million kilometers) — almost 20 times closer than Earth, which orbits the sun at around 93 million miles (150 million km). The planet's sibling, Kepler-20f, makes a full orbit every 19.6 days, at a distance of 10.3 million miles (16.6 million km).
The first alien planet was discovered in 1996, and the first planet found through the transit method came just 11 years ago. Both of those planets were roughly the size of Jupiter.
Earlier this month, the Kepler team announced another landmark find, the first planet known to occupy the habitable zonearound its star where liquid water, and perhaps life, could exist. That planet, called Kepler-22b, is about 2.4 times as wide as Earth.
Kepler Space Telescope
To discover the new planets, Fressin and his colleagues used NASA's Kepler space telescope, which noticed the tiny dips in the parent star's brightness when the planets passed in front of it, blocking some of its light (this is called the transit method). The researchers then used ground-based observatories to confirm that the planets actually exist by measuring minute wobbles in the star's position caused by gravitational tugs from its planets.
Stares fixedly at a patch corresponding to 1/400th of the sky
Looks at more than 155,000 stars
Has so far found 2,326 candidate planets
Among them are 207 Earth-sized planets, 10 of which are in the "habitable zone" where liquid water can exist
