Slowly orbiting the Sun, its eyes fixed on a single region of our galaxy, the Kepler space observatory quietly goes about its business, scouring the Milky Way for Earth-like planets.

The spacecraft, launched in 2009, is a specially designed telescope known as a photometer, or light meter. The telescope monitors the brightness of stars, waiting for them to dim – evidence of an orbiting planet. A dimming star is the result of a planet passing between the star and the observer, an event known as a transit. These can occasionally be glimpsed from the Earth, as Mercury and Venus cross between the Sun and us.

When the Kepler detects these transits in distant stars, it transfers the information back to Earth. Scientists can then use a series of observations and calculations to work out the characteristics of the planet, including its size and its distance from the star. But what are they searching for?

The mission has a number of objectives, but one of its main goals is to identify planets that are between one half and twice the size of Earth, and within the habitable zone – the term given to the region around a star in which planets can sustain liquid water. These are the planets that, in theory, could support life as we know it.

For the entire duration of its mission, the Kepler will keep its lens trained on a single star field, in a nearby region of our Milky Way galaxy, monitoring over 100,000 stars simultaneously. In the years that the telescope has been active, it has identified 2,740 candidate planets. Whilst only 105 of these have been confirmed, scientists optimistically estimate that 90% of the remaining candidates will also prove to be planets.

Some of these identified planets are Earth-like, similar in size to our planet and within their star’s habitable zone. One of the most recent discoveries is perhaps the most similar planet to Earth yet found.

The discovery of this planet, officially known as KOI (Kepler Object of Interest) 172.02, was announced just this month, along with 460 other candidate planets. KOI 172.02 is only slightly larger than the Earth (it has a radius 1.5 times that of the Earth), and orbits a star similar to our Sun, although it is slightly closer to its star – its year is just 242 days.

Although further study is required to confirm that this object is indeed what they suspect it to be, scientists are excited by the discovery. The prospect of another planet as habitable as Earth is an idea that is increasingly moving from science fiction to science fact, with another recent study from the Kepler team estimating the Milky Way to hold 17 billion Earth-sized planets in orbits that could be within habitable zones.

As Steve Howell, a member of the Kepler Science Team, said in a recent statement: “It is no longer a question of will we find a true Earth analogue, but a question of when.”